Computed tomography analysis of the cranium of Champsosaurus lindoei and implications for the choristoderan neomorphic ossification

Abstract

Choristoderes are extinct neodiapsid reptiles that are well known for their unusual cranial anatomy, possessing an elongated snout and expanded temporal arches. Although choristodere skulls are well described externally, their internal anatomy remains unknown. An internal description was needed to shed light on peculiarities of the choristodere skull, such as paired gaps on the ventral surface of the skull that may pertain to the fenestra ovalis, and a putative neomorphic ossification in the lateral wall of the braincase. Our goals were: (i) to describe the cranial elements of Champsosaurus lindoei in three dimensions; (ii) to describe paired gaps on the ventral surface of the skull to determine if these are indeed the fenestrae ovales; (iii) to illustrate the morphology of the putative neomorphic bone; and (iv) to consider the possible developmental and functional origins of the neomorph. We examined the cranial anatomy of the choristodere Champsosaurus lindoei (CMN 8920) using high‐resolution micro‐computed tomography scanning. We found that the paired gaps on the ventral surface of the skull do pertain to the fenestrae ovales, an unusual arrangement that may be convergent with some plesiosaurs, some aistopods, and some urodeles. The implications of this morphology in Champsosaurus are unknown and will be the subject of future work. We found that the neomorphic bone is a distinct ossification, but is not part of the wall of the brain cavity or the auditory capsule. Variation in the developmental pathways of cranial bones in living amniotes was surveyed to determine how the neomorphic bone may have developed. We found that the chondrocranium and splanchnocranium show little to no variation across amniotes, and the neomorphic bone is therefore most likely to have developed from the dermatocranium; however, the stapes is a pre‐existing cranial element that is undescribed in choristoderes and may be homologous with the neomorphic bone. If the neomorphic bone is not homologous with the stapes, the neomorph likely developed from the dermatocranium through incomplete fusion of ossification centres from a pre‐existing bone, most likely the parietal. Based on the apparent morphology of the neomorph in Coeruleodraco, the neomorph was probably too small to play a significant structural role in the skull of early choristoderes and it may have arisen through non‐adaptive means. In neochoristoderes, such as Champsosaurus, the neomorph was likely recruited to support the expanded temporal arches.

The internal cranial anatomy of Champsosaurus (Choristodera: Champsosauridae): Implications for neurosensory function

“…Supporting the hypothesis that Champsosaurus were atympanic is the inferred structural role of the stapes in Champsosaurus, where the stapes (likely homologous with the choristoderan neomorphic bone 56 ) is well sutured to the surrounding bones (primarily the quadrate, prootic, and opisthotic) and could not have articulated with a tympanum. In modern atympanic taxa, such as Sphenodon and Serpentes, the stapes articulates with the quadrate, and sound detection is limited to low frequency vibrations 85,86 that are better able to be conducted through the skull and transmit to the inner ear 86 .…”

“…A canal originates at the posterior wall of the otic capsule and extends posteriorly through the opisthotic. We previously interpreted this as the passage for the glossopharyngeal nerve 25,56 (CN IX). More likely, this canal carried the perilymphatic duct ( Fig.…”

A neomorphic ossification connecting the braincase, squamosal, and quadrate in choristoderan reptiles: insights from µCT data

“…2b). Unlike in Champsosaurus lindoei (Dudgeon et al 2020: fig. 1), the pterygoid of Coeruleodraco jurassicus has a short posterior process that does not extend beyond the parietal table.…”

“…1), the pterygoid of Coeruleodraco jurassicus has a short posterior process that does not extend beyond the parietal table. Ventrally, the neomorph has a distinct projection, but it is unclear whether this ventral process (or the ventral margin) contacts the opisthotic as in C. lindoei (Dudgeon et al 2020: fig. 1).…”

“…Based on another C. lindoei specimen (RTMP 87.36.41), Gao and Fox (1998) identified a large and elongated neomorph forming the pterygoquadrate foramen with the quadrate. Contrary to Gao and Fox (1998), a recent study of C. lindoei (CMN 8920) showed the pterygoquadrate foramen was enclosed within the neomorph (Dudgeon et al 2020). In C. gigas, the neomorph was suggested as a part of the parietal (Erickson 1972).…”