The Anatomy and Systematics of Eustreptospondylus Oxoniensis, A Theropod Dinosaur from the Middle Jurassic of Oxfordshire, England

ABSTRACT Recent work on theropod phylogeny has concentrated on the interrelationships of taxa that lie close to the ancestry of birds (coelurosaurs), whereas only a small number of studies have investigated the evolution of more primitive theropods (e.g. basal tetanurans). Ghost lineages implied by theropod phylogenies suggest that the Middle Jurassic was an important time in tetanuran evolution, witnessing the initial radiation and diversification of the clade. However, Middle Jurassic theropod specimens are rare and often incomplete. The holotype specimen of Eustreptospondylus oxoniensis, from the Middle Oxford Clay (upper Callovian) of Oxfordshire, England, represents the most complete Middle Jurassic theropod specimen from Europe. This taxon therefore has the potential to shed much needed light on basal tetanuran evolution at a critical time in the clade's history. Although several previous authors considered the anatomy and systematics of Eustreptospondylus, none of these accounts provided a comprehensive description and their utility is therefore limited. Here, we provide a detailed redescription of Eustreptospondylus and confirm its phylogenetic position as a basal member of Spinosauroidea. The taxon exhibits several anatomical features that appear to be incipient versions of the highly specialized character states found in more derived members of the clade (e.g. development of the premaxillary/maxillary embayment). The results of this work also suggest that Spinosauroidea may have originated in the Middle Jurassic of Europe, later dispersing to Gondwana.