hugs86 said:
The only characteristics that I know of the archosaurs have and other reptiles don't is just the two fenestrae and possible parental care. I haven't really been able to find much else, does anyone know what other morphologies distinguish this group?
Correction: Pneumatisized bones are found in sourischians dinosaurs, not just theropods. Sauropods had them, too.
Wedel MJ, 2009. Evidence for bird-like air sacs in Saurischian dinosaurs. J. Exp Zool. 311A:1-18.
There is a free link to the above article somewhere on the Net but I forgot the address.
http://en.wikipedia.org/wiki/Skeletal_pneumaticity
“Skeletal pneumaticity
…
Pneumatization occurs in the skulls of mammals, crocodilians and birds among extant groups, as well as extinct archosaurs including the dinosaurs and pterosaurs. Pneumatic spaces include the paranasal sinuses and some of the mastoid cells.”
Arcosaurs includes birds, crocodilians, dinosaurs and pterosaurs. Some pneumatized bones in extant arcosaurs (birds and crocodilians) have connections to the air passage ways for respiration.. Some paleontologists claim evidence that pneumatisized bones in dinosaurs also were connected to the air passages
http://www.ucmp.berkeley.edu/science/profiles/wedel_0609.php
“Inflatable dinosaurs — a pool toy from Walmart, right? It turns out that there's more than a grain of scientific truth in a T. rex floatie. T. rex's bones contained large airspaces connected to air sacs within its body cavity. In fact, more than 10% of T. rex's body volume may have been made up of "inflatable" air space. In scientific jargon, this is called pneumaticity. Biologists have long known that theropods (the dinosaur clade containing T. rex, Velociraptor, and birds — shown below) had pneumatic bones. But UCMP's Matt Wedel is pioneering studies of pneumaticity in sauropods — the clade containing the "biggest of the big": Apatosaurus, Diplodocus, and the 50 ton Sauroposeidon.”
Note that the pneumatization of bones in mammals is completely different from the pneumatization of bones in arcosaurs. In extant arcosaurs, there are air sacs in many of the pneumatisized bones connected to the lungs and air passages. Mammals don’t have air sacs connected to bones. Of course, many vertebrates have some pneumatisized bones that include bone-marrow.
With extant animals, one can determine accurately whether the pneumatisized bones have air-sacs. With extinct animals, it is harder to determine with reliability the presence of air-sacs in pneumatisized-bones. So the theory isn’t accepted by all scientists. However, I think the theory that dinosaurs and pterosaurs had air-sacs in their bones has entered “mainstream science.”
So I would list air-sacs in pneumatisized bones as a possible primitive-trait for arcosaurs. According to this theory, the first use of air-sacs in pneumatisized-bones was for respiration. The largest dinosaurs found them useful for lightning the load on their bodies. The birds find it useful for lightning their bodies for flight. Crocodilians like lightened bodies for buoyancy in the water. However, the primary advantage of air-sacs in pneumatisized-bones is in many cases still respiration.