The earliest known sauropod dinosaur and the first steps towards sauropod locomotion

Abstract

A partial dinosaur skeleton from the Upper Triassic (Norian) sediments of South Africa is described and named Antetonitrus ingenipes. It provides the first informative look at a basal sauropod that was beginning to show adaptations towards graviportal quadrupedalism such as an elongated forelimb, a modified femoral architecture, a shortened metatarsus and a changed distribution of weight across the foot. These adaptations allowed the clade to produce the largest-ever terrestrial animals. However, A. ingenipes lacked specializations of the hand found in more derived sauropods that indicate it retained the ability to grasp. Antetonitrus is older than the recently described Isanosaurus from Thailand and is the oldest known definitive sauropod.

The evolution of ornithischian quadrupedality

“…Similarly, it has been proposed that radius/humerus length ratios were lower in quadrupeds (Galton 1970), but these differences are marginal with extensive overlap in this ratio between quadrupedal and bipedal taxa (Maidment and Barrett 2014). Some features that have been proposed to characterise quadrupedality in sauropodomorphs, such as the presence of a facet on the distal radius for reception of the ulna (Yates and Kitching 2003), are present in a wide range of both quadrupedal and bipedal ornithischians and may not be of help in determining stance outside of Sauropodomorpha.…”

“…Nevertheless, several features do appear to be strongly correlated with quadrupedal stance, including: the presence of an anterolateral process on the ulna (Bonnan 2003;Yates and Kitching 2003;Yates et al 2010); hoof-like manual ungual phalanges (e.g. Galton 1970;Norman 1980); a femur that is longer than the tibia (Colbert 1964;Galton 1970); a femur that is straight in lateral view (Yates and Kitching 2003;Chinnery 2004); a pes that is relatively shorter relative to tibia ?…”

“…Large body size is correlated with quadrupedality in sauropods (e.g. Carrano 2005;Sander et al 2011), but this group achieved quadrupedal stance at similar body masses to those attained by some bipedal basal sauropodomorphs (for example, Riojasaurus and Antetonitrus both have femoral lengths of approximately 780-790 mm, but the former is bipedal and the latter quadrupedal: see Bonaparte 1972;Yates and Kitching 2003). Moreover, at least one dinosaur taxon, the basal sauropodomorph Massospondylus, exhibits the contrary trend of abandoning quadrupedality as body mass increased (Reisz et al 2005).…”

“…Galton 1970;Norman 1980); a femur that is longer than the tibia (Colbert 1964;Galton 1970); a femur that is straight in lateral view (Yates and Kitching 2003;Chinnery 2004); a pes that is relatively shorter relative to tibia ? femur length (Colbert 1964;Galton 1970;Yates and Kitching 2003) [although both of the latter features could be related to increased body size (Maidment and Barrett 2014)]; reduction of the fourth trochanter (Galton 1970;Yates and Kitching 2003); a transversely expanded iliac blade (Carrano 2005;Maidment and Barrett 2014); and higher forelimb/hind limb length ratios than those in bipeds (Galton 1970), although the utility of this feature is somewhat equivocal (Maidment and Barrett 2014). The biomechanical significance of some of these characters remains untested: for example, it is unclear what functional changes result from reduction of the fourth trochanter.…”

“…In two cases (Sauropodomorpha and Thyreophora), these reversions occurred early in dinosaur history only * 15-20 Ma after the origin of Dinosauria, sauropodomorphs being the first to adopt secondary quadrupedalism, in the Late Triassic to Early Jurassic (e.g. Yates and Kitching 2003;Bonnan and Yates 2007;Yates et al 2010), while the first quadrupedal thyreophoran is known from the basal Jurassic (Owen 1861). The other two reversions, in Ceratopsia and Hadrosauriformes, happened much later in time, during the late Early to early Late Cretaceous (Sereno 1999;Weishampel et al 2004).…”