Sunday, September 23, 2007

Eustreptospondylus oxoniensis

This is one of my favorite dinosaurs - Eustreptospondylus oxoniensis. It's also perhaps one of the most complete theropod fossils from Britain. Eustreptospondylus is a Middle Jurassic 'megalosaur' from the Oxford Clay of Oxfordshire. Now, I'm not much of a historian so I won't claim to know all the details, but it was known for a long time as 'Streptospondylus' but was subsequently renamed by Walker in 1964 in his paper 'Triassic reptiles from the Elgin area: Ornithosuchus and the origin of carnosaurs'.

Ornithosuchu
s is nowadays regarded as an archosaur so for some time, I was baffled as to why Walker addressed the question of carnosaur origins by redescribing a Triassic archosaur. However, it appeared I was just being lazy (as usual) as I had not even read the abstract. It is made quite clear that Walker considered Ornithosuchus to be a theropod dinosaur, a primitive carnosaur close to the ancestry of Megalosauridae and Tyrannosauridae. Thus, Walker proposed a modified classification of the Infra-order Carnosauria* within Sub-order Theropoda; comprised of the Superfamily Megalosauroidea and Superfamily Tyrannosauroidea which includes the Families Ornithosuchidae, Spinosauridae, and Tyrannosauridae.

In the process of establishing this classification, Walker naturally compared Ornithosuchus with other supposed 'Triassic carnosaurs' but also with Jurassic and Cretaceous 'carnosaurs'. Thus, much of the British 'carnosaur' materials were re-examined and several new names emerged including Eustreptospondylus oxoniensis, E. divesensis, and Metriacanthosaurus.

*Carnosauria used to include all the large theropods but has diminished substantially in taxonomic range in the last few years - I think it only includes the Allosauroidea and Spinosauroidea if these two form a monophyletic Carnosauria to begin with.

Reference:
Walker, A. D. 1964. Triassic reptiles from the Elgin area: Ornithosuchus and the origin of carnosaurs. Phil. Trans. R. Soc. Lond. 248: 53-134.