Abstract
The braincase of the Lower Cretaceous hybodont shark Tribodus limae is examined using high-resolution CT scanning, and its internal and external morphology is described based on three-dimensional 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.
Highlights
Interrelationships of extinct chondrichthyans have only recently begun to be under¬ stood with clarity
Knowledge of fossil chondrichthyan cranial morphology, in particular, was impeded until recently by a lack of wellpreserved material as well as the means to examine the few that did exist without destroying the specimens in the process
Studies of well-preserved fossil chondrichthyan braincases not based on CT scans (e.g., Pucapampella: Maisey, 2001b; Maisey and Anderson, 2001; Orthacanthus and Tamiobatis: Schaeffer, 1981; Egertonodus basanus: Maisey, 1983; Hybodus reticulatus: Maisey, 1987; Synechodus: Maisey, 1985; Akmonistion: Coates and Sequeira, 1998) have provided additional information of value for comparative studies
Summary
Interrelationships of extinct chondrichthyans have only recently begun to be under¬ stood with clarity. Serial grinding/sectioning techniques (e.g., Stensio, 1927; Poplin and de Ricqles, 1970; Jarvik, 1980) were once the methods of choice for examining fossil shark braincases, and were still widely used until fairly recently (e.g., Schaeffer, 1981). These methods resulted in numerous slabs or slices that make it difficult to reconstruct the original three-dimensional fossil, they provided the best view of internal structures available. Studies of well-preserved fossil chondrichthyan braincases not based on CT scans (e.g., Pucapampella: Maisey, 2001b; Maisey and Anderson, 2001; Orthacanthus and Tamiobatis: Schaeffer, 1981; Egertonodus basanus: Maisey, 1983; Hybodus reticulatus: Maisey, 1987; Synechodus: Maisey, 1985; Akmonistion: Coates and Sequeira, 1998) have provided additional information of value for comparative studies
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
