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Cynodont lower jaw, variation in musculoskeletal configuration (free pdf)



Ben Creisler
bcreisler@gmail.com

A new paper with  free pdf:

D. A. Reed, J. Iriarte-Diaz and T. G. H. Diekwisch (2015)
A three dimensional free body analysis describing variation in the
musculoskeletal configuration of the cynodont lower jaw.
Evolution & Development 18(1): 41–53
DOI: 10.1111/ede.12171
http://onlinelibrary.wiley.com/doi/10.1111/ede.12171/abstract
http://onlinelibrary.wiley.com/doi/10.1111/ede.12171/epdf



The evolution of the middle ear from the cynodont craniomandibular
bones is one of the key mammalian innovations, and the mechanics
underlying this anatomical transformation represents an intriguing
paradox. Because the jaw joint of nonmammalian cynodonts was
functionally coupled to the inner ear, auditory performance would
favor low joint reaction forces. However, this could not be achieved
at the expense of feeding performance, favoring high bite forces. The
balance of these two seemingly incompatible performance criteria in
the context of the morphological diversity of the cynodont lower jaw
is poorly understood. Here we derive a series of equations using three
dimensional free body analysis that describe the relationship between
the orientation and position of the jaw elevator muscles, the position
of the jaw articulation relative to the bite point, the joint reaction
forces and the bite force in the lower jaw of the nonmammalian
cynodont Probainognathus. These equations permit the effects of
variation in each variable to be tested independently, yielding three
terms that act to limit joint reaction forces without substantially
impacting bite force: the reorientation of the resultant muscle force
more vertically, shifting the position of the bite point medial to the
jaw articulation, and elevating the jaw articulation above the level
with the tooth row only when the muscles are oriented principally in
the anterior direction. The predictions from our equations provide
insights for functional interpretations of patterns of morphological
diversity in the cynodont fossil record. They also illustrate that the
musculoskeletal configuration of the cynodont lower jaw can be
evolutionarily labile without negatively impacting the dual
performance criteria of the auditory and feeding system.

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