Evolution of the presacral vertebrae of Sauropodomorpha.

This study demonstrates that the presacral vertebrae represent a rich sourc e of information for understanding the evolution and systematics of sauropo domorphs. In the basal dinosauromorph Marasuchus three morphological types of vertebrae are recognized, a condition further developed by sauropodomorp hs. Within Prosauropoda, the melanorosaurid Riojasaurus shows the most prim itive condition in the number and organization of the cervical vertebrae: i t has 9 cervicals, the first 5 of which exhibit the cervical morphological type, whereas the last 4 correspond to the dorsal morphological type. Plate osaurus is more derived than Riojasaurus in characters of the neck. The pri mitive sauropod-type of vertebra is more advanced than the typical prosauro pod vertebra, but it shows more primitive characters than the cetiosaurid t ype. It is considered that the presacrals of cetiosaurid type correspond to an evolutionary stage that might have made gigantism possible, triggering the adaptive radiation recorded in the Upper Jurassic of North America, Afr ica and Asia. In diplodocids the presacrals show clear relationships to the cetiosaurid type, but there are a higher number of cervicals, and opisthoc oely of centra is more developed. Significant differences in the cervical v er vertebrae of Apatosaurus with respect to those of Diplodocus, Camarasaur us and other sauropods suggest that Apatosaurus should be placed in a famil y of its own: Apatosauridae nov. Presacrals of brachiosaurid type, examined in Brachiosaurus brancai only because the generic placement of B. altithor ax is doubtful, posess several characters, such as opisthocoely extending t o the last dorsal, that are more advanced than in Patagosaurus and Diplodoc us. Other characters of the brachiosaurid type of vertebrae are more primit ive than those of the Diplodocus, thus suggesting origin from a condition m ore primitive than that exemplified by the cetiosaurid type. Restudy of pre sacrals of camarasaurid type does not confirm previous interpretations of t hese vertebrae as primitive; furthermore, several features are more derived than in Diplodocus. Also, it is proposed that the dicraeosaurid type might have evolved independently of other presacral types, the cetiosaurid type probably representing the ancestral condition. Presacrals of Haplocanthosau rus have a distinct morphology; thus, this genus is included in Haplocantho sauridae nov. Titanosaurs exhibit several morphological types of presacrals , indicating several levels of organization. Distinctive features are prese nt in the cervical region of members of Titanosauridae s.slr., e.g., a uniq ue design of the infra-postzygapophyseal constriction, the reduced distance between the centrum and the zygapophyses, the near absence of neural spine s. In general, Malawisaurus, Andesaurus and Argentinosaurus are more primit ive than Titanosauridae s.str. Tentative interpretation of the significance of several complex characters involving presacral vertebrae, such as the s igmoid neck, the dorsoventral extension of the dorsals, the transversely wi de neural spines, the relation between the number of cervicals and dorsals, the pleurocoels and the supraneural cavity, are discussed. In addition, a new prosauropod genus and species, Lessemsaurus sauropoides, diagnosed by i ts high neural arches and infrapostzygapophyseal constriction of the poster ior cervicals is erected.