Ammosaurus ("sand lizard") is a genus of sauropodomorph dinosaur from the Early Jurassic Period of North America. At 4 meters (13 feet) in length, it was small compared to some other members of its suborder, which included the largest animals ever to walk the Earth. It was a versatile animal, able to move both bipedally and quadrupedally, and may have been omnivorous.

Naming

The generic name Ammosaurus is derived from the Greek words ammos ("sandy ground") and sauros ("lizard"), referring to the sandstone in which it was found and its reptilian nature. There is one currently valid species (A. major), which is so named because it is larger than Anchisaurus, of which it was originally considered a second species. Famous American paleontologist Othniel Charles Marsh created this specific name in 1889. In 1891, Marsh created the new genus Ammosaurus for this species and he subsequently named another species (Ammosaurus solus) in 1892, although scientists now consider it synonymous with A. major.

Synonym with Anchisaurus?

The relationships of Ammosaurus with other dinosaurs are highly uncertain at this time. It is an early member of the suborder Sauropodomorpha and is most closely related to Anchisaurus, with which it may actually be synonymous. Different paleontologists consider Anchisaurus to be either a basal prosauropod (Galton & Upchurch, 2004) or a basal sauropod (Yates & Kitching, 2003; Yates, 2004).

Marsh originally described Ammosaurus major as Anchisaurus major, although he removed it to its own new genus only 2 years later. However, some recent studies have suggested that Ammosaurus and Anchisaurus are the same animal after all (Sereno, 1999; Yates, 2004). Other scientists prefer to keep the two genera separate due to anatomical differences in the pelvis and hind foot, although the two animals are still considered sister taxa (Galton & Upchurch, 2004).

Fossil discoveries

Fossils of Ammosaurus were originally discovered in the Portland Formation of the Newark Supergroup in the U.S. state of Connecticut. This formation preserves an arid environment with strong wet and dry seasons, from the Pliensbachian through to Toarcian stages of the Early Jurassic Period, roughly 190 to 176 million years ago. The original specimens were recovered from a sandstone quarry, which was used in the construction of the South Manchester Bridge in Connecticut. In fact, the holotype specimen was discovered by quarry workers. Unfortunately, it consists of only the rear half of the skeleton, as the block containing the front half had already been installed in the bridge. In 1969, the bridge was demolished, and some Ammosaurus remains were recovered. Three other incomplete skeletons of different ages are also known from Connecticut, but there is no known skull (Weishampel & Young, 1996).

Ammosaurus outside Connecticut

Ammosaurus remains have been reported from other parts of North America, but may not represent the species A. major, if they represent the genus at all.

The Navajo Sandstone of Arizona is the same age as the Portland Formation, and has produced prosauropod remains that have been referred to as Ammosaurus (Galton, 1971). However, it is possible that these actually belong to the genus Massospondylus, otherwise known only from South Africa (Galton & Upchurch, 2004).

In the Canadian province of Nova Scotia, scientists have unearthed prosauropods from the McCoy Brook Formation, which is about 200 to 197 million years old, from the Early Jurassic Hettangian stage. The Nova Scotia material provides clues about the diet of these animals. A large number of gastroliths, stones swallowed to grind up plant material in the gut, were found in the abdomen, as well as bone from the skull of a small lizard, Clevosaurus. This indicates that these dinosaurs were omnivorous, with a diet mainly consisting of plants but with an occasional supplement of meat (Shubin et al., 1994). However, these remains have never been fully described or illustrated and are only tentatively referred to Ammosaurus. Further study may either confirm or falsify this hypothesis.

References

• Galton, P.M. 1971. The prosauropod dinosaur Ammosaurus, the crocodile Postosuchus, and their bearing on the age of the Navajo Sandstone of Northeastern Arizona. Journal of Palaeontology 45: 781-795.
• Galton, P.M. & Upchurch, P. 2004. Prosauropoda. In: Weishampel, D.B., Dodson, P, & Osmolska, H. (Eds.) The Dinosauria (2nd Edition). Berkeley: University of California Press. Pp. 232-258.
• Marsh, O.C. 1889. Notice of new American dinosaurs. American Journal of Science Series 3, 37: 331-336.
• Marsh, O.C. 1891. Notice of new vertebrate fossils. American Journal of Science Series 3, 42: 265-269.
• Shubin, N.H., Olson, P.E., & Sues, H.-D. 1994. Early Jurassic small tetrapods from the McCoy Brook Formation of Nova Scotia, Canada. In: Fraser, N.C. & Sues, H.-D. (Eds.). In the Shadow of Dinosaurs: Early Mesozoic Tetrapods. Cambridge: Cambridge University Press. Pp. 244-250.
• Weishampel, D.B. & Young, L.O. 1996. Dinosaurs of the East Coast. Baltimore: Johns Hopkins University Press. 275 pp.
• Yates, A.M. & Kitching, J.W. 2003. The earliest known sauropod dinosaur and the first steps towards sauropod locomotion. Proceedings of the Royal Society of London 270: 1753–1758.
• Yates, A. M. 2004. Anchisaurus polyzelus (Hitchcock): the smallest known sauropod dinosaur and the evolution of gigantism among sauropodomorph dinosaurs. Postilla 230: 1-58.