Thursday, March 11, 2010

The Marshosaurus of Benson isn't Marshosaurus

I'm finishing up integrating Benson's (2010) and Benson et al.'s (2010) characters/codings in my saurischian supermatrix.  One thing I found that's great about Benson's matrix is that it codes the currently undescribed partial skeleton that's been referred to Marshosaurus (CMNH 21704).  It consists of a posterior skull, most of the presacrals, the scapula and a humerus, which were originally reported by Chure et al. in 1993 and later referred tentatively to Marshosaurus by Chure et al. (1997).  Its referral is only tentative since no elements overlap with the type material, which consists of pelvic and snout elements.   Benson finds Marshosaurus to be related to taxa which he recovers as megalosauroids (Piatnitzkysaurus, Condorraptor, Xuanhanosaurus), but which are placed even more basally in most phylogenies.  I found that when I coded Marshosaurus for my supermatrix, it ended up as a sinraptorid (only including a few characters from CMNH 21704).  So why the difference?  Well, it seems Benson didn't code basically any pelvic characters for Marshosaurus (except a lack of pelvic fusion, and pubic peduncle size).  Now maybe he thinks the pelvis belongs to a different taxon than the cranial, axial and forelimb elements, since there's no overlap.  But there's a slight problem with this.  The holotype of Marshosaurus is an ilium!  The other pelvic elements fit it perfectly and some are even thought to come from the same individual.  So Benson's Marshosaurus OTU excludes the only material we know for sure is Marshosaurus.  Would the pelvic elements influence its position?  We'll have to see what happens once all that data is in my supermatrix.

Benson, 2010. A description of Megalosaurus bucklandii (Dinosauria: Theropoda) from the Bathonian of the UK and the relationships of Middle Jurassic theropods. Zoological Journal of the Linnean Society. DOI 10.1111/j.1096-3642.2009.00569.x

Benson, Brusatte and Carrano, 2010. A new clade of large-bodied predatory dinosaurs (Theropoda: Allosauroidea) that survived to the latest Mesozoic. Naturwissenschaften. 97, 71-78.

Chure, Britt and Madsen, 1997. A new specimen of Marshosaurus bicentesimus (Theropoda) from the Morrison Formation (Late Jurassic) of Dinosaur National Monument. Journal of Vertebrate Paleontology. 17(3) 38A.

Chure, Madsen and Britt, 1993. New data on theropod dinosaurs from the Late Jurassic Morrison FM. (MF). Journal of Vertebrate Paleontology. 13(3) 30A.

1 comment:

  1. Very very very interesting!
    I entered the Benson's Marshosaurus in my data matrix assuming it was based on new parts of the holotype, but reading this post, I would code it separately. (I also coded two Marshosaurus OTUs in my analysis: one based on the holotypic pelvic elements only and one based on the cranial elements in the original holotype paper; in order to test what happens if the two OTUs are considered distinct taxa or a single animal. I was not very convinced by Madsen assumption that they belong to a single taxon).