There’s something I need to get off my chest. It’s been bugging me for about a year now ever since a good friend and colleague of mine, Dr. Julia Schultz, pointed it out to me while we were both postdocs at the University of Chicago and my mind kind of imploded.
“Propalinal” jaw movement is not a thing.
It’s a term that a LOT of people, including myself, have used to generally describe the jaw motions of certain herbivorous dinosaurs and other extinct critters, like dicynodonts and edaphosaurids. It describes a fore-aft motion of the jaw while the teeth are in occlusion. (Notice right off the bat that I said, “while the teeth are in occlusion”…This is important.) It is a combination of the word “proal” (which means a forward or rostral movement of the jaw while in occlusion) ad the word “palinal” (which means a backward or caudal motion of the jaw while in occlusion).
Now, these two terms, “proal” and “palinal” are completely valid. There is clear evidence for both of these.
In terms of proal motion, the main animals known to implement it are many proboscideans (yup, elephants push their food forward), rodents (in really, REALLY quick cycles), kangaroos, and the tuatara (Jones et al., 2009) — (yeah, that one surprises me, too, but it’s true!)
In terms of palinal motion, it seems, oddly enough, that the only animals that implemented PRIMARILY palinal motion are all extinct. These include multituberculates, haramyidans, and gondwanatherians (three non-therian mammal groups) (Schultz et al., 2014), dicynodonts (Angielczyk, 2004 – here termed “propalinal” but describing characters for a palinal motion), and, yes you guessed it, a LOT of herbivorous dinosaurs, primarily ornithischians (Mallon and Anderson, 2014; Varriale, 2016; Nabavizadeh and Weishampel, 2016). How do we know this? From tooth wear studies. All tooth wear studies that have been done have either shown proal or palinal motions, but never an indication of a “propalinal” motion. You always see a leading and a trailing edge of a tooth scratch, with no indication of a scratch going back and forth while in occlusion.
And if you think about it, it makes sense. In order to have a forward and backward motion of the jaw, you need more or less equal opposing muscle forces to effortlessly move the jaw forward and backward with the same amount of force. But lets look at what we really see: in rodents, we have a specialized muscle called zygomaticomandibularis that actually extends rostrodorsally toward the infraorbital foramen in many cases (especially hystricomorph rodents, like guinea pigs and capybaras) (Cox and Jeffery, 2011). This gives a clear rostrally-oriented vector for proal motion, which is what is seen in the tooth wear! In dinosaurs, most of the muscle are oriented caudodorsally (with a possible forward extension of muscle that will be described in an upcoming paper), with the exception of pterygoideus musculature, which is more for stabilization and slight translational rotations of the jaw and is not anchored rostrally enough to produce palinal motion. Pterygoideus muscles would also have been instrumental in the return vector for restarting a chewing cycle, but you don’t need nearly as much of a mechanical advantage and muscle force to do that). And guess what: their tooth wear only show palinal motion! Likewise, the musculature in dicynodonts and edaphosaurids would only produce palinal motion as well.
Why is it that the only animals that produce palinal motion are all extinct? I honestly can’t answer that confidently (although, I have some ideas), but it’s a great mystery to delve into. But the fact of the matter is, “propaliny” is just a term people (again, including myself) have used to describe a general uncertainty of forward or backward motion. And don’t get me wrong: it is totally fine in that context. But with knowledge of tooth wear now being a great resource, I suggest putting an end to this and picking which one it is — Proal or palinal? Because there is no microwear evidence to suggest “propaliny” in anything we know of (at least to my knowledge and to the knowledge of many I have talked to about this -– if you have differing info, please direct me to the source!) Until then, proal and palinal are two completely separate jaw motions. Lets treat them as such!
More about this in upcoming papers, so stay tuned!
Angielczyk, Kenneth D. “Phylogenetic evidence for and implications of a dual origin of propaliny in anomodont therapsids (Synapsida).” Paleobiology 30.2 (2004): 268-296.
Cox, Philip G., and Nathan Jeffery. “Reviewing the morphology of the jaw‐closing musculature in squirrels, rats, and guinea pigs with contrast‐enhanced microCT.” The Anatomical Record 294.6 (2011): 915-928.
Jones, Marc EH, et al. “The head and neck muscles associated with feeding in Sphenodon (Reptilia: Lepidosauria: Rhynchocephalia).” Palaeontologia Electronica 12.2 (2009).
Mallon, Jordan C., and Jason S. Anderson. “The functional and palaeoecological implications of tooth morphology and wear for the megaherbivorous dinosaurs from the Dinosaur Park Formation (upper Campanian) of Alberta, Canada.” PloS one 9.6 (2014): e98605.
Nabavizadeh, Ali, and David B. Weishampel. “The predentary bone and its significance in the evolution of feeding mechanisms in ornithischian dinosaurs.” The Anatomical Record 299.10 (2016): 1358-1388.
Schultz, Julia A., et al. “Dental function and diet of Vintana sertichi (Mammalia, Gondwanatheria) from the Late Cretaceous of Madagascar.” Journal of Vertebrate Paleontology 34.s1 (2014): 182-202.
Varriale, Frank J. “Dental microwear reveals mammal-like chewing in the neoceratopsian dinosaur Leptoceratops gracilis.” PeerJ 4 (2016): e2132.