Thursday, August 21, 2008

Archosauria, I Presume?

Is that an external mandibular fenestra I see? How well-known is the skull of Dimorphodon? I've always questioned the relationship between archosaurs and pterosaurs based on the apparent lack of mandibular fenestrae, but if Dimorphodon actually has such a feature, then I may have to change my tune!


Neil said...

Here's Romer (1956) on pterosaur jaws:

"The external fenestra, however, has disappeared in most cases, although it is present in some of the more primitive rhamphorhynchids, as Dimorphodon:"

(not the most up-to-date reference I know, but it's what I could reach without getting up...)

Neil said...

and here's a nice drawing of an non-reconstructed dimorpho skull:

(don't I have anything better to writing about some obscure group of marine hellasaurs?)

Nick Gardner said...

Why would you leave it up to a single character that has been reversed many times throughout Archosauria?

Benton, IIRC, offered many characters supporting the placement of Pterosauria within Archosauria. Though his analysis did not include any proposed alternative sister taxa for pterosaurs (such as prolacertiforms), but Senter's did, and still recovered pterosaurs as archosaurs. :-P

Zach said...

Yeah, I probably shouldn't have said "one of the ONLY" characters. Unwin brought up an interesting point in his recent book: If pterosaurs were interested in making themselves more lightweight, why close off a fenestra?"

I also think that Bennett's treatment of the group in his 1996 investigation of pterosaur phylogenetics makes a good case for convergence between the hindlimb skeletons of pterosaurs and archosaurs. And when you remove the hindlimb elements from the character matrix, it's tough to call pterosaurs archosaurs.

Meh. I'm on the fence with pterosaurs, as there's good evidence from both "sides." I just wish somebody would find a doggone pterosaurian Archaeopteryx! :-)

Senter did a pterosaur cladogram? This I need to see!

Christopher said...

If pterosaurs aren't archosaurs proper then they are surely archosauriformes, not a terribly huge distinction. But seriously, if they aren't archosaurs then they convergently evolved a lot of features.

That Unwin quote is kind of a straw man. First of all its basal feature for the archosauria and therefore a plesiomorphic condition in pterosaurs (if they are archosaurs.) Second fenestrae aren't weight saving features in any case, they have other functions mechanical in most instances. The mandibular fenestrae gives jaw muscles room to expand and also apparently plays a role in the ability of the jaw rami to bulge outwardly. Now take a look at an average pterosaur jaw, they are extremely shallow, all the post dentary bones are all bunched up way at the back. Now put a big hole back there, the small post dentary bones are now even smaller, not a particularly strong condition. So the situation exists where selective pressure would easily select for a reduction in the mandibular fenestrae in favor of increased strength in the posterior dentary.

And, no, Senter didn't do a pterosaur cladogram, just included them in a larger analysis, see:

Senter, P. 2005. Phylogenetic taxonomy and the names of the major archosaurian (Reptilia) clades. PaleoBios 25(2):1-7.

Senter, P. 2004. Phylogeny of Drepanosauridae (Reptilia, Diapsida). Journal of Systematic Palaeontology 2:257-268.