| Antetonitrus | |
|---|---|
 | |
| Holotype | |
Scientific classification | |
| Domain: | Eukaryota |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Clade: | Dinosauria |
| Clade: | Saurischia |
| Clade: | †Sauropodomorpha |
| Clade: | †Sauropoda |
| Family: | †Lessemsauridae |
| Genus: | †Antetonitrus |
| Species: | †A. ingenipes |
| Binomial name | |
| †Antetonitrus ingenipes | |
Antetonitrus is agenus ofsauropodiformdinosaur found in theEarly JurassicElliot Formation ofSouth Africa. The only species isAntetonitrus ingenipes. Sometimes considered a basalsauropod, it is crucial for the understanding of the origin and early evolution of this group. It was aquadrupedalherbivore, like its later relatives, but shows primitive adaptations to use the forelimbs for grasping, instead of purely for weight support.[1]
Adam Yates, anAustralian expert on earlysauropodomorphs,[2] namedAntetonitrus in a 2003 report co-authored bySouth AfricanJames Kitching. The name is derived from theLatinante- ("before") andtonitrus ("thunder"), which refers to its existence, before other known sauropods, specificallyBrontosaurus ("thunder lizard"). The one known species ofAntetonitrus is calledA. ingenipes, from the Latiningens ("massive") andpes ("foot"), because it shows the beginning of the development of feet designed solely to support weight.[1]
Thefossils now known asAntetonitrus were discovered by Kitching in 1981 in theFree State of South Africa,[2] and were stored in theBernard Price Institute where they were labeled asEuskelosaurus. Yates recognized them as a separate taxon and published a description several years later.[1] Theholotype, or original specimen, consists of severalvertebrae 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.[1]
The holotype specimen may have measured 8–10 metres (26–33 ft) in length, 1.5–2 metres (4.9–6.6 ft) in hip height and 5.6 metric tons (6.2 short tons) in body mass.[1][3] However, theneural arches of the vertebrae were not fused with thecentra, indicating that this individual perhaps was not fully grown.[1]
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.[1] Like sauropods, its forelimbs were much longer relative to its hind legs than earlier animals, while themetatarsus was shortened.[1] However, the first digit of the hand, also called the "thumb" orpollex, was still twisted and flexible, capable of grasping against the hand. In morederived sauropods, the wrist bones are large and thick, arranged in such a way as to lock the hand into a permanentlypronated position for full-time weight support, and the hand is incapable of grasping.[1]
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.[1] The vertebrae bear highneural spines and well developedhyposphene-hypantrum articulations which add rigidity to the trunk.[1] 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.[1] The femur was slightly sigmoidal (S-curved) in lateral view rather than straight as in other sauropods.[1]
Despite being a true sauropod, the long bone development ofAntetonitrus 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.[4]
Acladistic analysis by Yates and Kitching recognizesAntetonitrus as abasal sauropod, occupying a position between more derived animals such asIsanosaurus orVulcanodon, and more basal sauropods likeMelanorosaurus.[1] The back vertebrae are extremely similar toLessemsaurus from South America, while the limb bones are similar toBlikanasaurus, another stocky early sauropod from South Africa. However, these animals were not included in a cladistic analysis withAntetonitrus because they are poorly known.[1] Apaldetti et al. (2018) erectedLessemsauridae, aclade containingAntetonitrus,Lessemsaurus andIngentia. Their cladogram is reproduced below:[5]
The following cladogram shows the position ofAntetonitrus within Massopoda, according to Oliver W. M. Rauhut and colleagues, 2020:[6]
WhileAntetonitrus is not the earliest sauropod from aphylogenetic standpoint, it is currently one of the oldest known sauropodchronologically, or rather tied for that distinction with other early sauropods from the same formation, likeMelanorosaurus andBlikanasaurus. Fossils of these animals were recovered from theElliot Formation. Initially it was thought to have been recovered from theLower Elliot Formation which dates to theNorian stage of theLate Triassic, or approximately 221 to 210million years ago. Later studies indicate that it was actually recovered from the Early Jurassic Upper Elliot Formation.[7][8] BeforeAntetonitrus and the other animals recovered from the Elliot Formation were recognized as sauropods, the oldest known sauropod had beenIsanosaurus from theRhaetian stage ofThailand.[9]
Early sauropods and theirsauropodomorph relatives were found around the world as all of the continents were at the time united into the single supercontinent,Pangaea, which madedispersal across the entire terrestrial world possible.
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