Friday, March 27, 2009

paleo Query: Basal Most, Earliest Archosaur?

While pondering the alternate Permian that we are working on, I was wondering...what is the basal most archosaur? I know its supposed to have appeared prior to the PT Extinction absed on a passing reference. However, what is it? What fossil has been identified as the basal most archosaur and how derived in that sucker relative to other diapsids?

Can anyone point me to a paper?


220mya said...

Archosauria is defined as the closest common ancestor and all its descendants of birds + crocodiles (or Pseudosuchia/Crurotarsi + Ornithosuchia/Ornithodira depending on the clade names you want to use). The oldest taxon within this group is Arizonasaurus from the Anisian of the Moenkopi Fm of Arizona. The oldest archosauriform is Archosaurus from the latest Permian of Russia, but this is a proterosuchid that is outside of Archosauria proper.

Unknown said...

It kinda depends on what definition you're using for the Archosauria. For the show, we used a definition that actually excluded Euparkeria, and in doing so, allowed the Archosauria to emergence in the lower Triassic (which also happens even if Euparkeria is included).

220mya said...

Most folks use Archosauria to mean birds + crocs though, so by that definition Euparkeria is a basal archosauriform outside of Archosauria. A few folks use Archosauria to mean "everything with an antorbital fenestra", and by that definition, all archosauriforms are archosaurs.

I forgot to give a reference that Will requested:

Nesbitt, S.J. 2003. Arizonasaurus and its implications for archosaur divergence. Proceedings of the Royal Society of London, Biological Sciences 270 (Suppl. 2):S234-S237. DOI: 10.1098/rsbl.2003.0066

Metalraptor said...

Some of the proterosuchids were Late Permian in age. I think Ward mentions a Late Permian archosaurimorph in his book Gorgon, which might be a proterosuchid too.