The phylogenetic relationships of sauropod dinosaurs

Abstract

A data-matrix of 205 osteological characters for 26 sauropod taxa is subjected to cladistic analysis. Two most parsimonious trees are produced, differing only in the relationships betweenEuhelopus,OmeisaurusandMamenchisaurus. The monophyly of the Euhelopodidae (includingShunosaurus) is supported by seven synapomorphies. The Cetiosauridae (Patagosaurus,CetiosaurusandHaplocanthosaurus) is paraphyletic with respect to the Neosauropoda. The latter clade divides into two major radiations—the «Brachiosauria» (Camarasaurus, brachiosaurids and titanosauroids), and the Diplodocoidea (nemegtosaurids, dicraeosaurids, diplodocids andRebbachisaurus). Further evidence for the inclusion ofOpisthocoelicaudiain the Titanosauroidea is presented.Phuwiangosaurus, a problematic sauropod from Thailand, may represent one of the most plesiomorphic titanosauroids. ‘Peg’-like teeth have evolved at least twice within the Sauropoda. The postspinal lamina, on the neural spines of middle and caudal dorsal vertebrae, represents a neomorph rather than a fusion of pre-existing structures. Forked chevrons may have evolved convergently in the Euhelopodidae and the diplodocid-dicraeosaurid clade, or they may have been acquired early in sauropod evolution and subsequently lost in the «Brachiosauria». The strengths and weaknesses of the data-matrix and tree topologies are explored using bootstrapping, decay analysis and randomization tests. Several nodes are only poorly supported, but this seems to reflect the large proportion of missing data in the matrix (46%), rather than an abnormally high level of homoplasy. The results of the randomization tests indicate that the data-matrix probably contains a strong phylogenetic ‘signal’. The relationships of some forms, such asHaplocanthosaurus, are influenced by the inclusion or exclusion of certain taxa with unusual combinations of character states. Such a result suggests that there are dangers inherent in the view that ‘higher’ level sauropod phylogeny can be accurately reconstructed using only a small number of well-known taxa.