Anchisaurus

Discovery and naming

Sauropodomorph remains were first documented in North America in 1818, when bones nearly 6 ft long were uncovered by Solomon Ellsworth Jr. while excavating a well with gunpowder in East Windsor, Connecticut. Initially thought to be human, the presence of tail vertebrae disproved that idea; these bones are now considered those of an indeterminate sauropodomorph possibly related to plateosaurian prosauropods. In 1855, the type specimen of Anchisaurus polyzelus, AM 41/109, was found by William Smith in Springfield, Massachusetts during blasting for a well at the Springfield Armory.

Both the East Windsor and Springfield specimens were severely damaged by blasting, and many bones were discarded or kept by onlookers, leaving only incomplete remains. In 1863, Edward Hitchcock Jr described the Springfield remains in a supplement to his father's work on fossil footprints, suggesting they explained certain reptile tracks. He contacted Richard Owen, who advised naming the finds as a new genus and suggested the name Megadactylus, meaning 'large finger' in Greek, referring to the animal's enormous thumb.

Hitchcock Jr chose the specific name polyzelus, 'much sought for' in Greek, because his father had long sought the track-maker's identity. In 1877, Othniel Charles Marsh noted that Megadactylus was preoccupied by a lizard subgenus, and in 1882 he replaced it with Amphisaurus, meaning 'near saurian.' In 1885, Marsh discovered that Amphisaurus was also preoccupied, by an anthracosaurian, and again replaced it with Anchisaurus, with the same meaning.

Description

Anchisaurus was a rather small dinosaur, with a length of just over 2 m and a weight around 27 kg, which explains its initial misidentification as human bones. Marsh's species A. major was larger, from 2.5 to 4 m and up to 70 lb. Gregory S. Paul estimated its length at 2.2 m and weight at 20 kg in 2010. Based on cf. Otozoum tracks and four known specimens, Yates estimated Anchisaurus could measure up to 4.9 m, and exceptionally large individuals based on the largest Eubrontes footprint may have reached 6 m.

Paleobiology

Fossils of Anchisaurus were originally discovered in the Portland Formation of the Newark Supergroup in the Connecticut River Valley, an arid environment with strong wet and dry seasons from the Hettangian into the Sinemurian, between about 200 and 195 million years ago. As an herbivore, Anchisaurus swallowed gastroliths to help break down plant matter in its stomach. Herbivorous dinosaurs needed a large gut positioned in front of the pelvis, making bipedal balance increasingly difficult as they grew larger, leading to the quadrupedal stance of later sauropods.

Prosauropods like Anchisaurus represented a middle phase between early bipedal herbivores and giant sauropods. Its teeth, used to rip food, were spoon-shaped, and it had fewer, more widely spaced teeth than true prosauropods, as well as narrower feet. Anchisaurus would have spent most of its time on four legs but could rear up on its hind legs to reach higher plants. As a facultative biped, its front legs were multi-purpose: as hands, they could turn inward for grasping, with a simple reversible first finger like a thumb; as feet, the five toes could be placed flat and were strong at the ankle.

Multiple specimens are preserved as partially articulated skeletons despite being in a high-energy riverine environment, leading to speculation that Anchisaurus may have had a burrowing lifestyle, allowing entombment within burrows and minimal disarticulation. Further support comes from its close size and proportion similarity to the confirmed burrowing dinosaur Oryctodromeus.