Sunday, February 17, 2008

Torvosaurus

I initially named this sketch Megalosaurus but then remembered that I've been relying on the proportions of Torvosaurus for the reconstruction, so it's been renamed to Torvosaurus. Torvosaurus is a North American "megalosaur" popularly used to aid in the reconstructions of the English Megalosaurus mostly because the long-held assumption that these two taxa are closely related. However, more recent phylogenetic analyses show that the traditional monophyletic Megalosauridae does not seem to exist anymore but rather a paraphyletic "Megalosauridae" with a paraphyletic grade of "megalosaurs" leading up to the Spinosauridae. Or something like that...there seems to be quite a lot of confusion in this area of the theropod phylogeny probably because of the lack of good specimens. Although, in a consensus tree of published trees, a fair chunk of the traditional "megalosaurs" still seem to come together in a smaller but yet monophyletic Megalosauridae.

Anyway, I quite like these basal tetanurans as they are so enigmatic. Megalosaurus has historically been used as a "waste-basket" taxon, i.e. if the affinities of a new taxon is indeterminable, then it is attributed to the genus "Megalosaurus". Some very famous taxa such as Dilophosaurus or Eustreptospondylus were initially attributed to Megalosaurus. In the 1960's to the 1970's many new Chinese theropods were also given the name Megalosaurus, which were subsequently reassigned to new genera. Because of this, Megalosaurus had the longest temporal range of all theropod genera starting at the Rhaetian in the Triassic ("M". cambrensis = Zanclodon cambrensis) all the way up to the Cretaceous ("M". crenatissimus = Majungasaurus crenatissimus). Of course Megalosaurus proper is fairly restricted to the Middle Jurassic.

Torvosaurus on the other hand is from the Late Jurassic of western North America and Portugal. In both settings, Torvosaurus was contemporaneous with other theropods: Allosaurus, A. fragilis in N. America and A. europaeus in Portugal; and Ceratosaurus in N. America. As with Ceratosaurus, Torvosaurus is relatively rare compared to Allosaurus.

1 comment:

Zach Miller said...

And according to Sereno, it formed an outgroup to the Spinosauridae, in a larger monophyletic clade called Spinosauroidea.

I don't know how sold I am on this idea. Thomas Holtz thinks that Megaraptor is a basal (but late-surviving) spinosauroid, which is just all kinds of awesome.