Evolución de las vértebras presacras en Sauropodomorpha

José F. Bonaparte


This study demonstrates that the presacral vertebrae represent a rich source of information for understanding the evolution and systematics of sauropodomorphs. In the basal dinosauromorph lvlorasuchus three morphological types of vertebrae are recognized, a condition further developed by sauropodomorphs. Within Prosauropoda, the melanorosaurid Rioiasaurus shows the most primitive condition in the number and organization of the cervical vertebrae: it has 9 cervicals, the first 5 of which exhibit the cervical morphological type, whereas the last 4 correspond to the dorsal morphological type. Plateosaurus is more derived than Rioiasaurus in characters of the neck. The primitive sauropod-type of vertebra is more advanced than the typical prosauropod vértebra, but it shows more primitive characters than the cetiosaurid type. It is considered that the presacrals of cetiosaurid type correspond to an evolutionary stage that rnight have made gigantism possible, triggering the adaptive radiation recorded in the Upper Jurassic of North America, Africa and Asia. In diplodocids the presacrals show clear relationships to the cetiosaurid type, but there are a higher number of cervicals, and opisthocoely of centra is more developed. Significant differences in the cervical vertebrae of Apatosaurus with respect to those of Diplodocus, Camarasaurus and other sauropods suggest that Apatosaurus should be placed in a family of its own: Apatosauridae nov. Presacrals of brachiosaurid type, examined in Brachiosaurus brancai only beca use the generic placement of B. altithorax is doubtful, poses s several characters, such as opisthocoely extending to the last dorsal, that are more advanced than in Patagosaurus and Diplodocus. Other characters of the brachiosaurid type of vertebrae are more primitive than those of the Diplodocus, thus suggesting origin from a condition more primitive than that exemplified by the cetiosaurid type. Restudy of presacrals of camarasaurid type does not confirm previous interpretations of these vertebrae as primitive; furtherrnore, 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 Haplocanthosaurus have a distinct morphology; thus, this genus is included in Haplocanthosauridae nov. Tiianosaurs exhibit several morphological types of presacrals, indicating several levels of organization. Distinctive features are present in the cervical region of members of Titanosauridae s.str., e.g., a unique design of the infrapostzygapophyseal constriction, the reduced distance between the centrum and the zygapophyses, the near absence of neural spines. In general, Molaunsourus, Andesaurus and Argentinosaurus are more primitive than Titanosauridae s.str. Tentative interpretation of the significance of several complex characters involving presacral vertebrae, such as the sigmoid neck, the dorsoventral extension of the dorsals, the transversely wide 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 its high neural arches and infrapostzygapophyseal constriction of the posterior cervicals is erected.

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