Morphology of the Braincase in the Cretaceous Hybodont Shark Tribodus limae (Chondrichthyes: Elasmobranchii), Based on CT Scanning

Abstract

The braincase of the Lower Cretaceous hybodont shark Tribodus limae is examined using highresolution CT scanning, and its internal and external morphology is described based on threedimensional digital reconstructions. This study represents the first in-depth examination of a hybodont braincase using CT scanning and digital imaging technology. The braincase of an additional Lower Cretaceous hybodont, Egertonodus basanus, is also digitally reconstructed and compared to Tribodus. A reconstruction of cranial nerves and blood vessels in Tribodus is presented on the basis of preserved foramina. The braincase of Tribodus shares many features with those of Egertonodus and neoselachians, providing further support for the sister-group relationship between hybodonts and extant elasmobranchs. CT scans confirm that in both Tribodus and Egertonodus the glossopharyngeal and vagus canals converge and exit from a common foramen (also found in Chlamydoselachus). In both of the hybodonts examined, the trochlear nerve exits the braincase anterior to the optic nerve, a possible hybodont synapomorphy. Separate foramina for the two rami of the octaval nerve are present in both Tribodus and Egertonodus, and may represent an additional hybodont synapomorphy. Also, both taxa have three separate foramina for the trigeminal, facial, and anterodorsal lateral line nerves, apparently including an individual foramen for the superficial ophthalmic complex. However, the basicranial arterial system in Tribodus differs considerably from that of Egertonodus in that in the former the internal carotid arteries enter the braincase much farther posteriorly and through paired foramina rather than a single median foramen. The median ventral basicranial process in Tribodus is similar in structure and position to median ventral processes seen in some extant neoselachians (e.g., Etmopterus) and in embryonic Torpedo (although in the latter this structure disappears during development), and thus may have had a similar ontogenetic origin.

The buccohypophyseal canal is an ancestral vertebrate trait maintained by modulation in sonic hedgehog signaling

“…In neoselachians and holocephalans the dorsal and lateral dorsal aortas are free below the neurocranium, whereas in some basal chondrichthyans the dorsal aorta is enclosed in the basicranial canal (for example, Cobelodus [39]) and in all basal chondrichthyans the lateral dorsal aortas run into the basicranial canals (for example, Cladodoides [37]). In addition, in neoselachians the posterior part of the basicranial arterial system forms a bell-shaped circuit due to the laterally curving position of the lateral dorsal aortas posteriorly and the V-shaped disposition of the internal carotid arteries anteriorly [79], whereas in basal chondrichthyans the basicranial circuit is narrower, more elongated and lacks the posterior bell-shaped curve. In recent holocephalans, as well as in some primitive holocephalans (for example, Iniopera [60]), the internal carotid arteries abort during ontogeny [5].…”

Modern Imaging Techniques as a Window to Prehistoric Auditory Worlds

“…X-ray CT has, however, been used to investigate the skulls of several Paleozoic and Mesozoic chondrichthyans (Pradel et al, 2009;Lane, 2010;Maisey & Lane, 2010;Fig. 3a), but investigations of the inner ear have often centered on morphological description, with an emphasis on resolving the systematics of the clades concerned.…”

“…They found that phonoreception-related characters are present in Mesozoic . ca, ampulla for the caudal semicircular canal; ca, cochlear aqueduct; cc, cochlear canal; cscc, caudal semicircular canal; ds, dorsal sinus; fr, fenestra rotunda; fv, fenestra vestibuli; la, ampulla for the lateral semicircular canal; lscc, lateral semicircular canal; ra, ampulla for the rostral semicircular canal; rscc, rostral semicircular canal; sac, saccule; st, stapes; u, utricle hybodont sharks [Tribodus limae and Egertonodus basanus; see Lane (2010) for detailed description], but are absent from osteichthyans, living and extinct holocephalans, and some Paleozoic chondrichthyans, such as the Early Devonian Pucapampella (Maisey & Anderson, 2001). From this evidence Maisey and Lane (2010) concluded that the labyrinth of living elasmobranchs is highly specialized for low frequency semidirectional sound detection, and that the adaptation must have arisen only once in chondrichthyans some time after the split between elasmobranchs and chimeroids.…”

A reconstruction of the palaeoecology and environmental dynamics of the Bahariya Formation of Egypt

“…While dismissed as a nomen dubium by some (Rees & Underwood, 2002), Werner (1990), Dutheil (1999), Lane (2010) & Cavin et al (2010 consider A. aegyptiacus valid and this opinion is accepted herein. Squatina sp (Dumeril, 1806;Slaughter & Thurmond, 1974;Underwood & Mitchel, 1999) is also known from Bahariya.…”