Chondrocranial Morphology of the Salmon Shark, Lamna ditropis, and the Porbeagle, L. nasus (Lamnidae)
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.
Lamna ditropis, USNM 404353, dried adult chondrocranium, collected on beach at Port Moller, Alaska. Anterior to left. Scale bar = 5 cm.
Lamna nasus, AMNH FF 20426, dried adult chondrocranium, provenance unknown. Anterior to left. Scale bar = 5 cm.
Lamna ditropis, USNM 404353, tomographic volume renderings of chondrocranium in anterior view, with the rostrum sliced transversely at successively more posterior levels. Note the radial structure and concentric banding of hypercalcified tissue forming the rostral node, with outer layers overlapping and extending beyond inner layers. Also note the presence of paired lumina of the lateral rostral bars within the rostral node in D and E. Scale bar = 20 mm.
Lamna ditropis, USNM 404353, rostrum sliced sagitally (A) and horizontally (B–D) at successively lower levels. Note overlapping, concentric layers and the convergence of the lumina of lateral and ventral rostral bars within the rostral node. Scale bar = 20 mm.
Lamna nasus, AMNH FF 20426, tomographic volume renderings of sliced rostrum. (A–E) Anterior view of chondrocranium, with the rostrum sliced transversely at successively more posterior levels; (F) oblique rostro-caudal slice through the ventral and left lateral rostral bars. Rostral bars remain separated along much of the rostrum and display little evidence of concentric layers within the hypercalcified tissue (cf. L. ditropis). Scale bar = 20 mm.
Tomographic segmentation of the chondrocranium endolymphatic region in Lamna nasus (A, B) and L. ditropis (C, D). A and C are dorsal views, B and D are ventral (anterior to top). Note that the median dorsal foramen is separated from the endolymphatic ducts internally by the chondrified medial wall of the otic capsule, and is located much farther anteriorly than the perilymphatic fenestrae. A posterior endolymphatic ridge surrounds the posterior and lateral sides of the perilymphatic fenestrae. Scale bar = 10 mm.
Right scleral capsule of Lamna nasus, AMNH FF 20426, in lateral (A) and medial (B) views. Anterior is to right in A, to left in B. Small rectangle in A represents area seen in C. The entire outer surface of the capsule is covered by tesserate calcified cartilage. On the inner surface, only the margins of foramina are lined by tessellated (e.g., the optic foramen, shown in C), while the remainder (covered by the tapetum and retina in life) consists of smooth, unmineralized cartilage. The external surface of the scleral capsule in L. nasus is thus mineralized in identical fashion to most of the chondral endoskeleton in elasmobranchs. The capsule is pierced by the optic foramen, plus three small vascular foramina dorsally, posteriorly, and ventrally. The optic foramen and ventral vascular foramen are connected by the ocular ridge. Scale bars: (A, B) = 3 cm; (C) = 5 mm.
Contributor Notes
Associate Editor: D. Buth.