Chondrocranial Morphology of the Salmon Shark and the Porbeagle

Adult chondrocranial morphology is poorly documented in modern lamniform sharks. Here it is described for the two extant species of Lamna, L. nasus and L. ditropis. Several features are noted and discussed, including hypercalcification of the rostral cartilages, the ethmoidal region associated with ligamentous jaw attachments, and the configuration of the postorbital region. The small polygonal blocks of calcified cartilage (tesserae) that typically line the chondrichthyan endoskeleton are greatly thickened in the rostral bars of Lamna, forming radial columns; furthermore, concentric banding of the columnar calcification in the rostrum of L. ditropis suggests periodic accretion of new calcified tissue below the perichondrium.

The postorbital region in Lamna is notable for lacking a postorbital process, and in having the supraorbital shelf continuing posteriorly as a supra-otical shelf, which extends as far as the hyomandibular fossa. The orbit in Lamna is apparently not drained in the usual elasmobranch manner (i.e., via a lateral head vein, located lateral and dorsal to the hyomandibula). Instead, the vein passes ventral and medial to the hyomandibula, via a large suborbital fenestra which also houses the orbital artery. This arrangement may be phylogenetically informative because it also occurs in other lamnids, although its wider distribution among lamniforms has yet to be determined.

Copeia: September 2013, Vol. 2013, No. 3, pp. 378-389.

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