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Evolutionary trends in jaw adductor mechanics of ornithischian dinosaurs

Ben Creisler

A new paper:

Ali Nabavizadeh (2015)
Evolutionary trends in the jaw adductor mechanics of ornithischian dinosaurs.
The Anatomical Record (advance online publication)
DOI: 10.1002/ar.23306

Jaw mechanics in ornithischian dinosaurs have been widely studied for
well over a century. Most of these studies, however, use only one or
few taxa within a given ornithischian clade as a model for feeding
mechanics across the entire clade. In this study, mandibular
mechanical advantages among 52 ornithischian genera spanning all
subclades are calculated using 2D lever arm methods. These lever arm
calculations estimate the effect of jaw shape and difference in
adductor muscle line of action on relative bite forces along the jaw.
Results show major instances of overlap between taxa in tooth
positions at which there was highest mechanical advantage. A
relatively low bite force is seen across the tooth row among
thyreophorans (e.g., stegosaurs and ankylosaurs), with variation among
taxa. A convergent transition occurs from a more evenly distributed
bite force along the jaw in basal ornithopods and basal
marginocephalians to a strong distal bite force in hadrosaurids and
ceratopsids, respectively. Accordingly, adductor muscle vector angles
show repeated trends from a mid-range caudodorsal orientation in basal
ornithischians to a decrease in vector angles indicating more caudally
oriented jaw movements in derived taxa (e.g., derived thyreophorans,
basal ornithopods, lambeosaurines, pachycephalosaurs, and derived
ceratopsids). Analyses of hypothetical jaw morphologies were also
performed, indicating that both the coronoid process and lowered jaw
joint increase moment arm length therefore increasing mechanical
advantage of the jaw apparatus. Adaptive trends in craniomandibular
anatomy show that ornithischians evolved more complex feeding
apparatuses within different clades as well as morphological
convergences between clades.