27 - Comparative alveolar anatomy in crocodylians and mammals using micro-CT imaging and formalization of the term alveolar trough

Poster Board Number: 27
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Co-authors:

Haley O'Brien - Associate Professor, Cellular and Molecular Medicine, University of Arizona; Paul Gignac - Associate Professor, Cellular and Molecular Medicine, University of Arizona

Abstract Body :Crocodylians and mammals share thecodont tooth implantation, wherein bony sockets (alveoli) enclose tooth roots, supporting the dentition and providing the anchor for the periodontal ligament attachment. Mammalian alveoli are composed of a thin socket of remodeled Haversian bone, with cancellous bone filling the space between the jaw elements and sockets. Meanwhile, crocodylian alveolar bone type (based on yearling caimans) is identified as lamellar bundle bone with interalveolar plates isolating tooth roots. Nonetheless, crocodylians and some mammals show an apparently common "alveolar trough" phenotype during development, in which multiple roots reside in continuous sockets. However, the term has been used to describe a variety of phenotypes, which may not be consistent, leaving unaddressed whether or not these phenotypes are homologous in extant thecodonts. This knowledge gap exists, in part, because no ontogenetically complete, anatomical description of alveoli in modern crocodylians has been undertaken. Here, we used X-ray microcomputed tomography imaging to capture alveolar anatomy in two crocodylian species over ontogeny (Alligator mississippiensis, Crocodylus acutus) and compared it to a sample of four mammalian taxa (Rattus norvegicus, Odocoileus hemionus, Didelphis virginiana, and Tursiops truncatus) to assess the utility and precision of the term “alveolar trough.” We identified multiple configurations of the alveolar trough within individual crocodylians. Both Alligator and Crocodylus exhibit a “mammal-like” socket structure made of a cortical socket with trabecular infill to the dentary along the mesial curved portion of the dentary. However, more distally, the alveoli show a unique trough of thick cortical bone. This represents a departure from the thecodont phenotype established by observation of mammals alone. Contrasting this newfound variation to the context of usage for "alveolar trough" across the past eight decades, we find the term to be anatomically imprecise. We propose a basic formal definition for alveolar trough, and provide an exemplar set of modifiers to describe the disparity found in the alveolar anatomy found of extant as well as extinct thecodont clades.