EVOLUTION OF HERBIVORY, NECK ELONGATION, AND LOCOMOTION IN SAUROPOD DINOSAURS

Sauropod dinosaurs represent the upper limit for body size in terrestrial vertebrates, which are known to vary across four orders of magnitude. Despite the biomechanical, physiological, and developmental implications of their body size, sauropods were the dominant herbivores throughout most of the Mesozoic. The evolutionary history of this geographically widespread, morphologically diverse clade included major transitions in the cranial, vertebral, and limb skeleton.

A cladistic analysis of 25 sauropod genera scored for 222 characters resolves several early-appearing taxa as outgroups to Neosauropoda, a clade that first appears in the Late Jurassic and persists until the latest Cretaceous. Neosauropoda comprises two lineages (Macronaria, Diplodocoidea) whose monophyly and interrelationships are supported by synapomorphies localized in different regions of the skeleton. Large-nostrilled macronarians are characterized by a preponderance of features from the vertebral column and limb skeleton, whereas top-nostrilled diplodocoids are diagnosed by changes in the skull and vertebral column. These differences in character support cannot be explained by missing data or peculiarities of the data matrix itself. Rather, these data suggest that the divergence and subsequent diversification of Macronaria and Diplodocoidea were shaped by innovations focused in different regions of the skeleton.

Early sauropod evolution involved the acquisition of herbivorous specializations, neck elongation, and a graviportal limb posture. These early features are not reversed within the group, but appear to have been modified within the two main neosauropod lineages. Changes in the macronarian limb skeleton are associated with adoption of a "wide-gauge" limb posture in Saltasauridae, in which the manus and pes were positioned at a distance from the body axis during the step cycle. Changes in the skull and dentition of diplodocoids is associated with enhanced tooth replacement and acquisition of a dental battery within Rebbachisauridae. Saltasauridae and Rebbachisauridae represent the latest surviving sauropod lineages. The latter persisted until the Cenomanian in Africa and the Campanian in South America, the former until the Maastrichtian in India, South America, North America, and Europe.