Early Evolution and Higher-Level Phylogeny of Sauropod Dinosaurs

ABSTRACT

Although sauropods played a major role in terrestrial ecosystems during much of the Mesozoic Era, little effort has been directed toward diagnosing Sauropoda and establishing higher-level interrelationships among sauropods. As a consequence, the origin and evolution of major skeletal adaptations in sauropods has remained largely speculative.

The cladistic analysis presented here focuses on higher-level relationships among sauropods. Based on 109 characters (32 cranial, 24 axial, 53 appendicular) for 10 sauropod taxa, the most parsimonious arrangement places four genera (Vulcanodon, Shunosaurus, Barapasaurus, and Omeisaurus) as a sequence of sister-taxa to a group of advanced sauropods, defined here as Neosauropoda. Neosauropoda, in turn, is composed of the sister-clades Diplodocoidea and Macronaria; the latter is a new taxon that includes Haplocanthosaurus, Camarasaurus, and Titanosauriformes. Titanosauriformes includes Brachiosauridae and Somphospondyli, a new taxon uniting Euhelopus and Titanosauria. Among macronarians, the position of Haplocanthosaurus is the least stable as a result of the absence of cranial remains.

The basic structure of the phylogeny is resilient to various tests and establishes the evolutionary sequence of many functionally significant sauropod adaptations, such as the digitigrade posture of the manus in neosauropods. Other characteristic sauropod adaptations, such as narrow tooth crowns, increases in length and number of cervical vertebrae, and bifid neural spines, are shown to have evolved more than once. As these results underscore, the higher-level phylogeny of sauropods must be based on a broad sampling of character data.

The fossil record of sauropods, although relatively limited during the early phase of the radiation (Late Triassic through Early Jurassic), nonetheless indicates that all major clades were established prior to the Late Jurassic, when substantial faunal interchange among major continental regions was still possible. The functional, temporal, and biogeographic implications of the higher-level phylogeny of sauropods are explored.