Antetonitrus

''Antetonitrus'' is a genus of sauropodiform dinosaur found in the Early Jurassic Elliot Formation of South Africa. The only species is ''Antetonitrus ingenipes''. Sometimes considered a basal sauropod, it is crucial for the understanding of the origin and early evolution of this group. It was a quadrupedal herbivore, like its later relatives, but shows primitive adaptations to use the forelimbs for grasping, instead of purely for weight support.

Discovery and naming

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"). The one known species of ''Antetonitrus'' is called ''A. ingenipes'', from the Latin ''ingens'' ("massive") and ''pes'' ("foot"), because it shows the beginning of the development of feet designed solely to support weight.

The fossils now known as ''Antetonitrus'' were 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 hind limb, all presumed to be from one individual. Five more limb bones from another smaller individual were also referred to the genus.

Description

The holotype specimen may have measured in length, in hip height and in body mass. However, the neural arches of the vertebrae were not fused with the centra, indicating that this individual perhaps was not fully grown.

''Antetonitrus'' shows several features which appear to be similar to those of later 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 hind legs than earlier animals, while the metatarsus was shortened. 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.

''Antetonitrus'' already shows adaptations for an increasing body size as seen in all later sauropods: The wrist bones were broader and thicker to support more weight, whereas the femur was elliptical in cross section. The vertebrae bear high neural spines and well developed hyposphene-hypantrum articulations which add rigidity to the trunk. The first toe of the hind foot already bears a large claw longer than the first metatarsal; however, this claw was not sickle shaped as seen in later sauropods. The femur was slightly sigmoidal (S-curved) in lateral view rather than straight as in other sauropods.

Despite being a true sauropod, the long bone development of ''Antetonitrus'' was more characteristic of basal sauropodomorphs, with subadults possessing, as derived sauropods do, highly vascularised fibrolamellar bone throughout their cortices, but with intermittent growth lines as seen in more basal sauropodomorphs instead of the absence of growth lines or presence of growth lines exclusively in the outer circumferential layer.

Classification

A cladistic analysis by Yates and Kitching recognizes ''Antetonitrus'' as a basal sauropod, occupying a position between more derived animals such as ''Isanosaurus'' or ''Vulcanodon'', and more basal sauropods like '' 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. Apaldetti et al. (2018) erected Lessemsauridae, a clade containing ''Antetonitrus'', '' Lessemsaurus'' and '' Ingentia''. Their cladogram is reproduced below:

The following cladogram shows the position of ''Antetonitrus'' within Massopoda, according to Oliver W. M. Rauhut and colleagues, 2020:

Paleogeography

While ''Antetonitrus'' is not the earliest sauropod from a phylogenetic standpoint, it is currently one of 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 Elliot Formation. Initially it was thought to have been recovered from the Lower Elliot Formation which dates to the Norian stage of the Late Triassic, or approximately 221 to 210 million years ago. Later studies indicate that it was actually recovered from the Early Jurassic Upper Elliot Formation. Before ''Antetonitrus'' and the other animals recovered from the Elliot Formation were recognized as sauropods, the oldest known sauropod had been ''Isanosaurus'' from the Rhaetian stage of Thailand.

Early sauropods and their sauropodomorph 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.

References

External links

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{{Taxonbar, from=Q772043

Sauropoda
Dinosaur genera
Hettangian dinosaurs
Elliot Formation
Dinosaurs of South Africa
Fossil taxa described in 2003
Taxa named by James Kitching