Antetonitrus

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iAntetonitrus
Fossil range: Late Triassic

Scientific classification

Kingdom:	Animalia
Phylum:	Chordata
Class:	Sauropsida
Superorder:	Dinosauria
Order:	Saurischia
Suborder:	Sauropodomorpha
Infraorder:	Sauropoda
Genus:	Antetonitrus
Species:	A. ingenipes

Binomial name

Antetonitrus ingenipes
Yates & Kitching, 2003

Antetonitrus (ant-EE-tohn-IET-rus; "before the thunder") is the oldest known genus of sauropod dinosaur, living during the Late Triassic Period of southern Africa. It was a quadrupedal herbivore, like many of its later relatives, although it was far smaller than some of them. Antetonitrus was the largest animal in its environment, reaching up to 33 feet (10 meters) long and weighing up to two tons, but still shows some primitive adaptations to use the forelimbs for grasping, instead of purely for weight support.

Adam Yates, an Australian expert on early sauropodomorphs, named Antetonitrus in a 2003 report co-authored by South African James Kitching. The name is derived from the Latin ante- ("before") and tonitrus ("thunder"), which refers to its existence before other known sauropods, specifically Brontosaurus ("thunder lizard"). Brontosaurus is actually a junior synonym of Apatosaurus, but the name is still used in popular culture, and sauropods are sometimes called "thunder lizards" in vernacular terms. The one known species of Antetonitrus is called A. ingenipes, from the Latin ingens ("massive") and pes ("foot"), because its shows the beginning of the development of feet designed solely to support weight.

The fossils now known as Antetonitrus were actually discovered by Kitching in 1981 in the Free State of South Africa, and were stored in the Bernard Price Institute where they were labeled as Euskelosaurus. Yates recognized them as a separate taxon and published a description several years later. The holotype, or original specimen, consists of several vertebrae and numerous bones from both forelimb and hindlimb, all presumed to be from one individual. Five more limb bones from another smaller individual were also referred to the genus.

Antetonitrus shows several features which appear to be approaching those of sauropods, but still retains some primitive features. Unlike most of its smaller and more lightly-built ancestors, Antetonitrus was primarily quadrupedal. Like sauropods, its forelimbs were much longer relative to its hindlegs than earlier animals, and the wrist bones were broader and thicker to support more weight. However, the first digit of the hand, also called the "thumb" or pollex, was still twisted and flexible, capable of grasping against the hand. In more derived sauropods, the wrist bones are large and thick, arranged in such a way as to lock the hand into a permanently pronated position for full-time weight support, and the hand is incapable of grasping.

A cladistic analysis recognizes Antetonitrus as a basal sauropod, occupying a position between more derived animals such as Isanosaurus or Vulcanodon, and more basal sauropods like Anchisaurus or Melanorosaurus. The back vertebrae are extremely similar to Lessemsaurus from South America, while the limb bones are similar to Blikanasaurus, another stocky early sauropod from South Africa. However, these animals were not included in a cladistic analysis with Antetonitrus because they are poorly known (Yates & Kitching, 2003).

While Antetonitrus is not the earliest sauropod from a phylogenetic standpoint, it currently the oldest known sauropod chronologically, or rather tied for that distinction with other early sauropods from the same formation, like Melanorosaurus and Blikanasaurus. Fossils of these animals were recovered from the Lower Elliot Formation, which dates to the Norian stage of the Late Triassic, or approximately 221 to 210 million years ago. Before Antetonitrus and the other Lower Elliot animals were recognized as sauropods, the oldest known sauropod had been Isanosaurus from the Rhaetian stage, a slightly younger segment of the Triassic, of Thailand (Buffetaut et al., 2000).

Early sauropods and their prosauropod relatives were found around the world as all of the continents were at the time united into the single supercontinent, Pangaea, which made dispersal across the entire terrestrial world possible.

[edit] References

Buffetaut, E., Suteethorn, V., Cuny, G., Tong, H., Le Loeuff, J., Khansubha, S. & Jongautchariyakul, S. 2000. The earliest known sauropod dinosaur. Nature 407: 72–74.
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 B: Biological Sciences 270: 1753-1758.