Sauropod dinosaur phylogeny: Critique and cladistic analysis

Abstract

Sauropoda is among the most diverse and widespread dinosaur lineages, having attained a near-global distribution by the Middle Jurassic that was built on throughout the Cretaceous. These gigantic herbivores are characterized by numerous skeletal specializations that accrued over a 140 million-year history. This fascinating evolutionary history has fuelled interest for more than a century, yet aspects of sauropod interrelationships remain unresolved. This paper presents a lower-level phylogenetic analysis of Sauropoda in two parts. First, the two most comprehensive analyses of Sauropoda are critiqued to identify points of agreement and difference and to create a core of character data for subsequent analyses. Second, a generic-level phylogenetic analysis of 234 characters in 27 sauropod taxa is presented that identifies well supported nodes as well as areas of poorer resolution. The analysis resolves six sauropod outgroups to Neosauropoda, which comprises the large-nostrilled clade Macronaria and the peg-toothed clade Diplodocoidea. Diplodocoidea includes Rebbachisauridae, Dicraeosauridae, and Diplodocidae, whose monophyly and interrelationships are supported largely by cranial and vertebral synapomorphies. In contrast, the arrangement of macronarians, particularly those of titanosaurs, are based on a preponderance of appendicular synapomorphies. The purported Chinese clade 'Euhelopodidae' is shown to comprise a polyphyletic array of basal sauropods and neosauropods. The synapomorphies supporting this topology allow more specific determination for the more than 50 fragmentary sauropod taxa not included in this analysis. Their distribution and phylogenetic affinities underscore the diversity of Titanosauria and the paucity of Late Triassic and Early Jurassic genera. The diversification of Titanosauria during the Cretaceous and origin of the sauropod body plan during the Late Triassic remain frontiers for future studies. © 2002 The Linnean Society of London.