Alveolar Trough: Formalizing an Arcus Alveolaris Analog in a Sauropsid Model

Abstract

Mammals and crocodylians share thecodont tooth implantation, wherein bony sockets (alveoli) enclose tooth roots. Mammalian alveoli are composed of a thin layer of remodeled haversian bone forming the socket, and cancellous bone filling the space between the socket and the jaw elements, together forming an arcus alveolaris. The sockets act as an anchoring attachment for the periodontal ligament and support the dentition, and recent research has shown that alveoli have a role in tooth formation. Complexity in mammal heterodonty depends, in part, on alveoli mechanically constraining the developing crown: high alveolar constraint produces more complex crowns. Previously, we have shown a significant relationship between alveolar and dental crown morphologies in Alligator mississippiensis: as each new tooth contributes tissue to the socket wall, the alveoli limit shape variation of subsequent crown generations. In yearling caimans, alveolar bone type is identified as lamellar bundle bone; however, there is to-date no complete morphological description of the alveolar anatomy of crocodylians. In this study, we describe the unique alveolar anatomy in extant crocodylians and evaluate the need for specific anatomical terminology based on deviations from the mammalian standard. From micro-computed tomography (μCT) data, we digitally reconstructed the jaws of embryonic, hatchling, juvenile, and subadult life stages of Alligator, parsing alveolar bone from other jaw elements. We compared these μCT datasets with those of model rodent species (Microtus, Rattus, Napaeozapus) and to the dental literature. Our results show alligators lack the mammalian-type arcus alveolus. Instead, they have a thick trough of lamellar bundle bone connected to adjacent jaw elements by thin osseous struts and direct fusion on buccal and lingual contacts. Notably, cancellous bone is absent. The trough is retained throughout ontogeny and is the foundation onto which interdental septa are added. Both trough and septa thicken with age as replacement-tooth size increases. Current anatomical terminology for alveoli fits the integrated mammalian jaw type well, but the crocodylian type is markedly different. The crocodylian alveolar condition is principally composed of a thickened trough of lamellar bundle bone, indicating a need for sauropsid-specific terminology to reflect this analogous configuration. The term alveolar trough is suggested as it has been used previously to describe alveolar features in fossil reptiles but lacks a formal definition, which we now erect. Alveus alveolaris (L., “alveolar trough”)—a thick channel of lamellar bundle bone in the jaws of, for example, crocodylians that provides the surface for gomphotic thecodonty and connects to jaw elements through direct fusion, but without cancellous bone. The delineation of the alveolar trough has far-reaching implications to dental form and function. Primarily, it calls attention to this apparently unique feature of crocodylian jaws, which should inform future research directions to more discretely address its development, function, and evolution. In addition, the trough—alongside interdental septa and rootedness—may help to clarify the role of tooth socket configurations as potential key innovations in tetrapod dental evolution.