The Phylogeny of Ceratosauria (Dinosauria: Theropoda)

摘要

Recent discoveries and analyses have drawn increased attention to Ceratosauria, a taxo-nomically and morphologically diverse group of basal theropods. By the time of its first appearance in the Late Jurassic, the group was probably globally distributed. This pattern eventually gave way to a primarily Gondwanan distribution by the Late Cretaceous. Ceratosaurs are one of several focal groups for studies of Cretaceous palaeobiogeography and their often bizarre morphological developments highlight their distinctiveness. Unfortunately, lack of phylogenetic resolution, shifting views of which taxa fall within Ceratosauria and minimal overlap in coverage between systematic studies, have made it difficult to explicate any of these important evolutionary patterns. Although many taxa are fragmentary, an increase in new, more complete forms has clarified much of ceratosaur anatomy, allowed the identification of additional materials and increased our ability to compare specimens and taxa. We studied nearly 40 ceratosaurs from the Late Jurassic-Late Cretaceous of North and South America, Europe, Africa, India and Madagascar, ultimately selecting 18 for a new cladistic analysis. The results suggest that Elaphrosaurus and its relatives are the most basal ceratosaurs, followed by Ceratosaurus and Noasauridae + Abelisauridae (= Abelisauroidea). Several additional forms were identified as noasaurids, including Genusaurus. Within Abelisauridae, our analysis reveals a clade including Majungasaurus and the Indian forms, as well as a more weakly supported clade comprising Carnotaurus and llokelesia. These results greatly clarify the sequence of character acquisition leading to, and within, Abelisauroidea. Thanks to new noasaurid materials (particularly Masiakasaurus), numerous formerly ambiguous characters can now be resolved as either abelisaurid, noasaurid or abelisauroid synapomorphies. Skull and forelimb shortening, for example, now appear to be features confined to Abelisauridae. Nevertheless, a great deal of phylogenetic resolution is lacking, particularly among noasaurids, which hampers attempts to glean meaningful biogeographical information from the phylogeny. As a result, temporal and geographical sampling biases are probably contributing to the apparent patterns in the data and we suggest that definitive answers must await new discoveries. None of the recent ceratosaurian discoveries bear directly on the controversy surrounding latest Cretaceous ceratosaur biogeography.