Iguanodon is a genus of ornithopod dinosaur that lived roughly halfway between the first of the swift bipedal hypsilophodontids and the ornithopods' culmination in the ducked-billed dinosaurs. Many species of Iguanodon have been named, dating from the Kimmeridgian age of the Late Jurassic Period to the Cenomanian age of the Late Cretaceous Period from Asia, Europe, and North America. However, research in the first decade of the 2000s suggests that there is only one well-substantiated species: I. bernissartensis, that lived from the Barremian to the early Aptian (Early Cretaceous) in Europe, between about 130 and 120 million years ago. Iguanodon's most distinctive features were its large thumb spikes, which were possibly used for defence against predators.
Discovered in 1822 and described three years later by English geologist Gideon Mantell, Iguanodon was the second dinosaur formally named, after Megalosaurus. Together with Megalosaurus and Hylaeosaurus, it was one of the three genera originally used to define Dinosauria. A large, bulky herbivore, Iguanodon is a member of Iguanodontia, along with the duck-billed hadrosaurs. The taxonomy of this genus continues to be a topic of study as new species are named or long-standing ones reassigned to other genera.
Scientific understanding of Iguanodon has evolved over time as new information has been obtained from the fossils. The numerous specimens of this genus, including nearly complete skeletons from two well-known bonebeds, have allowed researchers to make informed hypotheses regarding many aspects of the living animal, including feeding, movement, and social behaviour. As one of the first scientifically well-known dinosaurs, Iguanodon has occupied a small but notable place in the public's perception of dinosaurs, its artistic representation changing significantly in response to new interpretations of its remains.
Iguanodon was a bulky herbivore that could shift from bipedality to quadrupedality. The best-known species, I. bernissartensis, is estimated to have weighed about 3.08 tonnes (3.5 tons) on average, and measured about 10 metres long (32.8 ft) as an adult, with some specimens possibly as a long as 13 metres (42.6 ft). Other species were not as large; the similarly robust I. dawsoni is estimated at 8 metres long (26.2 ft), and its more lightly-built contemporary I. fittoni at 6 metres (19.7 ft). This genus had a large, tall but narrow skull, with a toothless beak probably covered with keratin, and teeth like those of an iguana, but much larger and more closely packed.
The arms were long (up to 75% the length of the legs in I. bernissartensis) and robust, with rather inflexible hands built so that the three central fingers could bear weight. The thumbs were conical spikes that stuck out away from the three main digits. In early restorations, the spike was placed on the animal's nose. Later fossils revealed the true nature of the thumb spikes, although their exact function is still debated. They could have been used for defense, or for foraging for food. The little finger was elongate and dextrous, and could have been used to manipulate objects. The legs were powerful, but not built for running, and there were three toes on each foot. The backbone and tail were supported and stiffened by ossified tendons, which were tendons that turned to bone during life (these rod-like bones are usually omitted from skeletal mounts and drawings). Overall, in body structure, it was not too dissimilar from its later relatives, the hadrosaurids.
Iguanodon gives its name to the unranked clade Iguanodontia, a very populous group of ornithopods with many species known from the Middle Jurasic to the Late Cretaceous. Aside from Iguanodon, the best-known members of the clade include Dryosaurus,Camptosaurus,Ouranosaurus, and the duck-bills, or hadrosaurs. In older sources, Iguanodontidae was shown as a distinct family. This family traditionally has been something of a wastebasket taxon, including ornithopods that were neither hypsilophodontids or hadrosaurids. In practice, animals like Callovosaurus, Camptosaurus,Craspedodon, Kangnasaurus,Mochlodon,Muttaburrasaurus, Ouranosaurus, and Probactrosaurus were usually assigned to this family. With the advent of cladistic analyses, Iguanodontidae as traditionally construed was shown to be paraphyletic, and these animals are recognized to fall at different points in relation to hadrosaurs on a cladogram, instead of in a single distinct clade. Essentially, the modern concept of Iguanodontidae currently includes only Iguanodon. Groups like Iguanodontoidea are still used as unranked clades in the scientific literature, though many traditional iguanodontids are now included in the superfamily Hadrosauroidea. Iguanodon lies between Camptosaurus and Ouranosaurus in cladograms, and is probably descended from a camptosaur-like animal. At one point, Jack Horner suggested, based mostly on skull features, that hadrosaurids actually formed two more distantly-related groups, with Iguanodon on the line to the flat-headed hadrosaurines, and Ouranosaurus on the line to the crested lambeosaurines, but his proposal has been rejected.
The discovery of Iguanodon has long been accompanied by a popular legend. The story goes that Gideon Mantell's wife, Mary Ann, discovered the first teeth of an Iguanodon in the strata of Tilgate Forest in Whitemans Green,Cuckfield, Sussex,England, in 1822 while her husband was visiting a patient. However, there is no evidence that Mantell took his wife with him while seeing patients. Furthermore, he admitted in 1851 that he himself had found the teeth. Not everyone agrees that the story is false, though. It is known from his note-books that Mantell first acquired large fossil bones from the quarry at Whitemans Green in 1820. Because also theropod teeth were found, thus belonging to carnivores, he at first interpreted these bones, which he tried to combine into a partial skeleton, as those of a giant crocodile. In 1821 Mantell mentioned the find of herbivorous teeth and began to consider the possibility that a large herbivorous reptile was present in the strata. However, in his 1822 publication Fossils of the South Downs he as yet did not dare to suggest a connection between the teeth and his very incomplete skeleton, presuming that his finds presented two large forms, one carnivorous ("an animal of the Lizard Tribe of enormous magnitude"), the other herbivorous. In May 1822 he first presented the herbivorous teeth to the Geological Society of London but the members, among which William Buckland, dismissed them as fish teeth or the incisors of a rhinoceros from a Tertiary stratum. On 23 June 1823 Charles Lyell showed some to George Cuvier, during a soiree in paris, but the famous French naturalist at once discarded them as those of a rhinoceros. Though Cuvier the very next day retracted, Lyell only reported the dismissal to Mantell, who became rather diffident about the issue. In 1824 Buckland described Megalosaurus and was on that occasion invited to visit Mantell's collection. Seeing the bones on 6 March he agreed that these were of some giant saurian — though still denying it was a herbivore. Emboldened nevertheless, Mantell again sent some teeth to Cuvier, who answered on 22 June 1824 that he had determined that they were reptilian and quite possibly belonged to a giant herbivore. In a new edition that year of his Recherches sur les Ossemens Fossiles Cuvier admitted his earlier mistake, leading to an immediate acceptance of Mantell, and his new saurian, in scientific circles. Mantell tried to further corroborate his theory by finding a modern-day parallel among extant reptiles. In September 1824 he visited the Royal College of London but at first failed to find comparable teeth. However, assistant-curator Samuel Stutchbury recognized that they resembled those of an iguana he had recently prepared, albeit twenty times longer. Mantell did not describe his findings until 10 February 1825, when he presented a paper on the remains to the Royal Geological Society of London..
In recognition of the resemblance of the teeth to those of the iguana, Mantell named his new genus Iguanodon or "iguana-tooth", from iguana and the Greek word odon, odontos ("tooth"). Based on isometric scaling, he estimated that the creature might have been up to 18 metres (60 ft) long, more than the 12 metres (40 ft) length of Megalosaurus. His initial idea for a name was Iguana-saurus ("Iguana lizard"), but his friend William Daniel Conybeare suggested that that name was more applicable to the iguana itself, so a better name would be Iguanoides ("Iguana-like") or Iguanodon. He neglected to add a specific name to form a proper binominal, so one was supplied in 1829 by Friedrich Holl: I. anglicum, which was later amended to I. anglicus
A better specimen was discovered in a quarry in Maidstone,Kent, in 1834, which Mantell soon acquired. He was able to identify it as an Iguanodon from its distinctive teeth. The Maidstone slab allowed the first skeletal reconstructions and artistic renderings of Iguanodon. As such, he made some mistakes, the most famous of which was the placement of what he thought was a horn on the nose. The discovery of much better specimens in later years revealed that the horn was actually a modified thumb. Still encased in rock, the Maidstone skeleton is currently displayed at the Natural History Museum in London. The borough of Maidstone commemorated this find by adding an Iguanodon as a supporter to their coat of arms in 1949. This specimen has become linked with the name I. mantelli, a species named in 1832 by Christian Erich Hermann von Meyer in place of I. anglicus, but it actually comes from a different formation than the original I. mantelli/I. anglicus material.
At the same time, tension began to build between Mantell and Richard Owen, an ambitious scientist with much better funding and society connections in the turbulent worlds of Reform Act -era British politics and science. Owen, a firm creationist, opposed the early versions of evolutionary science ("transmutationism") then being debated and used what he would soon coin as dinosaurs as a weapon in this conflict. With the paper describing Dinosauria, he scaled down dinosaurs from lengths of over 61 metres (200 ft), determined that they were not simply giant lizards, and put forward that they were advanced and mammal-like, characteristics given to them by God according to the understanding of the time, they could not have been "transmuted" from reptiles to mammal-like creatures
In 1849, a few years before his death in 1852, Mantell realized that Iguanodon was not a heavy, pachyderm-like animal, as Owen was putting forward, but had slender forelimbs; however, his passing left him unable to participate in the creation of the Crystal Palace dinosaur sculptures, and so Owen's vision of the dinosaurs became that seen by the public for decades. With Benjamin Waterhouse Hawkins, he had nearly two dozen lifesize sculptures of various prehistoric animals built out of concrete sculpted over a steel and brick framework; two Iguanodon, one standing and one resting on its belly, were included. Before the sculpture of the standing Iguanodon was completed, he held a banquet for twenty inside it.
The largest find of Iguanodon remains to date occurred in 1878 in a coal mine at Bernissart in Belgium, at a depth of 322 m (1056 ft). With the encouragement of Alphonse Briart, supervisor of mines at nearby Morlanwelz,Louis Dollo, with Louis de Pauw, oversaw excavation of the skeletons and reconstructed them. At least 38 Iguanodon individuals were uncovered, most of which were adults. Many of them went on public display beginning in 1882 and are still present for viewing; 11 are displayed as standing mounts, and 20 as they were (approximately) found. The exhibit makes an impressive display in the Royal Belgian Institute of Natural Sciences, in Brussels. A replica of one of these is on display at the Oxford University Museum of Natural History and at the Sedgwick Museum in Cambridge. Most of the remains were referred to a new species, I. bernissartensis, a larger and much more robust animal than the English remains had yet revealed, but one specimen was referred to the nebulous, gracile I. mantelli . The skeletons were some of the first complete dinosaur skeletons known. Found with the dinosaur skeletons were the remains of plants, fish, and other reptiles, including the crocodilian Bernissartia.
The science of conserving fossil remains was in its infancy, and was ill-equipped to deal with what soon became known as "pyrite disease". Pyrite in the bones was changing to iron sulphate, damaging the remains by causing them to crack and crumble. When in the ground, the bones were exposed to moisture that prevented this from happening, but when removed into the drier open air, the natural chemical conversion began to occur. Not knowing the true cause, and thinking it was an actual infection, the staff at the Museum in Brussels attempted to treat the problem with a combination of alcohol,arsenic, and shellac. This combination was intended to simultaneously penetrate (alcohol), kill any biological agent (arsenic), and harden (shellac) the fossils. This treatment had the unintended effect of sealing in moisture and extending the period of damage. Modern treatments instead involve either monitoring the humidity of fossil storage, or, for fresh specimens, preparing a special coating of polyethylene glycol that is then heated in a vacuum pump, so moisture is immediately removed and pore space is infiltrated with polyethelene glycol to seal and strengthen the fossil.
Dollo's specimens allowed him to show that Owen's prehistoric pachyderms were not correct for Iguanodon. He instead modelled the skeletal mounts after the emu and wallaby, and put the spike that had been on the nose firmly on the thumb. He was not completely correct, but he also had the disadvantage of being faced with some of the first complete dinosaur remains. A problem that was later recognized was the bend he introduced into the tail. This organ was more or less straight, as shown by the skeletons he was excavating, and the presence of ossified tendons. In fact, to get the bend in the tail for a more wallaby or kangaroo-like posture, the tail would have had to be broken. With its correct, straight tail and back, the animal would have walked with its body held horizontal to the ground, arms in place to support the body if needed.
Excavations at the quarry were stopped in 1881, although it was not exhausted of fossils, as recent drilling operations have shown. During World War I, when the town was occupied by German forces, preparations were made to reopen the mine for palaeontology, and Otto Jaekel was sent from Berlin to supervise. The Allies recaptured Bernissart just as the first fossiliferous layer was about to be uncovered. Further attempts to reopen the mine were hindered by financial problems and were stopped altogether in 1921 when the mine flooded.
Research on Iguanodon decreased during the early part of the 20th century as World Wars and the great Depression enveloped Europe. A new species that would become the subject of much study and taxonomic controversy, I. atherfieldensis, was named in 1925 by R.W.Hooley, for a specimen collected at Atherfield Point on the Isle of Wight. However, what had been a European genus was now being found worldwide, with material in Africa (teeth from Tunisia and elsewhere in the Sahara),Mongolia (I. orientalis), and the United States in North America (I. ottingeri from Utah). Another North American species, from South Dakota, once assigned to Iguanodon as I. lakotaensis, has since been reclassified as the genus Dakotadon.
Iguanodon was not part of the initial work of the dinosaur renaissance that began with the description of Deinonychus in 1969, but it was not neglected for long. David B.Weishampel's work on ornithopod feeding mechanisms provided a better understanding of how it fed, and David B.Norman's work on numerous aspects of the genus has made it one of the best-known dinosaurs.In addition, a further find of numerous Iguanodon skeletons, in Nehden,Nordrhein-Westphalen,Germany, has provided evidence for gregariousness in this genus, as the animals in this a really-restricted find appear to have been killed by flash floods. At least 15 individuals, from 2 to 8 metres long (6.6 to 26.2 ft), have been found here, although at least some of them are gracile iguanodontians and belong to the related Mantellisaurus or Dollodon(described as I. atherfieldensis, at that time believed to be another species of Iguanodon).
Iguanodon material has also been used in the search for dinosaur DNA and other biomolecules. In research by Graham Embery et al, Iguanodon bones were processed to look for remnant proteins. In this research, identifiable remains of typical bone proteins, such as phosphoproteins and proteoglycans, were found in a rib.
Because Iguanodon is one of the first dinosaur genera to have been named, numerous species have been assigned to it. While never becoming the wastebasket taxon several other early genera of dinosaurs became (such as Megalosaurus and Pelorosaurus), Iguanodon has had a complicated history, and its taxonomy continues to undergo revisions. Remains of the best-known species have come from Belgium,England, Germany, Spain, and France. Remains of similar animals possibly belonging to this genus have been found in Tunisia and Mongolia, and a distinct species is present in Utah, USA. Gregory Paul has recommended limiting use of I. bernissartensis to the Bernissart finds, and using I. sp. (meaning undetermined species) for robust iguanodontian remains from Barremian-age rocks of Europe. Thus, after thorough restudy, what had been seen as a quintessentially British dinosaur may in fact be poorly known from England.
I. anglicus was the original type species, but the holotype was based on a single tooth and only partial remains of the species have been recovered since. In March 2000, the International Commission on on Zoological Nomenclature changed the type species to the much better known I. bernissartensis. The original Iguanodon tooth is held at Te Papa Tongarewa, the national museum of New Zealand in Wellington, although it is not on display. The fossil arrived in New Zealand following the move of Gideon Mantell's son Walter there; after the elder Mantell's death, his fossils went to Walter.
Only a few of the many species assigned to Iguanodon are still considered to be valid, and only one may fall within the genus Iguanodon. I. bernissartensis, described by George Albert Boulenger in 1881, is the neotype for the genus. This species is best known for the many skeletons discovered in Bernissart, but is also known from remains across Europe. David Norman suggested that it includes the dubious Mongolian I. orientalis, but this has not been followed by other researchers.
Two species described by Richard Lydekker in the late 1800s are valid, but rarely discussed. I. dawsoni, described by Lydekker in 1889, is known from two partial skeletons found in East Sussex, England, from the middle Valanginian-age Lower Cretaceous Wadhurt Clay. I. fittoni was also described by Lydekker, in 1888. Like I. dawsoni, this species is known from the Wadhurst Clay of East Sussex. Remains from Spain may also pertain to it. Norman (2004) wrote that three partial skeletons are known for it, but this is an error. The two species are separated on the basis of vertebral and pelvic characters, size, and build. For example, I. dawsoni was more robust than I. fittoni, with large Camptosaurus-like vertebrae featuring short neural spines, whereas I. fittoni is known for its "long, narrow, and steeply inclined neural spines". Neither of these species may actually pertain to Iguanodon.
Two species of Iguanodon named by Richard Owen have since been reassigned to other genera. I. hoggi (also spelled I. boggii or hoggii), named by Owen for a lower jaw from the Tithonian-Berriasian-age Upper Jurassic-Lower Cretaceous Purbeck Beds of Dorset in 1874, has been reassigned to Camptosaurus by David Norman and Paul Barrett, although Paul (2008) and Carpenter and Wilson (2008) have argued against this. I. major, a vertebra from the Isle of Wight described by Owen in 1842 as a species of Streptospondylus, is a nomen dubium which is now thought to be a synonym of I. anglicus, although it may be its own species.
Other than the two species described by Owen which have been reassigned to other genera, fourteen other species have since been reclassified. Iguanodon albinus (or Albisaurus scutifer), described by Czech paleontologist Antonin Fritsch (correctly Frič) in 1893, is a dubious nondinosaurian reptile now known as Albisaurus albinus I. atherfieldensis, described by R.W.Hooley in 1925, was smaller and less robust than I. bernissartensis, with longer neural spines. It was renamed Mantellisaurus atherfieldensis in 2007. I. exogyrarum (also spelled I. exogirarum or I. exogirarus) was described by Fritsch in 1878. It is a nomen dubium based on very poor material and has been reassigned, by George Olshevsky, to Ponerosteus. I. valdensis, described by Hulke in 1879 from vertebral and pelvic remains, was from the Barremian stage of the Isle of Wight. Originally named Vectisaurus, it may be a partially-grown specimen of Mantellisaurus atherfieldensis, or from an undetermined species of Mantellisaurus. I. gracilis, named by Lydekker in 1888 as the type species of Sphenospondylus and assigned to Iguanodon in 1969 by Rodney Steel, may belong to Mantellisaurus atherfieldensis.
I. foxii (also spelled I. foxi) was originally described by Thomas Henry Huxley in 1869 as the type species of Hypsilophodon; Owen (1873 or 1874) reassigned it to Iguanodon, but his assignment was soon overturned. I. hollingtoniensis (also spelled I. hollingtonensis), described by Lydekker in 1889, has been regarded as a synonym of I. fittoni, , although this assessment has not been evaluated in detail. Another specimen assigned to I. hollingtonensis by Richard Owen in 1874, with an unusual combination of hadrosaurid-like lower jaw and very robust forelimb, may represent an unnamed taxon. I. prestwichii (also spelled I. prestwichi), described by John Hulke in 1880, has been reassigned to Camptosaurus prestwichii. I. seeleyi (also spelled I. seelyi), described by Hulke two years after I. prestwichii, has been synonymized with I. bernissartensis, although this has been disputed. I. suessii, described by Emanuel Bunzel in 1871, has been reassigned to Mochlodon suessi.
I. lakotaensis was described by David B.Weishampel and Philip R.Bjork in 1989. The only well-accepted North American species of Iguanodon, I. lakotaensis was described from a partial skull from the Barremian-age Lower Cretaceous Lakota Formation of South Dakota. Its assignment has been controversial. Some researchers suggest that it was more basal than I. bernissartensis, and related to Theiophytalia, but David Norman has suggested that it was a synonym of I. bernissartensis.Gregory S.Paul has since given the species its own genus, Dakotadon
Iguanodon mantelli (also spelled I. manteli or I. mantellii), described by Christian Erich Hermann von Meyer in 1832, is actually based on the same material as I. anglicus. Several skeletons, however, including the Maidstone specimen and one of the Bernissart skeletons have been assigned here over the years, and their attribution is not complete. The gracile Bernissart skeleton, for example, has been reassigned, first to Mantellisaurus atherfieldensis, and upon further review, to its own genus and species, Dollodon bampingi. I. orientalis, described by A.K.Rozhdestvensky in 1952, was based on poor material, but a skull with a distinctive arched snout that had been assigned to it was renamed Altirhinus kurzanovi in 1998. At the same time, I. orientalis was considered to be a nomen dubium indistinguishable from I. bernissartensis. Harry Seeley described I. phillipsi in 1869, but later reassigned it to Priodontognathus.
Five Iguanodon species are considered to be nomina dubia or undescribed. I. anglicus, described by Friedrich Holl in 1829, is the original type species of Iguanodon, but, as discussed above, was replaced by I. bernissartensis. In the past, it has been spelled as I. angelicus (Lessem and Glut, 1993) and I. anglicum (Holl, 1829 emend. Bronn, 1850). It is known from teeth from the Valanginian-Barremian-age Lower Cretaceous Tilgate Forest of East Sussex, England. I. hillii, coined by Edwin Tully Newton in 1892 for a tooth from the early Cenomanian Upper Cretaceous Lower Chalk of Hertfordshire, is an early hadrosaurid of some sort. "I. mongolensis" (Whitfield, 1992) is a nomen nudum from a photo caption in a book, of remains that would later be named Altirhinus.
I. ottingeri, described by Peter Galton and James A.Jensen in 1979, is a nomen dubium based on teeth from the possibly Aptian-age lower Cedar Mountain Formation of Utah. I. praecursor (also spelled I. precursor), described by E. Sauvage in 1876 from teeth from an unnamed Kimmeridgian (Late Jurassic) formation in Pas-de-Calais,France, is actually a sauropod, sometimes assigned toNeosodon, although the two come from different formations.
Finally, several other poorly known genera and species are included with Iguanodon without being separate species, although their assignment is less certain with the renaming of I. atherfieldensis. These include Heterosaurus neocomiensis (Cornuel, 1850), Hikanodon (Keferstein, 1834), and Therosaurus (Fitzinger, 1840), and the species "Streptospondylus" recentior (Owen, 1851), "Cetiosaurus" brachyurus, and part of "C." brevis (Owen, 1842; "C." brevis is a chimera). The nomen nudum "Proiguanodon" (van den Broeck, 1900) also belongs here, and possibly the very obscure "Streptospondylus" grandis (Owen, 1851) and meyeri (Owen, 1854).
One of the first details noted about Iguanodon was that it had the teeth of a herbivorous reptile, although there has not always been consensus on how it ate. As Mantell noted, the remains he was working with were unlike any modern reptile, especially in the toothless, scoop-shaped form of the lower jaw symphysis, which he found best compared to that of the two-toed sloth and the extinct ground sloth Mylodon. He also suggested that Iguanodon had a prehensile tongue which could be used to gather food, like a giraffe. More complete remains have shown this to be an error; for example, the hyoid bones that supported the tongue are heavily built, implying a muscular, non-prehensile tongue used for moving food around in the mouth. The giraffe-tongue idea has also been incorrectly attributed to Dollo via a broken lower jaw.
Iguanodon teeth are, as the name suggests, like those of an iguana, but larger. Unlike hadrosaurids, which had columns of replacement teeth, Iguanodon only had one replacement tooth at a time for each position. The upper jaw held up to 29 teeth per side, with none at the front of the jaw, and the lower jaw 25; the numbers differ because teeth in the lower jaw are broader than those in the upper. Because the tooth rows are deeply inset from the outside of the jaws, and because of other anatomical details, it is believed that, as with most other ornithischians, Iguanodon had some sort of cheek-like structure, muscular or non-muscular, to retain food in the mouth.
The skull was structured in such a way that as it closed, the bones holding the teeth in the upper jaw would bow out. This would cause the lower surfaces of the upper jaw teeth to rub against the upper surface of the lower jaw's teeth, grinding anything caught in between and providing an action that is the rough equivalent of mammalian chewing. Because the teeth were always replaced, the animal could have used this mechanism throughout its life, and could eat tough plant material. Additionally, the front ends of the animal's jaws were toothless and tipped with bony nodes, both upper and lower, providing a rough margin that was likely covered and lengthened by a keratinous material to form a cropping beak for biting off twigs and shoots. Its food gathering would have been aided by its flexible little finger, which could have been used to manipulate objects, unlike the other fingers.
Exactly what Iguanodon ate with its well-developed jaws is not known. The size of the larger species, such as I. bernissartensis, would have allowed them access to food from ground level to tree foilage at 4–5 metres high (13–16.5 ft). A diet of horsetails,cycads, and conifers was suggested by David Norman, although iguanodonts in general have been tied to the advance of angiosperm plants in the Cretaceous due to the dinosaurs' inferred low browsing habits. Angiosperm growth, according to this hypothesis, would have been encouraged by iguanodont feeding because gymnosperms would be removed, allowing more space for the weed-like early angiosperms to grow. The evidence is not conclusive, though. Whatever its exact diet, due to its size and abundance, Iguanodon is regarded as a dominant medium to large herbivore for its ecological communities. In England, this included the small predator Aristosuchus, larger predators Eotyrannus,Baryonyx, and Neovenator, low-feeding herbivores Hypsilophodon and Valdosaurus, fellow "iguanodontid" Mantellisaurus, the armoured herbivore Polacanthus, and sauropods like Pelorosaurus.
Early fossil remains were fragmentary, which led to much speculation on the posture and nature of Iguanodon. As discussed, Iguanodon was initially portrayed as a quadrupedal horn-nosed beast. However as more bones were discovered, Mantell observed that the forelimbs were much smaller than the hindlimbs. His rival Owen was of the opinion it was a stumpy creature with four pillar-like legs. The job of overseeing the first lifesize reconstruction of dinosaurs was initially offered to Mantell, who declined due to poor health, and Owen's vision subsequently formed the basis on which the sculptures took shape. Its bipedal nature was revealed with the discovery of the Bernissart skeletons. However, it was depicted in an upright posture, with the tail dragging along the ground, acting as the third leg of a tripod.
During his re-examination of Iguanodon, David Norman was able to show that this posture was unlikely, due to the presence of a long tail stiffened with ossified tendons. To get the tripodal pose, the tail would literally have to be broken. Putting the animal in a horizontal posture makes many aspects of the arms and pectoral girdle more understandable. For example, the hand is relatively immobile, with the three central fingers grouped together, bearing hoof-like phalanges, and able to hyperextend. This would have allowed them to bear weight. The wrist is also relatively immobile, and the arms and shoulder bones robust. These features all suggest that the animal spent time on all fours.
Furthermore, it appears that Iguanodon became more quadrupedal as it got older and heavier;juvenile I. bernissartensis have shorter arms than adults (60% of hindlimb length versus 70% for adults). When walking as a quadruped, the animal's hands would have been held so that the palms faced each other, as shown by iguanodontian trackways and the anatomy of this genus' arms and hands. The three toed pes (foot) of Iguanodon was relatively long, and when walking, both the hand and the foot would have been used in a digitigrade fashion (on the fingers and toes). The maximum speed of Iguanodon has been estimated at 24 km/h (14.9 mph), which would have been as a biped; it would not have been able to gallop as a quadruped.
Large three-toed footprints are known in Early Cretaceous rocks of England, particularly Wealden beds on the Isle of Wight, and these trace fossils were originally difficult to interpret. Some authors associated them with dinosaurs early on. In 1846, E. Tagert went so far as to assign them to an ichnogenus he named Iguanodon, and Samuel Beckles noted in 1854 that they looked like bird tracks, but might have come from dinosaurs. The identity of the trackmakers was greatly clarified upon the discovery in 1857 of the hind leg of a young Iguanodon, with distinctly three-toed feet, showing that such dinosaurs could have made the tracks. Despite the lack of direct evidence, these tracks are often attributed to Iguanodon. A trackway in England shows what may be an Iguanodon moving on all fours, but the foot prints are poor, making a direct connection difficult. Tracks assigned to the ichnogenus Iguanodon are known from locations including places in Europe where the body fossil Iguanodon is known, to Spitsbergen,Svalbard, Norway.
The thumb spike is one of the most well-known features of Iguanodon. Although it was originally placed on the animal's nose by Mantell, the complete Bernissart specimens allowed Dollo to correctly place it on the hand, as a modified thumb. (This would not be the last time a dinosaur's modified thumb claw would be misinterpreted; Noasaurus,Baryonyx, and Megaraptor are examples since the 1980s where an enlarged thumb claw was first put on the foot, as in dromaeosaurids.)
This thumb is typically interpreted as a close-quarters stiletto-like weapon against predators, although it could also have been used to break into seeds and fruits, or against other Iguanodon. One author has suggested that the spike was attached to a venom gland, but this has not been accepted, as the spike was not hollow, nor were there any grooves on the spike for conducting venom.
Although sometimes interpreted as the result of a single catastrophe, the Bernissart finds instead are now interpreted as recording multiple events. According to this interpretation, at least three occasions of mortality are recorded, and though numerous individuals would have died in a geologically short time span (?10–100 years), this does not necessarily mean these Iguanodon were herding animals.
An argument against herding is that juvenile remains are very uncommon at this site, unlike modern cases with herd mortality. They more likely were the periodic victims of flash floods whose carcasses accumulated in a lake or marshy setting. The Nehden find, however, with its greater span of individual ages, more even mix of Dollodon or Mantellisaurus to Iguanodon bernissartensis, and confined geographic nature, may record mortality of herding animals migrating through rivers.
Unlike other purported herding dinosaurs (especially hadrosaurs and ceratopsids), there is no evidence that Iguanodon was s exually dimorphic, with one gender appreciably different from the other. At one time, it was suggested that the Bernissart I. "mantelli", or I. atherfieldensis (Dollodon and Mantellisaurus, respectively) represented a gender, possibly female, of the larger and more robust, possibly male, I. bernissartensis. However, this is not supported today.
Since its description in 1825, Iguanodon has been a feature of worldwide popular culture. Two lifesize reconstructions of Iguanodon built at the Crystal Palace in London in 1852 greatly contributed to the popularity of the genus. Their thumb spikes were mistaken for horns, and they were depicted as elephant-like quadrupeds, yet this was the first time an attempt was made at constructing full-size dinosaur models.
Several motion pitcures have featured Iguanodon. In the Disney film Dinosaur, an Iguanodon named Aladar served as the protagonist with three other iguanodonts as other main characters; a loosely-related ride of the same name at Disney's Animal Kingdom is based around bringing an Iguanodon back to the present. Iguanodon is one of the three dinosaur genera that inspired Godzilla; the other two were Tyrannosaurus and Stegosaurus. Iguanodon has also made appearances in some of the many Land Before time films, as well as episodes of the television series.Aside from appearances on the silver screen, Iguanodon has also been featured on the television documentary miniseries Walking with Dinosaurs (1999) produced by the BBC and played a starring role in Sir Arthur Conan Doyle's book, The Lost world. It also was present in Bob Bakker's Raptor Red (1995), as a Utahraptor prey item. A main belt asteroid, 1989 CB3, has been named 9941 Iguanodon in honour of the genus.
Because it is both one of the first dinosaurs described and one of the best-known dinosaurs, Iguanodon has been well-placed as a barometer of changing public and scientific perceptions on dinosaurs. Its reconstructions have gone through three stages: the elephantine quadrupedal horn-snouted reptile of the Victorians; a bipedal but still fundamentally reptilian animal using its tail to prop itself up; and finally, its current, more agile and dynamic representation, able to shift from two legs to all fours. The second representation dominated the twentieth century, but was slowly overturned during the 1960s.
Tuesday 21 July 2009
Wednesday 15 July 2009
Megalosaurus
Megalosaurus (meaning "Great Lizard", from Greek, μεγαλο-/megalo- meaning 'big', 'tall' or 'great' and σαυρος/sauros meaning 'lizard') is a genus of large meat-eating theropod dinosaurs of the Middle Jurassic period ( Bathonian) of Europe (Southern England, France,Portugal). It is significant as the first genus of dinosaur (outside of birds) to be described and named.
Megalosaurus was the first dinosaur to be described in the scientific literature. Part of a bone was recovered from a limestone quarry at Cornwell near Chipping Norton,Oxfordshire, England in 1676. The fragment was sent to Robert Plot, Professor of Chemistry at the University of Oxford and first curator of the Ashmolean Museum, who published a description in his Natural History of Oxfordshire in 1677. He correctly identified the bone as the lower extremity of the femur of a large animal and he recognized that it was too large to belong to any known species. He therefore concluded it to be the thigh bone of a giant human, such as those mentioned in the bible. The bone has since been lost but the illustration is detailed enough to identify it clearly as the femur of a Megalosaurus.
The Cornwell bone was described again by Richard Brookes in 1763. He called it "Scotum humanum," while comparing its appearance to a pair of human testicles. The label was not considered to be a proper Linnaean "name" for the animal in question at the time, and was not used in subsequent literature. Technically, though, the name was published after the advent of binominal nomenclature, and so if it was truly intended to represent the erection of a new genus it would have priority over Megalosaurus. However, the rules of the ICZN state that if a name falls into disuse for 50 years after publication, it is no longer in competition for priority. Therefore, the name Scrotum humanum would be a nomen oblitum, or "forgotten name" even if it had been a valid genus to begin with.
More discoveries were made, starting in 1815, again at the Stonesfield quarry. They were acquired by William Buckland, Professor of Geology at the University of Oxford and dean of Christ Church. He did not know to what animal the bones belonged but, in 1818, after the Napoleonic Wars, the French comparative anatomist Georges Cuvier visited Buckland in Oxford and realised that the bones belonged to a giant lizard-like creature. Buckland then published descriptions of the bones in Transactions of the Geological Society, in 1824 ( Physician James Parkinson had described them in an article in 1822).
By 1824, Buckland had a piece of a lower jaw with teeth, some vertebrae, and fragments of pelvis,scapula and hind limbs, probably not all from the same individual. Buckland identified the organism as being a giant animal related to the Sauria (lizards) and he placed it in the new genus Megalosaurus, estimating the animal to be 12 m long in life. In 1826, Ferdinand von Ritgen gave this dinosaur a complete binomial, Megalosaurus conybeari, which was not used by later authors and is now considered a nomen oblitum. A year later, in 1827,Gideon Mantell included Megalosaurus in his geological survey of southeastern England, and assigned the species its current binomial name, Megalosaurus bucklandii. It would not be until 1842 that Richard Owen coined the term 'dinosaur'.
In 1997, a famous group of fossilised footprints (ichnites) was found in a limestone quarry at Ardley, 20 km Northeast of Oxford,England. They were thought to have been made by Megalosaurus and possibly also some left by Cetiosaurus. There are replicas of some of these footprints, set across the lawn of Oxford University Museum of Natural History.
Since those first finds, many other Megalosaurus bones have been recovered but still no complete skeleton has been found. Therefore, the details of its physical appearance cannot be certain.
In 1852, Benjamin Waterhouse Hawkins was commissioned to a build a model of Megalosaurus for the exhibition of dinosaurs at the Crystal Palace, which is still there to this day. Early paleontologists, never having seen such a creature before, reconstructed it like the dragons of popular mythology, with a huge head and walking on all fours. It was not until the middle of the nineteenth century, when other theropods began to be discovered in North America, that a more accurate picture was developed. Some confusion still exists, for at one time (before classification of dinosaurs became the serious business it is today), all theropods from Europe were given the title Megalosaurus. Since then, these have mostly been reclassified but older papers can still cause confusion. For further confusion, the most reproduced anatomy diagram of a Megalosaurus' skeleton was produced before any vertebrae had been recovered. While drawing it,Friedrich von Huene ofUniversity of Tubingen,Germany, instead used the backbones of Altisinax, a mysterious big theropod known from high-spined dorsal vertebrae and at times classified as a spinosaur. Hence, many later drawings, based on his original, show Megalosaurus with a deep spinal ridge or even a small sail, like that of Spinosaurus.
In fact, Megalosaurus did have a relatively large head and the teeth were clearly that of a carnivore. However, the long tail would have balanced the body and head and so Megalosaurus is now restored as a bipedal beast—like all other theropods—about 9 meters in length. The structure of the cervical vertebrae suggests that its neck would have been very flexible. To support its weight of around one tonne, the legs were large and muscular. Like all theropods, it had three forward facing toes and a single reversed one. Although they had not reached the minuscule size of later theropods like Tyrannosaurus, the fore limbs of Megalosaurus were small and probably had three or four digits.
Living in what is now Europe, during the Jurassic Period (181 to 169 million years ago), Megalosaurus may have hunted stegosaurs and sauropods. Repeated descriptions of Megalosaurus hunting Iguanodon (another of the earliest dinosaurs named) through the forests that then covered the continent are probably inaccurate, because Iguanodon skeletons are found in much younger Early Cretaceous formations. No fossils assignable to Megalosaurus have been discovered in Africa, contrary to some outdated dinosaur books.
Although Megalosaurus was a powerful carnivore and could probably have attacked even the largest sauropods, it is also likely that it gained some of its food by scavenging. That is not to detract from its prowess as a hunter (Tyrannosaurus probably did much the same). Efficiency was necessary to feed such a large body.
There is a good descriptive display of Megalosaurus and of the history of discovery, in the Oxford University Museum of Natural History.
Megalosaurus was the first dinosaur to be described in the scientific literature. Part of a bone was recovered from a limestone quarry at Cornwell near Chipping Norton,Oxfordshire, England in 1676. The fragment was sent to Robert Plot, Professor of Chemistry at the University of Oxford and first curator of the Ashmolean Museum, who published a description in his Natural History of Oxfordshire in 1677. He correctly identified the bone as the lower extremity of the femur of a large animal and he recognized that it was too large to belong to any known species. He therefore concluded it to be the thigh bone of a giant human, such as those mentioned in the bible. The bone has since been lost but the illustration is detailed enough to identify it clearly as the femur of a Megalosaurus.
The Cornwell bone was described again by Richard Brookes in 1763. He called it "Scotum humanum," while comparing its appearance to a pair of human testicles. The label was not considered to be a proper Linnaean "name" for the animal in question at the time, and was not used in subsequent literature. Technically, though, the name was published after the advent of binominal nomenclature, and so if it was truly intended to represent the erection of a new genus it would have priority over Megalosaurus. However, the rules of the ICZN state that if a name falls into disuse for 50 years after publication, it is no longer in competition for priority. Therefore, the name Scrotum humanum would be a nomen oblitum, or "forgotten name" even if it had been a valid genus to begin with.
More discoveries were made, starting in 1815, again at the Stonesfield quarry. They were acquired by William Buckland, Professor of Geology at the University of Oxford and dean of Christ Church. He did not know to what animal the bones belonged but, in 1818, after the Napoleonic Wars, the French comparative anatomist Georges Cuvier visited Buckland in Oxford and realised that the bones belonged to a giant lizard-like creature. Buckland then published descriptions of the bones in Transactions of the Geological Society, in 1824 ( Physician James Parkinson had described them in an article in 1822).
By 1824, Buckland had a piece of a lower jaw with teeth, some vertebrae, and fragments of pelvis,scapula and hind limbs, probably not all from the same individual. Buckland identified the organism as being a giant animal related to the Sauria (lizards) and he placed it in the new genus Megalosaurus, estimating the animal to be 12 m long in life. In 1826, Ferdinand von Ritgen gave this dinosaur a complete binomial, Megalosaurus conybeari, which was not used by later authors and is now considered a nomen oblitum. A year later, in 1827,Gideon Mantell included Megalosaurus in his geological survey of southeastern England, and assigned the species its current binomial name, Megalosaurus bucklandii. It would not be until 1842 that Richard Owen coined the term 'dinosaur'.
In 1997, a famous group of fossilised footprints (ichnites) was found in a limestone quarry at Ardley, 20 km Northeast of Oxford,England. They were thought to have been made by Megalosaurus and possibly also some left by Cetiosaurus. There are replicas of some of these footprints, set across the lawn of Oxford University Museum of Natural History.
Since those first finds, many other Megalosaurus bones have been recovered but still no complete skeleton has been found. Therefore, the details of its physical appearance cannot be certain.
In 1852, Benjamin Waterhouse Hawkins was commissioned to a build a model of Megalosaurus for the exhibition of dinosaurs at the Crystal Palace, which is still there to this day. Early paleontologists, never having seen such a creature before, reconstructed it like the dragons of popular mythology, with a huge head and walking on all fours. It was not until the middle of the nineteenth century, when other theropods began to be discovered in North America, that a more accurate picture was developed. Some confusion still exists, for at one time (before classification of dinosaurs became the serious business it is today), all theropods from Europe were given the title Megalosaurus. Since then, these have mostly been reclassified but older papers can still cause confusion. For further confusion, the most reproduced anatomy diagram of a Megalosaurus' skeleton was produced before any vertebrae had been recovered. While drawing it,Friedrich von Huene ofUniversity of Tubingen,Germany, instead used the backbones of Altisinax, a mysterious big theropod known from high-spined dorsal vertebrae and at times classified as a spinosaur. Hence, many later drawings, based on his original, show Megalosaurus with a deep spinal ridge or even a small sail, like that of Spinosaurus.
In fact, Megalosaurus did have a relatively large head and the teeth were clearly that of a carnivore. However, the long tail would have balanced the body and head and so Megalosaurus is now restored as a bipedal beast—like all other theropods—about 9 meters in length. The structure of the cervical vertebrae suggests that its neck would have been very flexible. To support its weight of around one tonne, the legs were large and muscular. Like all theropods, it had three forward facing toes and a single reversed one. Although they had not reached the minuscule size of later theropods like Tyrannosaurus, the fore limbs of Megalosaurus were small and probably had three or four digits.
Living in what is now Europe, during the Jurassic Period (181 to 169 million years ago), Megalosaurus may have hunted stegosaurs and sauropods. Repeated descriptions of Megalosaurus hunting Iguanodon (another of the earliest dinosaurs named) through the forests that then covered the continent are probably inaccurate, because Iguanodon skeletons are found in much younger Early Cretaceous formations. No fossils assignable to Megalosaurus have been discovered in Africa, contrary to some outdated dinosaur books.
Although Megalosaurus was a powerful carnivore and could probably have attacked even the largest sauropods, it is also likely that it gained some of its food by scavenging. That is not to detract from its prowess as a hunter (Tyrannosaurus probably did much the same). Efficiency was necessary to feed such a large body.
There is a good descriptive display of Megalosaurus and of the history of discovery, in the Oxford University Museum of Natural History.
Tuesday 7 July 2009
Sauropodomorpha
The Sauropodomorpha were a group of long-necked, herbivorous dinosaurs that eventually dropped down on all fours and became the largest animals that ever walked the earth.
Sauropodomorphs were adapted to browsing higher than any other contemporary herbivore, giving them access to high tree foliage. This feeding strategy is supported by many of their defining characteristics, such as: a light, tiny skull on the end of a long neck (with ten or more elongated cervical vertebrae) and a counterbalancing long tail (with one to three extra sacral vertebrae).
Their teeth were weak, and shaped like leaves or spoons (lanceolate or spatulate). Instead of grinding teeth, they had stomach stones (gastroliths), similar to the gizzard stones of modern birds and crocodiles, to help digest tough plant fibers. The front of the upper mouth bends down in what may be a beak.
The earliest known sauropodomorph, Saturnalia, was small and slender (1.5 metres, or 5 feet long), but by the end of the Triassic they were the largest dinosaurs of their time, and in the Jurassic/Cretaceous they kept on growing. Ultimately the largest sauropods like the Supersaurus,Diplodocus hallorum, and Argentinosaurus reached 30–40 metres (100–130 ft) in length, and 60,000–100,000 kilograms (65–110 US short tons) or more in mass.
Initially bipedal, as their size increased they evolved to become graviportal quadrapeds (like elephants). The early sauropodomorphs were most likely omnivores as their shared common ancestor with the other saurischian lineage (the theropods) was a carnivore. Therefore their evolution to herbivory went hand in hand with their increasing size and neck length.
They also had large nostrils (nares), and retained a thumb (pollex) with a big claw which may have been used for defense — though their primary defensive adaptation was their extreme size.
Among the very first dinosaurs to evolve in the late Triassic Period, about 230 million years ago (Mya), they became the dominant herbivores by half way through the late Triassic (during the Norian stage). Their perceived decline in the early Cretaceous is most likely a bias in fossil sampling, as most fossils are known from Europe and North America. Sauropods were still the dominant herbivores in the Gondwana landmasses, however. The spread of flowering plants (angiosperms) and "advanced" ornithischians, another major group of herbivorous dinosaurs (noted for their highly developed chewing mechanisms) are most likely not a major factor in sauropod decline in the northern continents. Like all non-avian dinosaurs, the sauropodomorphs became extinct 65 Mya, during the Cretaceous-Tertiary extinction event.
The most basal sauropodomorph known, Saturnalia, was discovered in 1999, and is dated to the Carnian stage of the late Triassic. However, fragmentary remains from Madagascar may represent an even earlier sauropodomorph from the middle Triassic.
Sauropodomorpha is one of the two major clades within the order Saurischia.. The sauropodomorphs' sister group, the Theropoda, includes bipedal carnivores like Velociraptor and Tyrannosaurus. However, Sauropodomorpha also share a number of characteristics with the Ornithischia, so a small minority of paleontologists like Bakker place both sets of herbivores within Phytodinosauria (or Ornithischiformes).
In Linnaean taxonomy, Sauropodomorpha (which means "lizard feet forms") is either a suborder or is left unranked. It was originally established by Friedrich von Huene in 1932, who broke it into two groups: the basal forms within Prosauropoda, and their descendants, the giant Sauropoda.
Recent phylogenetic analyses by Adam Yates (2004, 2006) firmly places Sauropoda within a paraphyletic Prosauropoda. Also, finds of late Triassic sauropods demonstrate that there is no gap between the "prosauropod" and sauropod lineages.
Evidence against sauropod ancestry within Prosauropoda comes from the fact that prosauropods had a smaller outer toe on their hind feet than the sauropods. Many maintain that it is easier for digits to be reduced or lost during evolution than the reverse, however there is no evidence for this. The lengthening, or gaining of extra digits is common in marine reptiles, and within the theropods digit lengthening occurred at least once. Therefore using this as evidence against ancestral prosauropods is questionable.
While the sauropodomorphs are still grouped into prosauropods and sauropods for convenience, most modern classification schemes break the prosauropods into a half-dozen groups that evolved separately from one or more common ancestors. While they have a number of shared characteristics, the evolutionary requirements for giraffe-like browsing high in the trees may have caused convergent evolution, where similar traits evolve separately because they faced the same evolutionary pressure, instead of (homologous) traits derived from a shared ancestor.
Since the modern preference is for groups that are composed of all descendants of the same common ancestor (clades), instead of groups that exclude certain descendants of that ancestor (paraphyletic taxa), Prosauropoda is unpopular except as an informal collection of primitive (basal) sauropodomorphs. However, some like Michael Benton, consider the prosauropods and sauropods to be a distinct lineage descended from a common saurischian ancestor. While this is a minority view, supported by weak evidence, there is considerable support for a small, monophyletic Prosauropoda clade containing only smaller percentage of its previous members (taxa).
Saturnalia has the teeth, backbone,pelvis, and legs of traditional prosauropods, while lacking all of the unique sauropod characteristics. This lends some support to the prosauropod paraphyly theory, as it is the most basal sauropodomorph. However, it also lacks some of characteristics traditionally associated with Sauropodomorpha. Although, again being the most basal species this is not too surprising. The suggestion that the lack of some derived sauropodomorph characters in Saturnalia can be taken as evidence that Sauropodomorpha eis polyphyletic (evolved separately from different saurischian ancestors) has not been demonstrated by any cladistic analysis of sauropodomorphs.
Sauropodomorphs were adapted to browsing higher than any other contemporary herbivore, giving them access to high tree foliage. This feeding strategy is supported by many of their defining characteristics, such as: a light, tiny skull on the end of a long neck (with ten or more elongated cervical vertebrae) and a counterbalancing long tail (with one to three extra sacral vertebrae).
Their teeth were weak, and shaped like leaves or spoons (lanceolate or spatulate). Instead of grinding teeth, they had stomach stones (gastroliths), similar to the gizzard stones of modern birds and crocodiles, to help digest tough plant fibers. The front of the upper mouth bends down in what may be a beak.
The earliest known sauropodomorph, Saturnalia, was small and slender (1.5 metres, or 5 feet long), but by the end of the Triassic they were the largest dinosaurs of their time, and in the Jurassic/Cretaceous they kept on growing. Ultimately the largest sauropods like the Supersaurus,Diplodocus hallorum, and Argentinosaurus reached 30–40 metres (100–130 ft) in length, and 60,000–100,000 kilograms (65–110 US short tons) or more in mass.
Initially bipedal, as their size increased they evolved to become graviportal quadrapeds (like elephants). The early sauropodomorphs were most likely omnivores as their shared common ancestor with the other saurischian lineage (the theropods) was a carnivore. Therefore their evolution to herbivory went hand in hand with their increasing size and neck length.
They also had large nostrils (nares), and retained a thumb (pollex) with a big claw which may have been used for defense — though their primary defensive adaptation was their extreme size.
Among the very first dinosaurs to evolve in the late Triassic Period, about 230 million years ago (Mya), they became the dominant herbivores by half way through the late Triassic (during the Norian stage). Their perceived decline in the early Cretaceous is most likely a bias in fossil sampling, as most fossils are known from Europe and North America. Sauropods were still the dominant herbivores in the Gondwana landmasses, however. The spread of flowering plants (angiosperms) and "advanced" ornithischians, another major group of herbivorous dinosaurs (noted for their highly developed chewing mechanisms) are most likely not a major factor in sauropod decline in the northern continents. Like all non-avian dinosaurs, the sauropodomorphs became extinct 65 Mya, during the Cretaceous-Tertiary extinction event.
The most basal sauropodomorph known, Saturnalia, was discovered in 1999, and is dated to the Carnian stage of the late Triassic. However, fragmentary remains from Madagascar may represent an even earlier sauropodomorph from the middle Triassic.
Sauropodomorpha is one of the two major clades within the order Saurischia.. The sauropodomorphs' sister group, the Theropoda, includes bipedal carnivores like Velociraptor and Tyrannosaurus. However, Sauropodomorpha also share a number of characteristics with the Ornithischia, so a small minority of paleontologists like Bakker place both sets of herbivores within Phytodinosauria (or Ornithischiformes).
In Linnaean taxonomy, Sauropodomorpha (which means "lizard feet forms") is either a suborder or is left unranked. It was originally established by Friedrich von Huene in 1932, who broke it into two groups: the basal forms within Prosauropoda, and their descendants, the giant Sauropoda.
Recent phylogenetic analyses by Adam Yates (2004, 2006) firmly places Sauropoda within a paraphyletic Prosauropoda. Also, finds of late Triassic sauropods demonstrate that there is no gap between the "prosauropod" and sauropod lineages.
Evidence against sauropod ancestry within Prosauropoda comes from the fact that prosauropods had a smaller outer toe on their hind feet than the sauropods. Many maintain that it is easier for digits to be reduced or lost during evolution than the reverse, however there is no evidence for this. The lengthening, or gaining of extra digits is common in marine reptiles, and within the theropods digit lengthening occurred at least once. Therefore using this as evidence against ancestral prosauropods is questionable.
While the sauropodomorphs are still grouped into prosauropods and sauropods for convenience, most modern classification schemes break the prosauropods into a half-dozen groups that evolved separately from one or more common ancestors. While they have a number of shared characteristics, the evolutionary requirements for giraffe-like browsing high in the trees may have caused convergent evolution, where similar traits evolve separately because they faced the same evolutionary pressure, instead of (homologous) traits derived from a shared ancestor.
Since the modern preference is for groups that are composed of all descendants of the same common ancestor (clades), instead of groups that exclude certain descendants of that ancestor (paraphyletic taxa), Prosauropoda is unpopular except as an informal collection of primitive (basal) sauropodomorphs. However, some like Michael Benton, consider the prosauropods and sauropods to be a distinct lineage descended from a common saurischian ancestor. While this is a minority view, supported by weak evidence, there is considerable support for a small, monophyletic Prosauropoda clade containing only smaller percentage of its previous members (taxa).
Saturnalia has the teeth, backbone,pelvis, and legs of traditional prosauropods, while lacking all of the unique sauropod characteristics. This lends some support to the prosauropod paraphyly theory, as it is the most basal sauropodomorph. However, it also lacks some of characteristics traditionally associated with Sauropodomorpha. Although, again being the most basal species this is not too surprising. The suggestion that the lack of some derived sauropodomorph characters in Saturnalia can be taken as evidence that Sauropodomorpha eis polyphyletic (evolved separately from different saurischian ancestors) has not been demonstrated by any cladistic analysis of sauropodomorphs.
Archaeopteryx
Archaeopteryx, sometimes referred to by its German name Urvogel ("original bird" or "first bird"), is the earliest and most primitive bird known. The name is from the archaios meaning 'ancient' and,pteryx meaning 'feather' or 'wing'.
Archaeopteryx lived in the late Jurassic Period around 150–145 million years ago, in what is now southern Germany during a time when Europe was an archipelago of islands in a shallow warm tropical sea, much closer to the equator than it is now.
Similar in size and shape to a European Magpie, Archaeopteryx could grow to about 0.5 metres (1.6 ft) in length. Despite its small size, broad wings, and inferred ability to fly or glide, Archaeopteryx has more in common with small theropod dinosaurs than it does with modern birds. In particular, it shares the following features with the deinonychosaurs (dromaeosaurs and troodontids): jaws with sharp teeth, three fingers with claws, a long bony tail, hyperextensible second toes ("killing claw"), feathers (which also suggest homeothermy), and various skeletal features.
The features above make Archaeopteryx the first clear candidate for a transitional fossil between dinosaurs and birds.Thus, Archaeopteryx plays an important role not only in the study of the origin of birds but in the study of dinosaurs.
The first complete specimen of Archaeopteryx was announced in 1861, only two years after Charles Darwin published On the Origin of Species, and it became a key piece of evidence in the debate over evolution. Over the years, nine more fossils of Archaeopteryx have surfaced. Despite variation among these fossils, most experts regard all the remains that have been discovered as belonging to a single species, though this is still debated.
Many of these eleven fossils include impressions of feathers—among the oldest (if not the oldest) direct evidence of feathers. Moreover, because these feathers are an advanced form (flight feathers), these fossils are evidence that feathers had been evolving for quite some time.
Archaeopteryx was a primitive bird that lived during the Tithonian stage of the Jurassic Period,but a crimeajewel species, around 150–145 million years ago. The only specimens of Archaeopteryx that have been discovered come from Bavaria in southern Germany.
Archaeopteryx was roughly the size of a medium-sized modern-day bird, with broad wings that were rounded at the ends and a long tail compared to its body length. In all, Archaeopteryx could reach up to 500 millimeters (1.6 ft) in body length. Archaeopteryx feathers, although less documented than its other features, were very similar in structure and design to modern-day bird feathers. However, despite the presence of numerous avian features, Archaeopteryx had many theropod dinosaur characteristics. Unlike modern birds, Archaeopteryx had small teeth as well as a long bony tail, features which Archaeopteryx shared with other dinosaurs of the time.
Because it displays a number of features common to both birds and dinosaurs, Archaeopteryx has often been considered a link between them—possibly the first bird in its change from a land dweller to a bird. In the 1970s, John Ostrom, following T.H.Huxley's lead in 1868, argued that birds evolved from theropod dinosaurs and Archaeopteryx was a critical piece of evidence for this argument; it preserves a number of avian features, such as a wishbone, flight feathers, wings and a partially reversed first toe, and a number of dinosaur and theropod features. For instance, it has a long ascending process of the ankle bone, interdental plates, an obturator process of the ischium, and long chevrons in the tail. In particular, Ostrom found that Archaeopteryx was remarkably similar to the theropod family Dromaeosauridae.
The first remains of Archaeopteryx were discovered in 1861; just two years after Charles Darwin published On the Origin of Species. Archaeopteryx seemed to confirm Darwin's theories and has since become a key piece of evidence in the origin of birds, transitional fossils debate and the confirmation of evolution. Indeed, further research on dinosaurs from the Gobi Desert and China has since provided more evidence of a link between Archaeopteryx and the dinosaurs, such as the Chinese feathered dinosaurs. Archaeopteryx is close to the ancestry of modern birds, and it shows most of the features one would expect in an ancestral bird. However, it may not be the direct ancestor of living birds, and it is uncertain how much evolutionary divergence was already present among other birds at the time.
Specimens of Archaeopteryx were most notable for their well-developed flight feathers. They were markedly asymmetrical and showed the structure of flight feathers in modern birds, with vanes given stability by a barbule-barbule-barbicel arrangement. The tail feathers were less asymmetrical, again in line with the situation in modern birds and also had firm vanes. The thumb, however, did not yet bear a separately movable tuft of stiff feathers.
The body plumage of Archaeopteryx is less well documented and has only been properly researched in the well-preserved Berlin specimen. Thus, as more than one species seems to be involved, the research into the Berlin specimen's feathers does not necessarily hold true for the rest of the species of Archaeopteryx. In the Berlin specimen, there are "trousers" of well-developed feathers on the legs; some of these feathers seem to have a basic contour feather structure but are somewhat decomposed (they lack barbicels as in ratites), but in part they are firm and thus capable of supporting flight.
There was a patch of pennaceous feathers running along the back which was quite similar to the contour feathers of the body plumage of modern birds in being symmetrical and firm, though not as stiff as the flight-related feathers. Apart from that, the feather traces in the Berlin specimen are limited to a sort of "proto-down" not dissimilar to that found in the dinosaur Sinosauropteryx, being decomposed and fluffy, and possibly even appeared more like fur than like feathers in life (though not in their microscopic structure). These occur on the remainder of the body, as far as such structures are both preserved and not obliterated by preparation, and the lower neck.
However, there is no indication of feathering on the upper neck and head. While these may conceivably have been nude as in many closely related feathered dinosaurs for which good specimens are available, this may still be an artifact of preservation. It appears that most Archaeopteryx specimens became embedded in anoxic sediment after drifting some time on their back in the sea — the head and neck and the tail are generally bent downwards, which suggests that the specimens had just started to rot when they were embedded, with tendons and muscle relaxing so that the characteristic shape of the fossil specimens was achieved. This would mean that the skin was already softened and loose, which is bolstered by the fact that in some specimens the flight feathers were starting to detach at the point of embedding in the sediment. So it is hypothesized that the pertinent specimens moved along the sea bed in shallow water for some time before burial, the head and upper neck feathers sloughing off, while the more firmly attached tail feathers remained.
As in the wings of modern birds, the flight feathers of Archaeopteryx were highly asymmetrical and the tail feathers were rather broad. This implies that the wings and tail were used for lift generation. However, it is unclear whether Archaeopteryx was simply a glider or capable of flapping flight. The lack of a bony breastbone suggests that Archaeopteryx was not a very strong flier, but flight muscles might have attached to the thick, boomerang-shaped wishbone, the platelike coracoids, or perhaps to a cartilaginous sternum. The sideways orientation of the glenoid (shoulder) joint between scapula, coracoid and humerus—instead of the dorsally angled arrangement found in modern birds—suggests that Archaeopteryx was unable to lift its wings above its back, a requirement for the upstroke found in modern flapping flight. Thus, it seems likely that Archaeopteryx was indeed unable to use flapping flight as modern birds do, but it may well have utilized a downstroke-only flap-assisted gliding technique.
Archaeopteryx wings were relatively large, which would have resulted in a low stall speed and reduced turning radius. The short and rounded shape of the wings would have increased drag, but could also have improved Archaeopteryx' ability to fly through cluttered environments such as trees and brush (similar wing shapes are seen in birds which fly through trees and brush, such as crows and pheasants). The presence of "hind wings", asymmetrical flight feathers stemming from the legs similar to those seen in dromaeosaurids such as Microraptor, would also have added to the aerial mobility of Archaeopteryx. The first detailed study of the hind wings by Longrich in 2006 suggested that the structures formed up to 12% of the total airfoil. This would have reduced stall speed by up to 6% and turning radius by up to 12%.
In 2004, scientists analyzing a detailed CT scan of Archaeopteryx' braincase concluded that its brain was significantly larger than that of most dinosaurs, indicating that it possessed the brain size necessary for flying. The overall brain anatomy was reconstructed using the scan. The reconstruction showed that the regions associated with vision took up nearly one-third of the brain. Other well-developed areas involved hearing and muscle coordination. The skull scan also revealed the structure of the inner ear. The structure more closely resembles that of modern birds than the inner ear of reptiles. These characteristics taken together suggest that Archaeopteryx had the keen sense of hearing, balance, spatial perception and coordination needed to fly
Archaeopteryx continues to play an important part in scientific debates about the origin and evolution of birds. Some scientists see it as a semi-arboreal climbing animal, following the idea that birds evolved from tree-dwelling gliders (the "trees down" hypothesis for the evolution of flight proposed by O.C.Marsh). Other scientists see Archaeopteryx as running quickly along the ground, supporting the idea that birds evolved flight by running (the "ground up" hypothesis proposed by Samuel Wendell Williston). Still others suggest that Archaeopteryx might have been at home both in the trees and on the ground, like modern crows, and this latter view is what today is considered best-supported by morphological characters. Altogether, it appears that the species was not particularly specialized for running on the ground or for perching. Considering the current knowledge of flight-related morphology, a scenario outlined by Elżanowski in 2002, namely that Archaeopteryx used its wings mainly to escape predators by glides punctuated with shallow downstrokes to reach successively higher perches, and alternatively to cover longer distances by (mainly) gliding down from cliffs or treetops, appears quite reasonable .
The richness and diversity of the Solnhofen limestones in which all specimens of Archaeopteryx have been found have shed light on an ancient Jurassic Bavaria strikingly different from the present day. The latitude was similar to Florida, though the climate was likely to have been drier, as evidenced by fossils of plants with adaptations for arid conditions and lack of terrestrial sediments characteristic of rivers. Evidence of plants, though scarce, include cycads and conifers while animals found include a large number of insects, small lizards,pterosaurs and Compsognathus.
The excellent preservation of Archaeopteryx fossils and other terrestrial fossils found at Solnhofen indicates that they did not travel far before becoming preserved. The Archaeopteryx specimens found are likely therefore to have lived on the low islands surrounding the Solnhofen lagoon rather than been corpses that drifted in from further away. Archaeopteryx skeletons are considerably less numerous in the deposits of Solnhofen than those of pterosaurs such as Rhamphorhynchus, the group which dominated the niche currently occupied by seabirds, yet are common enough that it is unlikely that the specimens found are vagrants from the larger islands 50 km (31 miles) to the north.
The islands that surrounded the Solnhofen lagoon were low lying,semi-arid and sub-tropical with a long dry-season and little rain. The flora of these islands was adapted to these dry conditions and consisted mostly of low (3 m [10 ft]) shrubs. Contrary to reconstructions of Archaeopteryx climbing large trees, these seem to have been mostly absent from the islands; few trunks have been found in the sediments and fossilized tree pollen is also absent.
The lifestyle of Archaeopteryx is difficult to reconstruct and there are several theories regarding it. Some researchers suggest that it was primarily adapted to life on the ground, while other researchers suggest that it was principally arboreal. The absence of trees does not preclude Archaeopteryx from an arboreal lifestyle; several species of extant bird live exclusively in low shrubs. Various aspects of the morphology of Archaeopteryx point to either an arboreal or ground existence, the length of its legs, the elongation in its feet; and some authorities consider it likely to have been a generalist capable of feeding in both shrubs, open ground and even alongside the shores of the lagoon. It most likely hunted small prey, seizing it with its jaws if it was small enough or with its claws if it was larger.
Over the years, ten body fossil specimens of Archaeopteryx and a feather that may belong to it have been found. All of the fossils come from the limestone deposits, quarried for centuries, near Solnhofen, Germany.
The initial discovery, a single feather, was unearthed in 1860 and described a year later by Christian Erich Hermann von Meyer. It is currently located at the Humboldt Museum fur Naturkunde in Berlin. This is generally assigned to Archaeopteryx and was the initial holotype, but whether it actually is a feather of this species or another, as yet undiscovered, proto-bird is unknown. There are some indications it is indeed not from the same animal as most of the skeletons (the "typical" A. lithographica).
Soon after, the first skeleton, known as the London Specimen (BMNH 37001) was unearthed in 1861 near Langenaltheim, Germany and given to a local physician Karl Häberlein in return for medical services. He then sold it to the Natural History Museum in London, where it remains. Missing most of its head and neck, it was described in 1863 by Richard Owen as Archaeopteryx macrura, who assumed it did not belong to the same species as the feather. In the subsequent 4th edition of his On the Origin of Species (chap. 9 p.367), Charles Darwin described how some authors had maintained "that the whole class of birds came suddenly into existence during the eocene period; but now we know, on the authority of Professor Owen, that a bird certainly lived during the deposition of the upper greensand; and still more recently, that strange bird, the Archeopteryx, with a long lizard-like tail, bearing a pair of feathers on each joint, and with its wings furnished with two free claws, has been discovered in the oolitic slates of Solenhofen. Hardly any recent discovery shows more forcibly than this how little we as yet know of the former inhabitants of the world."
The Greek term "pteryx" (πτερυξ) primarily means "wing", but can also designate merely "feather". Von Meyer suggested this in his description. At first he referred to a single feather which appeared like a modern bird's remex (wing feather), but he had heard of and been shown a rough sketch of the London specimen, to which he referred as a "Skelet eines mit Federn bedeckten Thiers" ("skeleton of an animal covered in feathers"). In German, this ambiguity is resolved by the term Schwinge which does not necessarily mean a wing used for flying. Urschwinge was the favored translation of Archaeopteryx among German scholars in the late 19th century. In English, "ancient pinion" offers a rough approximation.
Since then nine specimens have been recovered: The Berlin Specimen (HMN 1880) was discovered in 1876 or 1877 on the Blumenberg near Eichstatt, Germany, by Jakob Niemeyer. He exchanged this precious fossil for a cow, with Johann Dörr. Placed on sale in 1881, with potential buyers including O.C.Marsh of Yale University's Peabody Museum, it was bought by the Humboldt Museum für Naturkunde, where it is now displayed. The transaction was financed by Ernst Werner von Siemens, founder of the famous company that bears his name.Described in 1884 by Wilhelm Dames, it is the most complete specimen, and the first with a complete head. Once classified as a new species, A. siemensii, a recent evaluation supports the A. siemensii species definition.
Composed of a torso, the Maxberg Specimen (S5) was discovered in 1956 or 1958 near Langenaltheim and described in 1959 by Heller. It is currently missing, though it was once exhibited at the Maxberg Museum in Solnhofen. It belonged to Efuard Opitsch, who loaned it to the museum. After his death in 1991, the specimen was discovered to be missing and may have been stolen or sold. The specimen is missing its head and tail, although the rest of the skeleton is mostly intact.
The Haarlem Specimen (TM 6428, also known as the Teyler Specimen) was discovered in 1855 near Riedenburg, Germany and described as a Pterodactylus crassipes in 1875 by von Meyer. It was reclassified in 1970 by John Ostrom and is currently located at the Teylers Museum in Haarlem, the Netherlands. It was the very first specimen, despite the classification error. It is also one of the least complete specimens, consisting mostly of limb bones and isolated cervical vertebrae and ribs.
The Eichstätt Specimen (JM 2257) was discovered in 1951 or 1955 near Workerszell, Germany and described by Peter Wellnhofer in 1974. Currently located at the Jura Museum in Eichstatt, Germany, it is the smallest specimen and has the second best head. It is possibly a separate genus (Jurapteryx recurva) or species (A. recurva).
The Solnhofen Specimen (BSP 1999) was discovered in the 1960s near Eichstatt, Germany and described in 1988 by Wellnhofer. Currently located at the Burgermeister-Muller-Museum in Solnhofen, it was originally classified as Compsognathus by an amateur collector. It is the largest specimen known and may belong to a separate genus and species, Wellnhoferia grandis. It is missing only portions of the neck, tail, backbone, and head.
The Munich Specimen (S6, formerly known as the Solnhofen-Aktien-Verein Specimen) was discovered in 1991 near Langenaltheim and described in 1993 by Wellnhofer. It is currently located at the Palaontologisches Museum Munchen in Munich. What was initially believed to be a bony sternum turned out to be part of the coracoid, but a cartilaginous sternum may have been present. Only the front of its face is missing. It may be a new species, A. bavarica.
An eighth, fragmentary specimen, the Bürgermeister-Müller Specimen was discovered in 1997 and it is currently kept at the Bürgermeister-Müller Museum. Other than the above remains discovered, a further fragmentary fossil was found in 2004.
Long in a private collection, the Thermopolis Specimen (WDC CSG 100) was discovered in Germany and described in 2005 by Mayr, Pohl, and Peters. Donated to the Wyoming Dinosaur Center in Thermopolis, Wyoming, it has the best-preserved head and feet; most of the neck and the lower jaw have not been preserved. The "Thermopolis" specimen was described in the December 2, 2005 Science journal article as "A well-preserved Archaeopteryx specimen with theropod features"; it shows that the Archaeopteryx lacked a reversed toe—a universal feature of birds—limiting its ability to perch on branches and implying a terrestrial or trunk-climbing lifestyle. This has been interpreted as evidence of theropod ancestry. The specimen also has a hyperextendible second toe. "Until now, the feature was thought to belong only to the species' close relatives, the deinonychosaurs." In 1988, Gregory S.Paul claimed to have found evidence of a hyperextensible toe, but this was not verified and accepted by other scientists until the Thermopolis specimen was described. This tenth and latest specimen was assigned to Archaeopteryx siemensii in 2007. The specimen itself, currently on loan to the Forschungsinstitut Senckenberg in Frankfurt, is considered the most complete and well preserved Archaeopteryx remains yet.
Some reports claim that they provide evidence to prove that Archaeopteryx is a fake. However, such reports are not confirmed. The issue is further discussed below.
Today, the fossils are usually assigned to a single species A. lithographica, but the taxonomic history is complicated. Dozens of names have been published for the handful of specimens, most of which are simply spelling errors (lapsus). Originally, the name A. lithographica only referred to the single feather described by von Meyer. In 1960, Swinton proposed that the name Archaeopteryx lithographica be officially transferred from the feather to the London specimen. The ICZN did suppress the plethora of alternative names initially proposed for the first skeleton specimens, which mainly resulted from the acrimonious dispute between von Meyer and his opponent Johann Andreas Wagner (whose Griphosaurus problematicus—"problematic riddle-lizard"—was a vitriolic sneer at von Meyer's Archaeopteryx). In addition, descriptions of Archaeopteryx fossils as pterosaurs before their true nature was realized were also suppressed.
The relationships of the specimens are problematic. Most subsequent specimens have been given their own species at one point or another. The Berlin specimen has been designated as Archaeornis siemensii, the Eichstätt specimen as Jurapteryx recurva, the Munich specimen as Archaeopteryx bavarica and the Solnhofen specimen was designated as Wellnhoferia grandis.
Recently, it has been argued that all the specimens belong to the same species. However, significant differences exist among the specimens. In particular, the Munich, Eichstätt, Solnhofen and Thermopolis specimens differ from the London, Berlin, and Haarlem specimens in being smaller or much larger, having different finger proportions, having more slender snouts, lined with forward-pointing teeth and possible presence of a sternum. These differences are as large as or larger than the differences seen today between adults of different bird species. However, it is also possible that these differences could be explained by different ages of the living birds.
Finally, it is worth noting that the feather, the first specimen of Archaeopteryx described, does not agree well with the flight-related feathers of Archaeopteryx. It certainly is a flight feather of a contemporary species, but its size and proportions indicate that it may belong to another, smaller species of feathered theropod, of which only this feather is so far known. As the feather was the original type specimen, this has created significant nomenclatorial confusion.
Beginning in 1985, a group including astronomer Fred Hoyle and physicist Lee Spetner published a series of papers claiming that the feathers on the Berlin and London specimens of Archaeopteryx were forged. Their claims were repudiated by Alan J.Charig and others at the British Museum (Natural History). Most of their evidence for a forgery was based on unfamiliarity with the processes of lithification; for example, they proposed that based on the difference in texture associated with the feathers, feather impressions were applied to a thin layer of cement, without realizing that feathers themselves would have caused a textural difference. They also expressed disbelief that slabs would split so smoothly, or that one half of a slab containing fossils would have good preservation, but not the counterslab. These, though, are common properties of Solnhofen fossils because the dead animals would fall onto hardened surfaces which would form a natural plane for the future slabs to split along, leaving the bulk of the fossil on one side and little on the other. They also misinterpreted the fossils, claiming that the tail was forged as one large feather, when this is visibly not the case.In addition, they claimed that the other specimens of Archaeopteryx known at the time did not have feathers, which is untrue; the Maxberg and Eichstätt specimens have obvious feathers. Finally, the motives they suggested for a forgery are not strong, and contradictory; one is that Richard Owen wanted to forge evidence in support of Charles Darwin's theory of evolution, which is unlikely given Owen's views toward Darwin and his theory. The other is that Owen wanted to set a trap for Darwin, hoping the latter would support the fossils so Owen could discredit him with the forgery; this is unlikely because Owen himself wrote a detailed paper on the London specimen, so such an action would certainly backfire.
Charig et al.. pointed to the presence of hairline cracks in the slabs running through both rock and fossil impressions, and mineral growth over the slabs that had occurred before discovery and preparation, as evidence that the feathers were original. Spetner et al.. then attempted to show that the cracks would have naturally propagated through their postulated cement layer, but neglected to account for the fact that the cracks were old and had been filled with calcite, and thus were not able to propagate. They also attempted to show the presence of cement on the London specimen through X-ray spectroscopy, and did find something that was not rock. However, it was not cement, either, and is most probably from a fragment of silicone rubber left behind when molds were made of the specimen. Their suggestions have not been taken seriously by paleontologists, as their evidence was largely based on misunderstandings of geology, and they never discussed the other feather-bearing specimens, which have increased in number since then.
Archaeopteryx lived in the late Jurassic Period around 150–145 million years ago, in what is now southern Germany during a time when Europe was an archipelago of islands in a shallow warm tropical sea, much closer to the equator than it is now.
Similar in size and shape to a European Magpie, Archaeopteryx could grow to about 0.5 metres (1.6 ft) in length. Despite its small size, broad wings, and inferred ability to fly or glide, Archaeopteryx has more in common with small theropod dinosaurs than it does with modern birds. In particular, it shares the following features with the deinonychosaurs (dromaeosaurs and troodontids): jaws with sharp teeth, three fingers with claws, a long bony tail, hyperextensible second toes ("killing claw"), feathers (which also suggest homeothermy), and various skeletal features.
The features above make Archaeopteryx the first clear candidate for a transitional fossil between dinosaurs and birds.Thus, Archaeopteryx plays an important role not only in the study of the origin of birds but in the study of dinosaurs.
The first complete specimen of Archaeopteryx was announced in 1861, only two years after Charles Darwin published On the Origin of Species, and it became a key piece of evidence in the debate over evolution. Over the years, nine more fossils of Archaeopteryx have surfaced. Despite variation among these fossils, most experts regard all the remains that have been discovered as belonging to a single species, though this is still debated.
Many of these eleven fossils include impressions of feathers—among the oldest (if not the oldest) direct evidence of feathers. Moreover, because these feathers are an advanced form (flight feathers), these fossils are evidence that feathers had been evolving for quite some time.
Archaeopteryx was a primitive bird that lived during the Tithonian stage of the Jurassic Period,but a crimeajewel species, around 150–145 million years ago. The only specimens of Archaeopteryx that have been discovered come from Bavaria in southern Germany.
Archaeopteryx was roughly the size of a medium-sized modern-day bird, with broad wings that were rounded at the ends and a long tail compared to its body length. In all, Archaeopteryx could reach up to 500 millimeters (1.6 ft) in body length. Archaeopteryx feathers, although less documented than its other features, were very similar in structure and design to modern-day bird feathers. However, despite the presence of numerous avian features, Archaeopteryx had many theropod dinosaur characteristics. Unlike modern birds, Archaeopteryx had small teeth as well as a long bony tail, features which Archaeopteryx shared with other dinosaurs of the time.
Because it displays a number of features common to both birds and dinosaurs, Archaeopteryx has often been considered a link between them—possibly the first bird in its change from a land dweller to a bird. In the 1970s, John Ostrom, following T.H.Huxley's lead in 1868, argued that birds evolved from theropod dinosaurs and Archaeopteryx was a critical piece of evidence for this argument; it preserves a number of avian features, such as a wishbone, flight feathers, wings and a partially reversed first toe, and a number of dinosaur and theropod features. For instance, it has a long ascending process of the ankle bone, interdental plates, an obturator process of the ischium, and long chevrons in the tail. In particular, Ostrom found that Archaeopteryx was remarkably similar to the theropod family Dromaeosauridae.
The first remains of Archaeopteryx were discovered in 1861; just two years after Charles Darwin published On the Origin of Species. Archaeopteryx seemed to confirm Darwin's theories and has since become a key piece of evidence in the origin of birds, transitional fossils debate and the confirmation of evolution. Indeed, further research on dinosaurs from the Gobi Desert and China has since provided more evidence of a link between Archaeopteryx and the dinosaurs, such as the Chinese feathered dinosaurs. Archaeopteryx is close to the ancestry of modern birds, and it shows most of the features one would expect in an ancestral bird. However, it may not be the direct ancestor of living birds, and it is uncertain how much evolutionary divergence was already present among other birds at the time.
Specimens of Archaeopteryx were most notable for their well-developed flight feathers. They were markedly asymmetrical and showed the structure of flight feathers in modern birds, with vanes given stability by a barbule-barbule-barbicel arrangement. The tail feathers were less asymmetrical, again in line with the situation in modern birds and also had firm vanes. The thumb, however, did not yet bear a separately movable tuft of stiff feathers.
The body plumage of Archaeopteryx is less well documented and has only been properly researched in the well-preserved Berlin specimen. Thus, as more than one species seems to be involved, the research into the Berlin specimen's feathers does not necessarily hold true for the rest of the species of Archaeopteryx. In the Berlin specimen, there are "trousers" of well-developed feathers on the legs; some of these feathers seem to have a basic contour feather structure but are somewhat decomposed (they lack barbicels as in ratites), but in part they are firm and thus capable of supporting flight.
There was a patch of pennaceous feathers running along the back which was quite similar to the contour feathers of the body plumage of modern birds in being symmetrical and firm, though not as stiff as the flight-related feathers. Apart from that, the feather traces in the Berlin specimen are limited to a sort of "proto-down" not dissimilar to that found in the dinosaur Sinosauropteryx, being decomposed and fluffy, and possibly even appeared more like fur than like feathers in life (though not in their microscopic structure). These occur on the remainder of the body, as far as such structures are both preserved and not obliterated by preparation, and the lower neck.
However, there is no indication of feathering on the upper neck and head. While these may conceivably have been nude as in many closely related feathered dinosaurs for which good specimens are available, this may still be an artifact of preservation. It appears that most Archaeopteryx specimens became embedded in anoxic sediment after drifting some time on their back in the sea — the head and neck and the tail are generally bent downwards, which suggests that the specimens had just started to rot when they were embedded, with tendons and muscle relaxing so that the characteristic shape of the fossil specimens was achieved. This would mean that the skin was already softened and loose, which is bolstered by the fact that in some specimens the flight feathers were starting to detach at the point of embedding in the sediment. So it is hypothesized that the pertinent specimens moved along the sea bed in shallow water for some time before burial, the head and upper neck feathers sloughing off, while the more firmly attached tail feathers remained.
As in the wings of modern birds, the flight feathers of Archaeopteryx were highly asymmetrical and the tail feathers were rather broad. This implies that the wings and tail were used for lift generation. However, it is unclear whether Archaeopteryx was simply a glider or capable of flapping flight. The lack of a bony breastbone suggests that Archaeopteryx was not a very strong flier, but flight muscles might have attached to the thick, boomerang-shaped wishbone, the platelike coracoids, or perhaps to a cartilaginous sternum. The sideways orientation of the glenoid (shoulder) joint between scapula, coracoid and humerus—instead of the dorsally angled arrangement found in modern birds—suggests that Archaeopteryx was unable to lift its wings above its back, a requirement for the upstroke found in modern flapping flight. Thus, it seems likely that Archaeopteryx was indeed unable to use flapping flight as modern birds do, but it may well have utilized a downstroke-only flap-assisted gliding technique.
Archaeopteryx wings were relatively large, which would have resulted in a low stall speed and reduced turning radius. The short and rounded shape of the wings would have increased drag, but could also have improved Archaeopteryx' ability to fly through cluttered environments such as trees and brush (similar wing shapes are seen in birds which fly through trees and brush, such as crows and pheasants). The presence of "hind wings", asymmetrical flight feathers stemming from the legs similar to those seen in dromaeosaurids such as Microraptor, would also have added to the aerial mobility of Archaeopteryx. The first detailed study of the hind wings by Longrich in 2006 suggested that the structures formed up to 12% of the total airfoil. This would have reduced stall speed by up to 6% and turning radius by up to 12%.
In 2004, scientists analyzing a detailed CT scan of Archaeopteryx' braincase concluded that its brain was significantly larger than that of most dinosaurs, indicating that it possessed the brain size necessary for flying. The overall brain anatomy was reconstructed using the scan. The reconstruction showed that the regions associated with vision took up nearly one-third of the brain. Other well-developed areas involved hearing and muscle coordination. The skull scan also revealed the structure of the inner ear. The structure more closely resembles that of modern birds than the inner ear of reptiles. These characteristics taken together suggest that Archaeopteryx had the keen sense of hearing, balance, spatial perception and coordination needed to fly
Archaeopteryx continues to play an important part in scientific debates about the origin and evolution of birds. Some scientists see it as a semi-arboreal climbing animal, following the idea that birds evolved from tree-dwelling gliders (the "trees down" hypothesis for the evolution of flight proposed by O.C.Marsh). Other scientists see Archaeopteryx as running quickly along the ground, supporting the idea that birds evolved flight by running (the "ground up" hypothesis proposed by Samuel Wendell Williston). Still others suggest that Archaeopteryx might have been at home both in the trees and on the ground, like modern crows, and this latter view is what today is considered best-supported by morphological characters. Altogether, it appears that the species was not particularly specialized for running on the ground or for perching. Considering the current knowledge of flight-related morphology, a scenario outlined by Elżanowski in 2002, namely that Archaeopteryx used its wings mainly to escape predators by glides punctuated with shallow downstrokes to reach successively higher perches, and alternatively to cover longer distances by (mainly) gliding down from cliffs or treetops, appears quite reasonable .
The richness and diversity of the Solnhofen limestones in which all specimens of Archaeopteryx have been found have shed light on an ancient Jurassic Bavaria strikingly different from the present day. The latitude was similar to Florida, though the climate was likely to have been drier, as evidenced by fossils of plants with adaptations for arid conditions and lack of terrestrial sediments characteristic of rivers. Evidence of plants, though scarce, include cycads and conifers while animals found include a large number of insects, small lizards,pterosaurs and Compsognathus.
The excellent preservation of Archaeopteryx fossils and other terrestrial fossils found at Solnhofen indicates that they did not travel far before becoming preserved. The Archaeopteryx specimens found are likely therefore to have lived on the low islands surrounding the Solnhofen lagoon rather than been corpses that drifted in from further away. Archaeopteryx skeletons are considerably less numerous in the deposits of Solnhofen than those of pterosaurs such as Rhamphorhynchus, the group which dominated the niche currently occupied by seabirds, yet are common enough that it is unlikely that the specimens found are vagrants from the larger islands 50 km (31 miles) to the north.
The islands that surrounded the Solnhofen lagoon were low lying,semi-arid and sub-tropical with a long dry-season and little rain. The flora of these islands was adapted to these dry conditions and consisted mostly of low (3 m [10 ft]) shrubs. Contrary to reconstructions of Archaeopteryx climbing large trees, these seem to have been mostly absent from the islands; few trunks have been found in the sediments and fossilized tree pollen is also absent.
The lifestyle of Archaeopteryx is difficult to reconstruct and there are several theories regarding it. Some researchers suggest that it was primarily adapted to life on the ground, while other researchers suggest that it was principally arboreal. The absence of trees does not preclude Archaeopteryx from an arboreal lifestyle; several species of extant bird live exclusively in low shrubs. Various aspects of the morphology of Archaeopteryx point to either an arboreal or ground existence, the length of its legs, the elongation in its feet; and some authorities consider it likely to have been a generalist capable of feeding in both shrubs, open ground and even alongside the shores of the lagoon. It most likely hunted small prey, seizing it with its jaws if it was small enough or with its claws if it was larger.
Over the years, ten body fossil specimens of Archaeopteryx and a feather that may belong to it have been found. All of the fossils come from the limestone deposits, quarried for centuries, near Solnhofen, Germany.
The initial discovery, a single feather, was unearthed in 1860 and described a year later by Christian Erich Hermann von Meyer. It is currently located at the Humboldt Museum fur Naturkunde in Berlin. This is generally assigned to Archaeopteryx and was the initial holotype, but whether it actually is a feather of this species or another, as yet undiscovered, proto-bird is unknown. There are some indications it is indeed not from the same animal as most of the skeletons (the "typical" A. lithographica).
Soon after, the first skeleton, known as the London Specimen (BMNH 37001) was unearthed in 1861 near Langenaltheim, Germany and given to a local physician Karl Häberlein in return for medical services. He then sold it to the Natural History Museum in London, where it remains. Missing most of its head and neck, it was described in 1863 by Richard Owen as Archaeopteryx macrura, who assumed it did not belong to the same species as the feather. In the subsequent 4th edition of his On the Origin of Species (chap. 9 p.367), Charles Darwin described how some authors had maintained "that the whole class of birds came suddenly into existence during the eocene period; but now we know, on the authority of Professor Owen, that a bird certainly lived during the deposition of the upper greensand; and still more recently, that strange bird, the Archeopteryx, with a long lizard-like tail, bearing a pair of feathers on each joint, and with its wings furnished with two free claws, has been discovered in the oolitic slates of Solenhofen. Hardly any recent discovery shows more forcibly than this how little we as yet know of the former inhabitants of the world."
The Greek term "pteryx" (πτερυξ) primarily means "wing", but can also designate merely "feather". Von Meyer suggested this in his description. At first he referred to a single feather which appeared like a modern bird's remex (wing feather), but he had heard of and been shown a rough sketch of the London specimen, to which he referred as a "Skelet eines mit Federn bedeckten Thiers" ("skeleton of an animal covered in feathers"). In German, this ambiguity is resolved by the term Schwinge which does not necessarily mean a wing used for flying. Urschwinge was the favored translation of Archaeopteryx among German scholars in the late 19th century. In English, "ancient pinion" offers a rough approximation.
Since then nine specimens have been recovered: The Berlin Specimen (HMN 1880) was discovered in 1876 or 1877 on the Blumenberg near Eichstatt, Germany, by Jakob Niemeyer. He exchanged this precious fossil for a cow, with Johann Dörr. Placed on sale in 1881, with potential buyers including O.C.Marsh of Yale University's Peabody Museum, it was bought by the Humboldt Museum für Naturkunde, where it is now displayed. The transaction was financed by Ernst Werner von Siemens, founder of the famous company that bears his name.Described in 1884 by Wilhelm Dames, it is the most complete specimen, and the first with a complete head. Once classified as a new species, A. siemensii, a recent evaluation supports the A. siemensii species definition.
Composed of a torso, the Maxberg Specimen (S5) was discovered in 1956 or 1958 near Langenaltheim and described in 1959 by Heller. It is currently missing, though it was once exhibited at the Maxberg Museum in Solnhofen. It belonged to Efuard Opitsch, who loaned it to the museum. After his death in 1991, the specimen was discovered to be missing and may have been stolen or sold. The specimen is missing its head and tail, although the rest of the skeleton is mostly intact.
The Haarlem Specimen (TM 6428, also known as the Teyler Specimen) was discovered in 1855 near Riedenburg, Germany and described as a Pterodactylus crassipes in 1875 by von Meyer. It was reclassified in 1970 by John Ostrom and is currently located at the Teylers Museum in Haarlem, the Netherlands. It was the very first specimen, despite the classification error. It is also one of the least complete specimens, consisting mostly of limb bones and isolated cervical vertebrae and ribs.
The Eichstätt Specimen (JM 2257) was discovered in 1951 or 1955 near Workerszell, Germany and described by Peter Wellnhofer in 1974. Currently located at the Jura Museum in Eichstatt, Germany, it is the smallest specimen and has the second best head. It is possibly a separate genus (Jurapteryx recurva) or species (A. recurva).
The Solnhofen Specimen (BSP 1999) was discovered in the 1960s near Eichstatt, Germany and described in 1988 by Wellnhofer. Currently located at the Burgermeister-Muller-Museum in Solnhofen, it was originally classified as Compsognathus by an amateur collector. It is the largest specimen known and may belong to a separate genus and species, Wellnhoferia grandis. It is missing only portions of the neck, tail, backbone, and head.
The Munich Specimen (S6, formerly known as the Solnhofen-Aktien-Verein Specimen) was discovered in 1991 near Langenaltheim and described in 1993 by Wellnhofer. It is currently located at the Palaontologisches Museum Munchen in Munich. What was initially believed to be a bony sternum turned out to be part of the coracoid, but a cartilaginous sternum may have been present. Only the front of its face is missing. It may be a new species, A. bavarica.
An eighth, fragmentary specimen, the Bürgermeister-Müller Specimen was discovered in 1997 and it is currently kept at the Bürgermeister-Müller Museum. Other than the above remains discovered, a further fragmentary fossil was found in 2004.
Long in a private collection, the Thermopolis Specimen (WDC CSG 100) was discovered in Germany and described in 2005 by Mayr, Pohl, and Peters. Donated to the Wyoming Dinosaur Center in Thermopolis, Wyoming, it has the best-preserved head and feet; most of the neck and the lower jaw have not been preserved. The "Thermopolis" specimen was described in the December 2, 2005 Science journal article as "A well-preserved Archaeopteryx specimen with theropod features"; it shows that the Archaeopteryx lacked a reversed toe—a universal feature of birds—limiting its ability to perch on branches and implying a terrestrial or trunk-climbing lifestyle. This has been interpreted as evidence of theropod ancestry. The specimen also has a hyperextendible second toe. "Until now, the feature was thought to belong only to the species' close relatives, the deinonychosaurs." In 1988, Gregory S.Paul claimed to have found evidence of a hyperextensible toe, but this was not verified and accepted by other scientists until the Thermopolis specimen was described. This tenth and latest specimen was assigned to Archaeopteryx siemensii in 2007. The specimen itself, currently on loan to the Forschungsinstitut Senckenberg in Frankfurt, is considered the most complete and well preserved Archaeopteryx remains yet.
Some reports claim that they provide evidence to prove that Archaeopteryx is a fake. However, such reports are not confirmed. The issue is further discussed below.
Today, the fossils are usually assigned to a single species A. lithographica, but the taxonomic history is complicated. Dozens of names have been published for the handful of specimens, most of which are simply spelling errors (lapsus). Originally, the name A. lithographica only referred to the single feather described by von Meyer. In 1960, Swinton proposed that the name Archaeopteryx lithographica be officially transferred from the feather to the London specimen. The ICZN did suppress the plethora of alternative names initially proposed for the first skeleton specimens, which mainly resulted from the acrimonious dispute between von Meyer and his opponent Johann Andreas Wagner (whose Griphosaurus problematicus—"problematic riddle-lizard"—was a vitriolic sneer at von Meyer's Archaeopteryx). In addition, descriptions of Archaeopteryx fossils as pterosaurs before their true nature was realized were also suppressed.
The relationships of the specimens are problematic. Most subsequent specimens have been given their own species at one point or another. The Berlin specimen has been designated as Archaeornis siemensii, the Eichstätt specimen as Jurapteryx recurva, the Munich specimen as Archaeopteryx bavarica and the Solnhofen specimen was designated as Wellnhoferia grandis.
Recently, it has been argued that all the specimens belong to the same species. However, significant differences exist among the specimens. In particular, the Munich, Eichstätt, Solnhofen and Thermopolis specimens differ from the London, Berlin, and Haarlem specimens in being smaller or much larger, having different finger proportions, having more slender snouts, lined with forward-pointing teeth and possible presence of a sternum. These differences are as large as or larger than the differences seen today between adults of different bird species. However, it is also possible that these differences could be explained by different ages of the living birds.
Finally, it is worth noting that the feather, the first specimen of Archaeopteryx described, does not agree well with the flight-related feathers of Archaeopteryx. It certainly is a flight feather of a contemporary species, but its size and proportions indicate that it may belong to another, smaller species of feathered theropod, of which only this feather is so far known. As the feather was the original type specimen, this has created significant nomenclatorial confusion.
Beginning in 1985, a group including astronomer Fred Hoyle and physicist Lee Spetner published a series of papers claiming that the feathers on the Berlin and London specimens of Archaeopteryx were forged. Their claims were repudiated by Alan J.Charig and others at the British Museum (Natural History). Most of their evidence for a forgery was based on unfamiliarity with the processes of lithification; for example, they proposed that based on the difference in texture associated with the feathers, feather impressions were applied to a thin layer of cement, without realizing that feathers themselves would have caused a textural difference. They also expressed disbelief that slabs would split so smoothly, or that one half of a slab containing fossils would have good preservation, but not the counterslab. These, though, are common properties of Solnhofen fossils because the dead animals would fall onto hardened surfaces which would form a natural plane for the future slabs to split along, leaving the bulk of the fossil on one side and little on the other. They also misinterpreted the fossils, claiming that the tail was forged as one large feather, when this is visibly not the case.In addition, they claimed that the other specimens of Archaeopteryx known at the time did not have feathers, which is untrue; the Maxberg and Eichstätt specimens have obvious feathers. Finally, the motives they suggested for a forgery are not strong, and contradictory; one is that Richard Owen wanted to forge evidence in support of Charles Darwin's theory of evolution, which is unlikely given Owen's views toward Darwin and his theory. The other is that Owen wanted to set a trap for Darwin, hoping the latter would support the fossils so Owen could discredit him with the forgery; this is unlikely because Owen himself wrote a detailed paper on the London specimen, so such an action would certainly backfire.
Charig et al.. pointed to the presence of hairline cracks in the slabs running through both rock and fossil impressions, and mineral growth over the slabs that had occurred before discovery and preparation, as evidence that the feathers were original. Spetner et al.. then attempted to show that the cracks would have naturally propagated through their postulated cement layer, but neglected to account for the fact that the cracks were old and had been filled with calcite, and thus were not able to propagate. They also attempted to show the presence of cement on the London specimen through X-ray spectroscopy, and did find something that was not rock. However, it was not cement, either, and is most probably from a fragment of silicone rubber left behind when molds were made of the specimen. Their suggestions have not been taken seriously by paleontologists, as their evidence was largely based on misunderstandings of geology, and they never discussed the other feather-bearing specimens, which have increased in number since then.
Subscribe to:
Posts (Atom)