Ornithischia is a large and incredibly diverse clade including a majority of the megaherbivorous dinosaur subclades, except sauropodomorphs and few non-avian theropods. It included the horned ceratopsian dinosaurs, like Triceratops, the dome-headed pachycephalosaurs, the armored dinosaurs, like Ankylosaurus, the plated and spiked dinosaurs, like Stegosaurus, and the duck-billed hadrosaurs and their relatives. For well over a century, researchers have explored various aspects of the feeding apparatus of this immensely diverse clade, with studies examining osteological correlates of feeding motions, joint morphology and its impact on type and range of motion as well as jaw muscle reconstruction and mechanical advantage.
In my new study, “Evolutionary Trends in the Jaw Adductor Mechanics of Ornithischian Dinosaurs”, published online in the journal Anatomical Record, I present a broad scale analysis of jaw mechanics throughout 52 genera of ornithischian dinosaurs (spanning all subclades) to explore evolutionary convergences and divergences in mechanical advantage of the jaws throughout different points along the tooth row. Using 2D lever arm mechanics to do so, this study sheds light on variations in tooth row length, jaw length, and angle and mechanical advantage of the main jaw adductor musculature. It also investigates the effect of having a coronoid eminence or process and a jaw joint that is lower than the tooth row, both of which are features common in a majority of ornithischians.
Although the mechanical advantage is in the group containing stegosaurs and ankylosaurs were found to be much lower relative to the most other ornithischians, it is also worth mentioning how diverse their the mechanics of their jaws are amongst each other as well. All of these difference shed light on various aspects of variations in jaw mechanics throughout the clade. The most exciting finding throughout all of this study, though, is the convergent evolution of a significantly high increase in mechanical advantage of the jaws in the large ceratopsian dinosaurs like Triceratops and all of the duck-billed hadrosaurs (derived ornithopods), both of which exhibit large dental batteries with hundreds of teeth. The basal members of each clade are much lower in mechanical advantage, with both basal ceratopsians and basal ornithopods having similar mechanical advantage results throughout the tooth row. However, each group of derived members of the clade is significantly higher than their basal members, but not significantly different from the derived members of the opposite clade. This indicates clear convergence in increase mandibular mechanics in both ceratopsids and hadrosaurids, which likely is an effect of changing landscapes in the Cretaceous. Perturbation analyses also examine hypothetical jaw morphologies with no coronoid process and/or no lowering of the jaw joint, both of which are shown to impact the increase in moment arm length and, in turn, mechanical advantage.
The paper then explores how these jaw adductor mechanics in ornithischians effect various hypotheses in feeding mechanisms, providing more clues into the 140 millions year reign of these animals. Hopefully we can find out a lot more about these bizarre and amazing creatures! Hooray for ornithischians!!
Nabavizadeh, A., 2016. Evolutionary trends in the jaw adductor mechanics of ornithischian dinosaurs. The Anatomical Record. DOI: 10.1002/ar.23306