Ontogeny of the Middle-Ear Air-Sinus System in Alligator mississippiensis (Archosauria: Crocodylia).

David L Dufeau,Lawrence M Witmer,Brian Lee Beatty

doi:10.1371/journal.pone.0137060

Abstract

Modern crocodylians, including Alligator mississippiensis, have a greatly elaborated system of pneumatic sinuses invading the cranium. These sinuses invade nearly all the bones of the chondrocranium and several bony elements of the splanchnocranium, but patterns of postnatal paratympanic sinus development are poorly understood and documented. Much of crocodylomorph—indeed archosaurian—evolution is characterized by the evolution of various paratympanic air sinuses, the homologies of which are poorly understood due in large part to the fact that individual sinuses tend to become confluent in adults, obscuring underlying patterns. This study seeks to explore the ontogeny of these sinuses primarily to clarify the anatomical relations of the individual sinuses before they become confluent and thus to provide the foundation for later studies testing hypotheses of homology across extant and extinct Archosauria. Ontogeny was assessed using computed tomography in a sample of 13 specimens covering an almost 19-fold increase in head size. The paratympanic sinus system comprises two major inflations of evaginated pharyngeal epithelium: the pharyngotympanic sinus, which communicates with the pharynx via the lateral (true) Eustachian tubes and forms the cavum tympanicum proprium, and the median pharyngeal sinus, which communicates with the pharynx via the median pharyngeal tube. Each of these primary inflations gives rise to a number of secondary inflations that further invade the bones of the skull. The primary sinuses and secondary diverticula are well developed in perinatal individuals of Alligator, but during ontogeny the number and relative volumes of the secondary diverticula are reduced. In addition to describing the morphological ontogeny of this sinus system, we provide some preliminary exploratory analyses of sinus function and allometry, rejecting the hypothesis that changes in the volume of the paratympanic sinuses are simply an allometric function of braincase volume, but instead support the hypothesis that these changes may be a function of the acoustic properties of the middle ear.

Highlights

Modern crocodylians have a remarkable proportion of their skulls occupied by large recesses (Fig 1, S1 File, 3D pdf of perinatal Alligator mississippiensis)
The middle ear is composed of a pneumatically inflated epithelial sinus that is situated lateral to the bones of the braincase, medial to the bony elements of the suspensorium, and constrained elsewhere by soft tissues
Crocodyliforms and bird-line dinosaurs, secondary outgrowths of the middle ear become insinuated within the bones of the braincase and the bones of the suspensorium

Summary

Introduction

Modern crocodylians have a remarkable proportion of their skulls occupied by large recesses (Fig 1, S1 File, 3D pdf of perinatal Alligator mississippiensis) These recesses, which infiltrate every skeletal element of the braincase, have been the subject of nearly two hundred years of interpretation regarding their form, function, and evolutionary trajectory [1,2,3,4,5,6]. Owen [2] observed that the median foramen, along with two foramina situated laterally to it (the true Eustachian canals), conduct the epithelium of the pharynx into the skull, where it becomes continuous with the epithelium of the tympanic cavity The nature of these braincase recesses, being evaginated pharyngeal epithelium that pneumatizes the skull and participate in the conformation of the middle ear, is well documented [3,5, 7,8,9]

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Conclusion