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

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.

Virtual reconstruction of the endocranial anatomy of the early Jurassic marine crocodylomorphPelagosaurus typus(Thalattosuchia)

“…The 3D morphology of the paratympanic sinuses has been described in Alligator mississippiensis (Witmer & Ridgely, 2008; Dufeau & Witmer, 2015) and Crocodylus johnstoni (Witmer et al, 2008); however, little is known about the system outside extant crocodylians (but see Bona, Degrange & Fernández, 2013). There are a number of differences between the paratympanic sinuses of Pelagosaurus typus (Fig.…”

“…4; dark purple) and other extant crocodylians (Witmer & Ridgely, 2008; Witmer et al, 2008; Dufeau & Witmer, 2015). Although Gavialis has a dorsal branch of the paratympanic sinus (Figs.…”

Major evolutionary transitions and innovations: the tympanic middle ear

“…In crocodilians, there is a complex network of airspaces that actually connect the ears on either side of the head by means of a bony canal that runs over the brain case [55]. A similar connection between the two ears occurs in birds.…”

Coupled ears in lizards and crocodilians

“…However, several bird-like dinosaurs from the late Cretaceous also demonstrate ventral and dorsal intertympanic pathways, with varying degrees of trabeculation (Starck 1994; Clark et al 2002; Kundrát and Janáček 2007). Cranial sinuses lighten the skull in addition to coupling the ears, and skull pneumaticity appears to have evolved multiple times among the archosaurs (Dufeau 2011; Dufeau and Witmer 2015), although bird skulls have changed more over evolutionary time. A recent series of papers from the Avian Phylogenomics Group has characterized the genomes of extant birds, and the American alligator, saltwater crocodile, and Indian gharial (Zhang et al 2014).…”

“…CT-based material has revealed the extent of the paratympanic sinuses that couple the middle ears (Witmer and Ridgely 2009; Dufeau 2011; Bierman et al 2014; Dufeau and Witmer 2015). In birds, the ventral interaural pathway, or interaural canal, is most prominent (Calford and Piddington 1988), since dorsal connections are via trabeculated bone (Larsen et al 1992).…”