The closure of the buccohypophyseal canal in chondrichthyans is associated with modifications of the anterior midline. (a) Detail of the palate in Doliodus problematicus, NBMG 10127, Lower Devonian of Canada; three-dimensional reconstruction based on X-ray microtomography, showing palatoquadrates (in purple) widely separated by the ethmoid region (in yellow, arrowhead) (after ). (b) Same specimen with teeth (red) in place; note the presence of teeth on the ethmoid keel. (c) Anterior view of the head of a hybodont shark, Egertonodus basanus (Egerton), NHM 6356, Lower Cretaceous, Wealden, Pevensey, England (after ). (d) Same specimen, with anterior tooth positions in red, and anterior parts of the palatoquadrates colored in yellow; the distance between the palatoquadrates is narrower than in Doliodus (arrowhead). (e) Anterior view of an articulated Permian shark braincase and jaws, Orthacanthus sp. (probably O. texensis), from Archer County, Texas (cast of MCZ 12872); the palatoquadrates are still separated by a narrow band of ethmoid bone (arrowhead). (f) Anterior view of an articulated extant shark braincase and jaws, Isurus oxyrinchus, showing a fusion of the two palatoquadrates (arrowhead), only separated by the palatonasal ligament.