Wednesday, March 19, 2008

The taxonomic status of Megalosaurus bucklandii

A new paper by Roger Benson of Cambridge University and colleagues discuss the taxonomic status of Megalosaurus bucklandii.

Every dinosaur fan knows of Megalosaurus, the first dinosaur to be named by William Buckland in 1824. Megalosaurus is also historically significant as being one of the taxa that Richard Owen based his Dinosauria in 1842, the other taxon obviously being Iguanodon. The type species M. bucklandii was erected by Gideon Mantell in 1827. Buckland's original description of Megalosaurus in 1824 is based on a series of syntypes, one of which is the famous dentary (fig). Over the years, many other large theropod specimens from the Middle Jurassic were "unjustifiably (Benson et al. 2008)" attributed to M. bucklandii. Over the years, many authors noted the possibility that the syntype series and all subsequent referred specimens may belong to different taxa all together. Thus M. bucklandii is suggested to only refer to the lectotype dentary.

According to Benson et al. (2008) the lectotype dentary allows for a diagnosis of M. bucklandii thus making it a valid taxon. Although the systematic position of M. bucklandii is unresolved, Benson et al. (2008) regard it as a possible member of the clade comprising of Ceratosauria and Tetanurae but not an abelisaurid or coelophysoid. Further, the characteristic dentary morphology present in all known spinosauroids is absent in the lectotype dentary. Thus, Benson et al. (2008) recommend the discontinuation of allying Megalosaurus with Spinosauroidea and also the practice of referring any other taxa as "megalosaurid".

So I guess we should stop using Megalosauridae and "megalosaurids" and start referring to these taxa (with the exclusion of Megalosaurus) as spinosauroids...

In my opinion, just because of its historical importance to dinosaur research, the most significant discovery in the future would be the discovery of a more complete specimen indisputably assignable to Megalosaurus bucklandii - that is, the associated specimen of a M. bucklandii dentary with other cranial and postcranial materials. The discovery of such a specimen is probably the only way to resolve the systematic position of M. bucklandii and the taxonomy of all other unassociated materials previously referred to M. bucklandii.

Benson, R. B. J., P. M. Barrett, P. Powell, and D. B. Norman. 2008. The taxonomic status of Megalosaurus bucklandii
(Dinosauria, Theropoda) from the Middle Jurassic of Oxfordshire, UK. Paleontology 51: 419-424

Abstract: The lectotype of the Middle Jurassic theropod dinosaur Megalosaurus bucklandii, a right dentary, can be diagnosed on the basis of two unique characters: a longitudinal groove on the ventral part of the lateral surface of the dentary and a slit-like anterior Meckelian foramen. This taxon, the first dinosaur to be scientifically described, is therefore valid. Currently, however, no further material can be referred to this species with any certainty. Megalosaurus bucklandii occupies an uncertain systematic position but is not an abelisaurid or coelophysoid. Additionally, it does not possess the diagnostic dentary characters that are present in all known spinosauroids. Owing to this uncertainty, use of the family Megalosauridae should be discontinued until such time as its systematic position becomes clearer.


Zach Miller said...

I'm not surprised that Megalosaurus is a chimaera, given the state of paleo in those days. And you're right, there's probably not going to be a whole lot of taxonomic re-shuffling. Everybody who used to be a megalosaur might either fall under Spinosauroidea or Sereno's Torvosauridae.

Mambo-Bob said...

Either way will be really cool - whether the traditional "megalosaurs" still form a monophyletic Torvosauridae (==Megalosauridae) or if there will be a paraphyletic grade of basal spinosauroids/spinosaurids...