Anchisaurus

Anchisaurus Temporal range: Early Jurassic,
Life restoration
Scientific classification
Domain:	Eukaryota
Kingdom:	Animalia
Phylum:	Chordata
Clade:	Dinosauria
Clade:	Saurischia
Clade:	†Sauropodomorpha
Clade:	†Sauropodiformes
Clade:	†Anchisauria Marsh, 1885
Genus:	†Anchisaurus Marsh, 1885
Species:	†A. polyzelus
Binomial name
†Anchisaurus polyzelus (Hitchcock, 1865)
Synonyms
Megadactylus polyzelus Hitchcock, 1865 (preoccupied) Amphisaurus polyzelus Marsh, 1882 (preoccupied) Yaleosaurus colurus Huene, 1932

Anchisaurus is a genus of basal sauropodomorph. An early herbivorous dinosaur, it lived during the Early Jurassic Period; more specifically, the Pliensbachian to Toarcian ages, 190 to 174 million years ago. Until recently it was classed as a member of Prosauropoda. The name comes from the Greek αγχι/agkhi anchi-; "near, close" + Greek σαυρος/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.

Description

Size of Anchisaurus, compared to a human.

Anchisaurus was a rather small dinosaur, with a length of just over 2 metres (6.6 ft), which helps explain why it was once mistaken for human bones. It probably weighed around 27 kilograms (60 lb). However, Marsh's species A. major (also known as Ammosaurus) was larger, from 2.5 to 4 metres (8 ft 2 in to 13 ft 1 in) and some estimates give it a weight of up to 70 pounds (32 kg). Gregory S. Paul estimated its length at 2.2 meters and its weight at 20 kg in 2010.

Discovery and naming

Anchisaurus skeleton restoration by O.C. Marsh.

Sauropodomorph remains were first discovered in North America in 1818, when some large bones were uncovered by Mr. Solomon Ellsworth Jr while with gunpowder excavating a well in East Windsor, Connecticut. At the time of their discovery it was thought that the bones might be those of a human, but the presence of tail vertebrae in 1821 falsified that idea. They are now recognized as those of an indeterminate sauropodomorph, possibly more closely related to the plateosaurian prosauropods.

In 1855, the original type specimen of Anchisaurus polyzelus, AM 41/109, which is housed at the Amherst College Museum of Natural History, was found by William Smith in Springfield, Massachusetts during blasting a well for the waterhouse at the Springfield Armory. 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 ichnologist Edward 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. He then contacted the British paleontologist Professor Richard Owen. Owen advised him to name the finds as a new genus. Owen suggested the name Megadactylus, "large finger" in Greek, in reference to the enormous thumb of the animal. Hitchcock Jr himself then chose the specific name polyzelus, "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.

In 1877, Professor Othniel Charles Marsh had noted that the name Megadactylus had been preoccupied by Megadactylus Fitzinger 1843, a subgenus of the lizard genus Stellio. In 1882, he replaced the name with Amphisaurus, "near saurian", probably referring to Marsh's interpretation of it as intermediate between primitive dinosaurs — at the time the British Palaeosaurus was an example of what was thought to be a primitive dinosaur — and more derived dinosaurs. In 1885, Marsh had discovered that this name also had been preoccupied, by the athracosaurian Amphisaurus Barkas 1870, and again replaced it by Anchisaurus, with the same meaning.

Meanwhile, nearly complete specimens had been found in Manchester, Connecticut. In 1884, a series of bridges was built over the Hop Creek. Sandstone blocks were sawed out of a local quarry north of Buckland 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 named Anchisaurus major, "the larger one", by Marsh in 1889. 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 made Anchisaurus major a separate genus, Ammosaurus, the "sand saurian". In the same publication he named YPM 1883/YPM 40313 as a new species of Anchisaurus, Anchisaurus colurus, "the mangled one". They served as the templates from which O.C. Marsh in 1893 restored the skeleton. The Manchester specimens are now considered conspecific with Anchisaurus polyzelus. The East Windsor and Manchester specimens are housed at the Peabody Museum of Natural History at Yale University.

The type species is Hitchcock's A. polyzelus. Marsh's A. major ("greater near lizard") is still often considered a valid species (as Ammosaurus) but his A. colurus (1891), once known as Yaleosaurus (von Huene, 1932), is now generally accepted as a female A. polyzelus, and his A. solus of 1892 is now reclassified as Ammosaurus major. However, Ammosaurus major itself may well be a synonym of A. polyzelus.

Broom named Gyposaurus capensis in 1911, from bones discovered in South Africa but Peter Galton renamed these A. capensis in 1976. This species has since been reclassified again and is probably a juvenile of Massospondylus carinatus. G. sinensis was also referred here, but appears to be a distinct animal.

Classification

Restoration by Lancelot Speed from 1905

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 a monophyletic sister-group to Sauropoda, and that Anchisaurus is instead closer to sauropods.

The family Anchisauridae was first proposed by Othniel Charles Marsh in 1885 and later defined as a clade consisting of Anchisaurus and its nearest relatives. However, it is not clear which other genera 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.

Paleobiology

Digesting plant matter is a much more intensive biochemical process than digesting meat. This herbivore swallowed gastroliths (gizzard stones) to help break down the food in its stomach. Herbivorous dinosaurs needed a huge gut. 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 the quadrupedal position that characterizes the later sauropods such as Diplodocus. Prosauropods represented a middle phase between the earliest bipedal herbivores and the later giant sauropods. Although it was not itself a prosauropod, Anchisaurus was mostly typical of this group, which flourished briefly during the late Triassic and early Jurassic. Anchisaurus teeth, used to rip food, were shaped like spoons. It had fewer and more widely spaced teeth than true prosauropods, and as Peter Galton and Michael Cluver observed, narrower feet. Anchisaurus would have spent most of its time on four legs but could have reared up on its hind legs to reach higher plants.

On the other hand, some paleontologists hypothesize Anchisaurus may also have eaten meat, as it was in the transition between these two ultimately distinct groups. The teeth were blunt but with file-like edges, suggesting mostly plant matter was eaten, and the jaw hinge was arranged in a way not entirely suited for tearing meat. Nevertheless, there is still some debate. The thumb had a large claw and the large eyes were not entirely on the side (as would be expected in an animal of a natural prey species).

As a facultative 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.