Saturday, May 7, 2011

Cosesaurus in Nesbitt's matrix

I haven't written the Cosesaurus entry for my site yet, but to me it always seemed the BANDits best bet, though instead they went with the weird Megalancosaurus and parafeathered LongisquamaCosesaurus looks outwardly like a little theropod, though of course it differs in the details.  Like Longisquama, its anatomy has not been redescribed since the 70s and the existing reconstructions involve lots of imagination.  The thing's preserved as a sandstone impression, and Ellenberger (1977) saw lots of feathers and drew numerous lines in and around the skull that have no guarantee of reflecting morphology.  Peters (2000) handled it better, but like in Longisquama identified antorbital fenestrae and suture details that Ellenberger didn't see, which makes me suspicious.  So again I coded it conservatively, using only the sutures agreed upon by both authors and Peters' interpretation of the pelvis.  I'm curious about including Cosesaurus not only because it's one of Peters' proposed non-archosaurian pterosaur relatives but also to address the argument that pterosaurs only clade with avemetatarsalians because there's nothing better in the matrix.  Would Cosesaurus also clade with avemetatarsalians despite its primitive anatomy?  Would it draw pterosaurs out of Archosauria?

The answer is that Cosesaurus emerges basal to Erythrosuchus, in a polytomy with Mesosuchus, Prolacerta, proterosuchids and higher archosauriforms.  If Longisquama is included as well, both clade with Prolacerta.  So it seems Nesbitt's matrix handles basal taxa just fine.  Only four steps move it sister to pterosaurs in Avemetatarsalia though, which is somewhat worrying.  It takes 22 more steps to move pterosaurs out with them in Prolacertiformes.

We'll see how this develops with the addition of unambiguous tanystropheid Langobardisaurus, a couple simiosaurs, Preondactylus and Pteranodon.  I'll also be checking the general pterosaur codings when coding them, to make sure Nesbitt didn't assume homology a priori by e.g. coding manual digit IV as unknown in amount of reduction.

1 comment:

  1. Does Cosesaurus look at ALL like Erythrosuchus? If not, consider it a red flag that there's something rotten in Denmark.