Anatomy and systematics of the prosauropod dinosaurThecodontosaurus antiquusfrom the upper Triassic of southwest England

Anomalously high variation in postnatal development is ancestral for dinosaurs but lost in birds

“…The Triassic sauropodomorphs Thecodontosaurus antiquus and Melanorosaurus readi have been interpreted as possessing robust/gracile variation in similarly sized limb elements (18,45), but given our interpretation of a similar ostensible dichotomy in early theropods, these taxa probably possess similar levels of variation in growth patterns. As predicted by our results, along with some morphological variation in Plateosaurus engelhardti (46), histological analysis of a large sample of limb elements (n = 33) of Plateosaurus demonstrated that size and histological maturity are poorly correlated in this taxon (22,25).…”

“…Most studies of the ontogeny of more-derived theropods (15)(16)(17) suggest that the low levels of variation that characterize avian ontogeny were present in close nonavian relatives as well. Closer to the origin of Dinosauria, morphological variation within species is widespread (18)(19)(20)(21)(22)(23)(24)(25)(26), but whether this variation is the result of taxonomic diversity, ontogeny, sexual dimorphism, or simple individual variation is not clear (22,23,27). Furthermore, close dinosaurian relatives or "dinosaur precursors" (e.g., Silesaurus, Asilisaurus) possess high intraspecific variation in growth sequences [i.e., sequence polymorphism (28)], suggesting that this condition could be ancestral for Dinosauria (21).…”

The origin and early radiation of dinosaurs

“…Similar assemblages have been reported from Greenland , and Dzik et al (2008) recently reported theropod dinosaurs from the latest Triassic of Poland. Poorly dated fissure fills from western Europe record the presence of sauropodomorphs and possible theropods (e.g., Benton et al, 2000;Yates, 2003a); these are generally thought to be latest Triassic to Early Jurassic in age (Whiteside and Marshall, 2008). Dinosaurs are conspicuously absent from the Late Triassic of Asia, but an extensive sauropodomorph-dominated assemblage is known from the Lower Jurassic Lufeng Formation of Yunnan, China; this assemblage also includes rare ornithischians and theropods (e.g., Luo and Wu, 1994).…”

“…Unfortunately, the specific interrelationships of these taxa are still controversial, and two main competing phylogenetic hypotheses have been proposed. The first hypothesis suggests that most or all basal sauropodomorphs (i.e., non-sauropod sauropodomorphs) form a monophyletic group that is the sister taxon to Sauropoda (e.g., Sereno, 1999;Benton et al, 2000;Yates and Kitching, 2003;Galton and Upchurch, 2004;Sereno, 2007b;Upchurch et al, 2007a). In contrast, other studies find these taxa as a largely paraphyletic grade where some basal sauropodomorphs are closer to sauropods than they are to each other (e.g., Yates, 2003aYates, ,b, 2007aSmith and Pol, 2007;Yates et al, 2010).…”

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

“…A further area of uncertainty and disagreement is the nature of the relationship between sauropods and the assemblage of early dinosaurs known collectively as the 'prosauropods'. Prosauropods are frequently thought of as forming a sister clade to the Sauropoda that appears long before the oldest known sauropods (Sereno 1999;Benton et al 2000), thus implying a long period of hidden evolution. However, some workers have suggested that certain 'prosauropods' are actually basal members of the Sauropoda (Yates 2003b), thus reducing the morphological and temporal gap between the two groups.…”