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:
- 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.
- 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.
- 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.
- 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.
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.