Thursday, January 8, 2009

The Death of the Kinetic Dinosaur Skull?

Cranial kinesis, in which moveable joints within the skull allow flexion and expansion during feeding, is well-documented in birds, snakes, many lizards, bony fish, and sharks. And of course, many readers of this blog know that lots of dinosaurs had cranial kinesis too. It allowed Tyrannosaurus to munch on big prey items and Edmontosaurus to chew its way through the Cretaceous landscape (see here for a nifty animation of how this should occur). Right?

Maybe not, after all. Casey Holliday and Larry Witmer just published their paper critically evaluating the evidence, "Cranial kinesis in dinosaurs: intracranial joints, protractor muscles, and their significance for cranial evolution in diapsids," in Journal of Vertebrate Paleontology. Irrefutable demonstration of cranial kinesis requires observation of living animals--and we just can't do that in dinosaurs. So, paleontologists look at the bony evidence in order to document skull function in non-avian dinosaurs (from here on out I'll just call them "dinosaurs"--yes, yes, I know that birds are dinosaurs too, but I don't want to weigh this post down too much).

Humans and other mammals fuse up their skulls pretty tightly--any open sutures between bones are so tightly interlocked that movement is effectively nil. Many other animals leave open sutures (often in the form of ball-and-socket joints)--and this is where kinesis takes place. Paleontologists have documented these open joints in dinosaur skulls, and used this as evidence of kinesis during feeding.

But, the story isn't quite that simple. Open joints are necessary for kinesis--but they aren't sufficient. And this is the core of Holliday and Witmer's argument. They lay out four criteria, present in modern tetrapods with cranial kinesis:
  1. A synovial basal joint. A synovial joint is basically a joint between bones with a fluid-filled cavity between them - a good example might be your jaw joint. The basal joint is between the pterygoid (a bone of the palate) and the basipterygoid process of the braincase. Pretty much all dinosaurs have this--and so do many modern reptiles and birds with both kinetic and akinetic skulls.
  2. A synovial otic joint. The otic joint is between the squamosal (a bone of the skull roof) and the quadrate (a bone of the jaw joint). Again, pretty much all dinosaurs, and many kinetic and akinetic modern animals, have a potentially mobile otic joint.
  3. Protractor musculature. These muscles attach to the bones of the basal and otic joints (and you need to move those bones for kinesis), but the muscles are present even in modern taxa with akinetic skulls (e.g., the tuatara). These protractors apparently varied in size across dinosaurs.
  4. Kinetically permissive linkages. This is a fancy way of saying that the skull is set up to allow movement (aside from the otic or basal joints). In modern animals, this takes the form of thin and flexible bones (as in the snout of some birds), missing bones (as in the loss of stabilizing cheek bones in lizards), and the addition of extra synovial joints, among other things. All modern animals with kinetic skulls have these--and dinosaurs lack them, in Holliday and Witmer's view.
All modern animals with kinetic skulls do fulfill all four criteria. Many dinosaurs possess two or three of these criteria, but none possess the whole shebang. Even the dinosaurs with the most supposed kinesis--such as T. rex--only fill criteria 1-3. Many of the supposed "extra" open joints in the skulls of animals such as Tyrannosaurus are of the sorts seen in modern animals with akinetic skulls. No major amounts of movement probably occurred here in T. rex, then. So, the conclusion is that dinosaurs as a whole just weren't capable of cranial kinesis.

At this point, I also want to mention the fact that a few other folks have started to question the kinetic skull idea, particularly for hadrosaurs. Robin Cuthbertson, Natalia Rybczynski, and others have all recently discussed this at SVP and in other venues. For a recent publication by these folks (including Holliday as a co-author), see their paper at Palaeontologia Electronica.

So why leave open sutures? Allowance of cranial growth probably played an important role. Maybe open sutures were helpful for absorbing shocks to the skull during feeding. This paper opens up a lot of interesting questions--and many of the answers will be found only with further study of modern animals.

Casey M. Holliday, Lawrence M. Witmer (2008). Cranial Kinesis in Dinosaurs: Intracranial Joints, Protractor Muscles, and Their Significance for Cranial Evolution and Function in Diapsids Journal of Vertebrate Paleontology, 28 (4), 1073-1088 DOI: 10.1671/0272-4634-28.4.1073

Casey has made the paper available as a PDF on his web page, along with high resolution versions of some of the figures.


Anonymous said...

So, dinosaurs, such as T. rex, don't truly have kinetic skulls? Very interesting. Apparently, as in the case of Prof. Witmer, CT scanning of fossils has proven to be a VERY valuable tool or Paleontologists.

Andy said...

My impression in reading the paper is that the bulk of the observations were in fact made from first-hand observations on physical specimens, supplemented by CT (Casey, care to chime in?). CT scans did serve to illustrate many of the specimens, however (and are good at getting a peek in articulated or annoyingly complete specimens). The most important thing here is not the CT, but careful anatomical observations.

Anonymous said...

Hi, All of the fossil specimens featured as figures in the paper we're obtained on loan and studied 1st hand. In many cases, identification or illustration could have been made without CT, these were highlighted by the photographs in one of the early figures. However CT really facilitated the ability to cut up the joints in many different ways so we could get a full 3D perspective on the joint morphologies, hence the sweet oblique horizontal section through the Carnegie Diplodocus skull. In some cases, such as the ankylosaur material, one really can't see the basal joints without CT. So the two methods, visual and virtual inspection are generally both necessary and often complementary.

Andy said...

Thanks, Casey!

Zach said...

Color me intrigued! I love it when old dogma comes down in favor of actual research.

Anonymous said...

Larry Witmer does a lot of work using CT Scanning. What kind of work do you do? Also, What would be the use of having your skull expand while chewing or eating.

Andy said...

I use CT scanning as part of my research toolkit, also - most of my data collection has focused on bovid mammals, with some occasional scanning of horned dinosaurs to supplement that (looking at the sinuses). In my lab, we're also working on a few other CT-related projects, drawing from the collections at my museum.

In terms of uses for skull expansion during chewing and eating, there are several hypotheses. And to be clear (Casey, again, is more of an expert on this than I am), it's not "expansion" so much as "movement." In lizards, it's suggested that the cranial kinesis might improve the mechanical advantage of some muscles, or allow more efficient chewing, or any other number of features. Keith Metzger had a nice review article on this whole topic a few years back, evaluating many suggested functional hypotheses for kinesis in lizards. Similar work has also been published for birds (and I found a few worthwhile refs in searching for "cranial kinesis" on Google).

Good comments all around! Thanks for chiming in.