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425 Basal Sauropodomorpha: The “Prosauropods” Adam M. Yates 21 Very early in the evolution of dinosaurs a branch of saurischians, known as the Sauropodomorpha, split away from the predatory Theropoda and began to specialize toward herbivory. The earliest sauropodomorphs that we know had already acquired small heads, moderately elongate necks, and leaf-shaped tooth crowns. These adaptations indicate that the ability to prey upon large vertebrates of similar body size had been abandoned, although some degree of omnivory was probably retained at this stage. Large body size soon followed, and by the middle of the Norian Stage (approximately 220 mya) sauropodomorphs had become the Earth’s dominant large terrestrial herbivores. Prior to the Norian the largest terrestrial herbivores were the stocky stahleckeriid dicynodonts that were unable to browse on vegetation over 1.5 m above the ground. Early sauropodomorphs, with their long necks, were the first herbivorous vertebrates that were capable of browsing among the trees. The sauropodomorph trend toward large-bodied herbivores culminated in the eusauropods and their close relatives, which may well have been in existence before the end of the Triassic Period. The early branching pattern of the Sauropodomorpha has always been a vexatious issue, in particular the precise relationship between the large columnar-limbed sauropods and the more basal sauropodomorphs. A consequence of this uncertainty is that the taxonomy of basal sauropodomorphs has remained in a state of flux. The classification of basal sauropodomorphs has had an unusually convoluted history, which is still far from settled. Early dinosaur studies placed the basal sauropodomorphs among the carnivorous theropods (e.g., Marsh 1881). In 1920 Friedrich von Huene invented the taxon Prosauropoda to encompass the early saurischians with long necks and small skulls. It is now clear that not all basal sauropodomorphs form an exclusive clade, and I will use the informal term basal sauropodomorph rather than Prosauropoda to refer to all nonsauropod sauropodomorphs. Huene can also be credited with first recognizing that the derived similarities between his prosauropods and sauropods indicate that they are more closely related to each other than to other dinosaur groups (Huene 1914). He coined the taxon Sauropodomorpha to encompass both sauropods and basal sauropodomorphs (Huene 1932). However, confusion with theropods persisted, and Huene abandoned Sauropodomorpha in his last work on saurischian classification, placing Prosauropoda, Sauropoda, and “Carnosauria” (large robust theropods that are no longer thought to form a clade) together in a more inclusive taxon he called Pachypodosauria (Huene History of Classification 21.1. Cladograms depicting the three major classes of results from recent phylogenetic analyses. (A) “Grand prosauropod monophyly” from Galton and Upchurch (2004). (B, C) Two versions of “Core prosauropod monophyly” from (B) Yates and Kitching (2003) and (C) Upchurch et al. (2007a). (D) “Extreme prosauropod paraphyly” from Yates (2007). P denotes the clade that conforms to the currently accepted definition of Prosauropoda. Note that in (D) it refers to a clade of low diversity that does not warrant recognition as a major unit of dinosaur diversity. S denotes the clade that conforms to the definition of Sauropoda used here. Adam M. Yates 426 1948). But even though it was not reflected in his classification, Huene had not abandoned his hypothesis of genealogical relationship between basal sauropodomorphs and sauropods. Underpinning the continued confusion between basal sauropodomorphs and theropods lay the so-called teratosaurids. These were thought to be dinosaurs with a postcranial skeleton identical to that of other basal sauropodomorphs but with a large robust skull armed with large carnosaur-like teeth. Thus it was thought that later “carnosaurs” had evolved from basal sauropodomorphs via the teratosaurids. It transpired that teratosaurids were chimaeras made of the postcrania of genuine basal sauropodomorphs that had been mixed with shed teeth or isolated skull parts of large carnivorous archosaurs, mostly rauisuchians (Benton 1986). Nevertheless, the continued confusion resulted in Romer’s 1956 classification where Prosauropoda was relegated to an infraorder within the suborder Theropoda while Sauropoda was elevated to the status of a suborder, on a par with Theropoda. Charig et al. (1965) revived Sauropodomorpha as a taxon, proposing that Prosauropoda and Sauropoda were two clades descended from a common quadrupedal pseudosuchian ancestor. Charig et al. recognized three families within Prosauropoda: Thecodontosauridae, Plateosauridae, and Melanorosauridae. This was little more than a size-based classification, with Thecodontosauridae encompassing the small, gracile species, Plateosauridae the medium-sized species, and Melanorosauridae the large robust species. No derived features were proposed to unite the members of any of these families. Since Charig et al. it has been...

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