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Alligator hyolingual apparatus and implications for feeding in extinct archosaurs

Ben Creisler

A new online paper:

Zhiheng Li and Julia A. Clarke (2015)
New insight into the anatomy of the hyolingual apparatus of Alligator
mississippiensis and implications for reconstructing feeding in
extinct archosaurs
Journal of Anatomy (advance online publication)
DOI: 10.1111/joa.12320

Anatomical studies of the cranium of crocodilians motivated by an
interest in its function in feeding largely focused on bite force, the
jaw apparatus and associated muscles innervated by the trigeminal
nerve. However, the ossified and cartilaginous elements of the hyoid
and the associated hyolingual muscles, innervated by the facial,
hypoglossal and glossopharyngeal nerves, received much less attention.
Crocodilians are known to retain what are ancestrally the ‘Rhythmic
Hyobranchial Behaviors’ such as buccal oscillation, but show
diminished freedom and movement for the hyobranchial apparatus and the
tongue in food transport and manipulation. Feeding among crocodilians,
generally on larger prey items than other reptilian outgroups,
involves passive transport of the food within the mouth. The tongue in
extant crocodilians is firmly attached to the buccal floor and shows
little movement during feeding. Here, we present a detailed anatomical
description of the myology of the hyolingual apparatus of Alligator
mississippiensis, utilizing contrast-enhanced micro-computed
tomography and dissection. We construct the first three-dimensional
(3D) description of hyolingual myology in Alligator mississippiensis
and discuss the detailed implications of these data for our
understanding of hyolingual muscle homology across Reptilia. These
anatomical data and an evaluation of the fossil record of hyoid
structures also shed light on the evolution of feeding in Reptilia.
Simplification of the hyoid occurs early in the evolution of
archosaurs. A hyoid with only one pair of ceratobranchials and a
weakly ossified or cartilaginous midline basihyal is ancestral to
Archosauriformes. The comparison with non-archosaurian reptilian
outgroup demonstrates that loss of the second set of ceratobranchials
as well as reduced ossification in basihyal occurred prior to the
origin of crown-clade archosaurs, crocodilians and birds. Early
modification in feeding ecology appears to characterize the early
evolution of the clade. Hyoid simplification has been linked to
ingestion of large prey items, and this shift in hyoid-related feeding
ecology may occur in early archosauriform evolution. A second
transformation in hyoid morphology occurs within the crocodilian stem
lineage after the split from birds. In Crocodyliformes, deflections in
the ceratobrachials become more pronounced. The morphology of the
hyoid in Archosauriformes indicates that aspects of the hyolingual
apparatus in extant crocodilians are derived, including a strong
deflection near the midpoint of the ceratobranchials, and their
condition should not be treated as ancestral for Archosauria.