Thursday, February 28, 2008

Ornithischian Eyeshades

Ever wonder what that prong of bone in the orbits of ornithischian dinosaurs is called? I've often wondered that myself, and I accidentally came across the answer this morning while reading through Currie & Carpenter's Acrocanthosaurus description.

It's the palpebral bone! And it evolved independantly among many crocodilians and ornithischians. The feature seems primitive to the Ornithischia, although many groups subsequently lost it. Others enlarged it, among them "hypsilophodonts" (above) and basal ceratopsians. In his Predatory Dinosaurs of the World, Greg Paul suggests that the palpebral would have given ornithischians "eagle eyes." The bones would have certainly been covered with skin, and would have given the eyes of many ornithischians a certain "evil" glare. What purpose the palpebral bones served is not known (and I've never really seen it discussed in the literature). And why did some groups need it while others lost it completely?


Emile said...

That must be why Cuvier's dwarf caiman is called Paleosuchus palpebrosus.

Maybe they protected their eyes from the sun in the same way eagle's eyes are protected? But then again, why do some have it and some don't? Eagles have palpebrals too, don't they?

Amanda said...

Very cooooool! Thanks...I didn't actually wonder about that bone because I'd never noticed, but now, I'm going to notice it everywhere.

Amanda said...

Oh...and about the comment you left at Laelaps about the banner. When you go into the "layout" section of the blogger dashboard, hit the edit button on the header section. There should be a place to upload a photo. I took a photo I'd taken and cropped it so that the dimensions seemed header-like. That's all there is to it!

Zach Miller said...

Thanks, Amanda!

Emile, eagles do have palpebrals, although the structure is probably not homologous. For a big bird of prey, hunting for rabbits in the sky, an eyeshade would be particularly beneficial. But a herbivorous ornithopod? Who knows? :-)

Alkalynic said...

To clarify, most of the groups that did not have palpebrals them probably didn't "lose" them as much they absorbed them into some of the other bones surrounding the orbit. Notice that many derived ornithischians do have prominent arches around their upper orbit which probably represent the absorbed supraorbital.