| Anchisaurus | |
|---|---|
 | |
| Neotype skeleton ofAnchisaurus YPM 1883/40313 | |
Scientific classification | |
| Domain: | Eukaryota |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Clade: | Dinosauria |
| Clade: | Saurischia |
| Clade: | †Sauropodomorpha |
| Clade: | †Sauropodiformes |
| Clade: | †Anchisauria |
| Genus: | †Anchisaurus Marsh, 1885 |
| Species: | †A. polyzelus |
| Binomial name | |
| †Anchisaurus polyzelus (Hitchcock, 1865) | |
| Synonyms | |
| |
Anchisaurus is a genus ofbasalsauropodomorphdinosaur.[1] It lived during the EarlyJurassicPeriod, and its fossils have been found in the red sandstone of the UpperPortland Formation,Northeastern United States, which was deposited from theHettangian age into theSinemurian age, between about 200 and 192 million years ago.[2] Until recently it was classed as a member ofProsauropoda. Thegenus nameAnchisaurus comes from the Greekαγχι (agkhi)anchi-; "near, close" + Greekσαυρος (sauros); "lizard".Anchisaurus was coined as a replacement name for "Amphisaurus", which was itself a replacement name for Hitchcock's "Megadactylus", both of which had already been used for other animals.
Sauropodomorph remains were first documented in North America in 1818, when some bones nearly 1.8 m (6 ft) in length were uncovered by Mr. Solomon Ellsworth Jr. while excavating a well with gunpowder inEast Windsor, Connecticut. At the time of their discovery it was thought that the bones might be those of a human,[3] but the presence of tail vertebrae falsified that idea. They are now recognized as those of an indeterminate sauropodomorph, possibly more closely related to the plateosaurian prosauropods.[4][5]
In 1855, the originaltype specimen ofAnchisaurus polyzelus, AM 41/109, which is housed at theAmherst College Museum of Natural History, was found by William Smith inSpringfield, Massachusetts during blasting a well for the waterhouse at theSpringfield Armory.[6] Unfortunately, both the East Windsor and Springfield specimens were severely damaged due to the blasting at the construction sites where they were found, and many of the bones were either accidentally thrown away by the workmen or kept by interested onlookers. As a result, these dinosaurs were only known from incomplete remains.
In 1863, the son of the ichnologistEdward Hitchcock, Edward Hitchcock Jr, described the Springfield remains in a supplement to his father's work on fossil footprints, suggesting they could explain a certain mysterious kind of reptile tracks.[6][7] He then contacted the British paleontologistRichard Owen. Owen advised him to name the finds as a new genus. Owen suggested the nameMegadactylus, "large finger" in Greek, in reference to the enormous thumb of the animal. Hitchcock Jr himself then chose thespecific namepolyzelus, "much sought for" in Greek, referring to the fact that his father had for many years vainly sought to discover the identity of the track-maker.[8]
In 1877, ProfessorOthniel Charles Marsh had noted that the nameMegadactylus had been preoccupied byMegadactylus Fitzinger 1843, asubgenus of the lizard genusStellio. In 1882, he replaced the name withAmphisaurus, "near saurian", probably referring to Marsh's interpretation of it as intermediate between primitive dinosaurs—at the time the BritishPalaeosaurus was an example of what was thought to be a primitive dinosaur—and more derived dinosaurs.[9] In 1885, Marsh had discovered that this name also had been preoccupied, by theanthracosaurianAmphisaurus Barkas 1870, and again replaced it byAnchisaurus, with the same meaning.[10]
Meanwhile, nearly complete specimens had been found inManchester, Connecticut. In 1884, a series of bridges was built over theHop Creek.Sandstone blocks were sawed out of Wolcott's Quarry north ofBuckland Station. On 20 October, an amateur paleontologist, Charles H. Owen, observed that a block had been removed containing the hind part of a skeleton. He warned Marsh who, using T. A. Bostwick as an intermediary, acquired the piece from the quarry owner, Charles O. Wolcott. Marsh tried to secure the front half of the skeleton but it had already been used in a bridge abutment. The specimen, YPM 208, was namedAnchisaurus major, "the larger one", by Marsh in 1889.[11] Eventually, when the bridge was demolished in August 1969,John Ostrom would save the front block. Subsequently, two other dinosaur fossils were located in the quarry. Six metres south of the original find a second skeleton was visible in the quarry face. It was removed as a single block and given the inventory number YPM 1883. In Yale, the part containing the skull was split off and became specimen YPM 40313. In 1891, Marsh madeAnchisaurus major a separate genus,Ammosaurus, the "sand saurian". In the same publication he named YPM 1883/YPM 40313 as a new species ofAnchisaurus,Anchisaurus colurus, "the mangled one".[12] They served as the templates from which O. C. Marsh in 1893 restored the skeleton.[13] The Manchester specimens are now considered conspecific withAnchisaurus polyzelus.[14] The East Windsor and Manchester specimens are housed at thePeabody Museum of Natural History at Yale University.
Thetype species is Hitchcock'sA. polyzelus. Marsh'sA. major (also known asAmmosaurus),A. solus, andA. colurus (also known asYaleosaurus), have since been recognized as synonyms ofA. polyzelus, their supposed differences being due to misinterpretation and different stages of growth.[14] In 2015, the ICZN formally made the more complete type specimen ofA. colurus theneotype of the genusAnchisaurus and the speciesA. polyzelus, renderingA. polyzelus andA. colurus objective synonyms (both names being based on exactly the same fossil).[15]
Broom namedGyposaurus capensis in 1911, from bones discovered in South Africa butPeter Galton renamed theseAnchisaurus capensis in 1976. This species has since been reclassified again and is probably a juvenile ofMassospondylus carinatus.G. sinensis was also referred here, but appears to be a distinct animal.
TheNavajo Sandstone ofArizona is the same age as the Portland Formation, and has producedprosauropod remains that have been referred to asAmmosaurus.[16] However, it is possible that these actually belong to the genusMassospondylus, otherwise known only from South Africa.[17]
In the easternCanadian province ofNova Scotia, scientists have unearthed prosauropods from theMcCoy Brook Formation, which is about 200 to 197 million years old, from the Early JurassicHettangian stage. The Nova Scotia material provides clues about the diet of these animals. A large number ofgastroliths, stones swallowed to grind up plant material in the gut, were found in the abdomen, as well as bone from the skull of a smallsphenodont,Clevosaurus. This indicates that these dinosaurs were omnivorous, with a diet mainly consisting of plants but with an occasional supplement of meat.[18] However, these remains have never been fully described or illustrated and were only tentatively referred toAmmosaurus. A further study identified them as a new taxon of sauropodomorph,Fendusaurus eldoni.[19]
Anchisaurus was a rather small dinosaur, with a length of just over 2 meters (6.6 ft), which helps explain why it was once mistaken for human bones.[20] It probably weighed around 27 kilograms (60 lb). However, Marsh's speciesA. major (also known asAmmosaurus) was larger, from 2.5 to 4 meters (8 ft 2 in to 13 ft 1 in) and some estimates give it a weight of up to 32 kilograms (70 lb).Gregory S. Paul estimated its length at 2.2 meters (7.2 ft) and its weight at 20 kilograms (44 lb) in 2010.[21]
According to the presence of cf.Otozoum tracks on the Connecticut Valley,Anchisaurus could reach even greater sizes.[22]Otozoum tracks were made by a semibipedal to quadrupedal sauropodomorph close to or on the line leading toward eusauropods.Anchisaurus is one of the two sauropodomorphs recognized in the zone. Based on the four known specimens ofAnchisaurus, Yates estimated that the animal measured up to 4.9 meters (16 ft) in length.[14] This matches well with the estimated average size of the adultEubrontes track-maker in the Hartford and Deerfield basins. Based on the largest knownEubrontes footprint, exceptionally large individuals ofAnchisaurus probably measured up to 6 meters (20 ft) in length.[22]
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Due to its primitive appearance,Anchisaurus was previously classified as a prosauropod, a member of a group of animals related to or ancestral to the sauropods. Recent investigations show that a group of traditional prosauropods form amonophyletic sister-group to Sauropoda, and thatAnchisaurus is instead closer to sauropods.[14]
The familyAnchisauridae was first proposed byOthniel Charles Marsh in 1885 and later defined as aclade consisting ofAnchisaurus and its nearest relatives. However, it is not clear which othergenera are included in the family; many of the dinosaurs once included have since been moved elsewhere, and the group is not used in most current taxonomies.[14]
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Fossils ofAnchisaurus were originally discovered in thePortland Formation of theNewark Supergroup in the Connecticut River Valley. This formation preserves an arid environment with strong wet and dry seasons, from theHettangian age into theSinemurian age, between about 200 and 195 million years ago.[2]
Digesting plant matter is a much more intensive biochemical process than digesting meat. This herbivore swallowedgastroliths (gizzard stones) to help break down the food in its stomach.[1] Herbivorous dinosaurs needed a hugegut. Since this had to be positioned in front of the pelvis, balancing on two legs became increasingly difficult, as dinosaurs became larger and they gradually evolved into thequadrupedal position that characterizes the later sauropods such asDiplodocus.[23] Prosauropods represented a middle phase between the earliest bipedal herbivores and the later giant sauropods. As a prosauropod,Anchisaurus was mostly typical of this group, which flourished briefly during the lateTriassic and early Jurassic.Anchisaurus teeth, used to rip food, were shaped like spoons.[1] It had fewer and more widely spaced teeth than true prosauropods, and as Peter Galton and Michael Cluver observed, narrower feet.[20]Anchisaurus would have spent most of its time on four legs but could have reared up on its hind legs to reach higher plants.
As afacultative biped,Anchisaurus had to have multi-purpose front legs. As 'hands', they could be turned inwards and be used for grasping. It had a simple reversible first 'finger', similar to a 'thumb'. As feet, the five toes could be placed flat against the floor and were strong at the ankle. This unspecialized design is typical of the early dinosaurs.
