And now for the specifics...

Chondrichthyan Synapomorphies

Tesserate Calcified Cartilage

Although the chondrichthyan endoskeleton is composed of cartilage and not bone, it does undergo a certain degree of mineralization. Chondrichthyans exhibit a unique mode of perichondral (= around cartilage) calcification that results in the appearance of a mineralized "honey-comb" or "prismatic" layer over the surface of the endoskeleton. This is called tesserate calcification. While calcification of cartilage is seen in other vertebrates, this prismatic pattern of mineralization observed in chondrichthyans is unique, and is not seen in any other animal group. Calcification of cartilage imparts additional strength and rigidity to the skeleton, without weighing it down excessively.

Claspers

Pectoral fin development in male chondrichthyans is modified, resulting in the formation of both a fin, and a clasper. Claspers (also called "mixopterygia") are sperm transfer organs that facilitate internal fertilization of females. Claspers extend from the base of the pelvic fin, and have both an internal skeletal support (derived from the cartilaginous radials of the pelvic fin) and a specialized musculature (to allow movement of the clasper, and to pump sperm). Claspers are unique to chondrichthyans, and are present in all extinct and extant males.



Xenacanthus showing modification of pelvic fin into a clasper (from Benton, 2005)

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  • Author: Andrew Gillis
    Last updated: 15 November 2004
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