Genus Staurotypus

Wagler, 1830
Giant musk turtles

Included in the genus Staurotypus are two large (to 37.9 cm) aquatic musk turtles of Mexico and northern Central America. The oval carapaces are strongly three-keeled and lack serrated marginals. There are 23 marginals, including the cervical scute. Seven neurals and ten pairs of peripheral bones are also present. The plastron is slightly hinged, and small and cruciform with a narrow, triangular posterior lobe. The anterior lobe is rounded. A large entoplastron is present. Only seven or eight scutes cover the plastron, and large axillary and inguinal scutes occur on the bridge. The head is large, with a projecting beak and only slightly hooked upper jaw. In the skull, the temporal region is moderately emarginated. The maxilla and quadratojugal touch, but the squamosal is separated from the postorbital. The quadrate does not enclose the stapes. There is a secondary palate, but no ridge is present on the triturating surface of the jaws.

Staurotypus is unique among kinosternids in that males of its two species, S. salvinii and S. triporcatus, possess sex chromosomes (Moon, 1974; Bull et al., 1974; Sites et al., 1979a). Males are heteromorphic (heterogametic, XY) while females are homomorphic (homogametic, XX). Moon (1974) and Bull et al. (1974) found that males have a single subtelocentric chromosome (X), bearing a secondary constriction in the long arm, which is the fourth largest and is homologous to one (Y) of three smaller telocentric chromosomes (Sites et al., 1979a reported the Y chromosome is acrocentric). Karyotypes of the females of both species possess two subtelocentric and two telocentric chromosomes. It is not known if the male's heteromorphic chromosomes actually influence sex determination, but they are definitely sex related since they do not occur in females. Sites et al. (1979a) conducted various staining and band-pattern determination studies on these chromosomes in S. salvinii, and concluded that, unlike previously described systems in most other vertebrates in which the Y or W is derived and the homomorphic sex represents the primitive condition, the opposite is true for S. salvinii. The X chromosome is derived, so the homomorphic female is more derived than the heteromorphic male. The male is actually intermediate between the female and the ancestral condition of other turtles. The 2n chromosome number is 54: S. salvinii has 24 macrochromosomes and 30 microchromosomes, S. triporcatus has 26 macrochromosomes and 28 microchromosomes (Moon, 1974; Killebrew, 1975b; Sites et al., 1979a).

Species identification
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