Anchisaurus, meaning “near lizard”, was an early herbivorous dinosaur that was previously classified as a primitive prosauropod dinosaur. It lived in the Early Jurassic Period, 200 to 188 million years ago. Anchisaurus was used as a replacement name for Amphisaurus, which itself was a replacement name for Megadactylus, both of which were already used for other animals. Anchisaurus was probably the first dinosaur discovered in North America. Nothing was known about dinosaurs in 1818 when the fossils were first discovered in Connecticut, and not until 1855, as many more bones were amassed, did anyone have any idea of what they were collecting. In 1855 the bones were recognized as at least reptilian.

The bones were given the name Megadactylus in 1865, and then changed to Anchisaurus, by Othniel Charles Marsh, in 1885. Unconfirmed discoveries of this genus have occurred in Nova Scotia and as far away as Africa and China, but the latter reports are debatable. Since not all remains have been collected for this dinosaur, reconstructions assumed that the neck and tail were prosauropod in nature. Recent analysis proves otherwise, and places the individual as a sauropod.

Anchisaurus was a small herbivorous dinosaur which was only a little over six and a half feet long. Originally, the remains were thought to be human bones until more fossils were recovered. It weighed about 60 pounds. The species A. major, recovered by O.C. Marsh, was somewhat larger (8 to 13 feet in length and weighed up to 70 pounds). Anchisaurus digested gizzard stones to help break down and digest plant matter that it ate. The gut of the specimen was huge, and the placement of it in front of the pelvis made it difficult for the dinosaur to keep its balance on two legs. As sauropod dinosaurs became larger, they gradually evolved into quadrupedal sauropods. Prosauropods represented the middle phase between early bipedal herbivores and later giant sauropods. Although Anchisaurus was not a true prosauropod, it had typical characteristics of prosauropods of the Late Triassic and Early Jurassic Periods. It used its spoon-shaped teeth to rip food. The teeth were more widely spaced apart than in true prosauropods, and it had narrower feet. Anchisaurus spent most of its time on four legs, but could rear up on its hind legs to reach higher plants.

Some paleontologists believe that Anchisaurus may have also eaten meat; however, the teeth were blunt with file-like edges, and the jaw was not entirely suited for tearing meat. The thumb had a large claw and the large eyes were not entirely on the side (as would be expected in predatory species). Anchisaurus was a quadrupedal/bipedal cross. It had multi-purpose front legs. Its front legs (or hands) could be turned inwards and be used for grabbing or grasping. It a simple reversible first finger, which was similar to a thumb. As feet, the five toes could be placed flat on the ground. It had strong ankles. These characteristics were typical of early dinosaurs.

A few species are known from this genus. Included is A. polyzelus, A. major, and possibly A. colurus. Other species names have been classified into other genus categories, such as Ammosaurus.