American musk turtles
The genus contains four species of North American musk turtles (Zug, 1986; Conant and Collins, 1991; Ernst et al., 1994). Musk turtles are small (to 17.6 cm) aquatic species with oblong to oval carapaces that range from flattened to high peaked. Either a medial keel or a medial keel and two lower dorsolateral keels (in juveniles) are present. The posterior carapacial rim is either smooth or only slightly serrate. The cervical scute is small, but rarely absent. Eleven pairs of marginals are present with the 10th and 11th pairs enlarged. The nuchal bone has costiform processes. The vertebrals are variable in shape and dimension, although the 1st is always long and narrow. Only 5-7 narrow neural bones are present, allowing the last two or three pairs of posterior and occasionally the 1st pair of anterior costals to meet dorsally at the carapacial midline. The 8th pair of costals are sometimes very small in S. odoratus. Normally the vertebral and pleural scutes are imbricate (overlap). The plastron is small, relatively narrow, somewhat cruciform in shape, and often has exposed skin along the scute seams. An inconspicuous and only slightly motile hinge is present between the pectoral and abdominal scutes; and the plastral hindlobe is longer and more narrow than the forelobe. Usually 11 plastral scutes are present, but only 10 in S. carinatus which lacks a gular. Pectoral scutes are usually triangle-shaped. The bridge is narrow, and bears both axillary and inguinal musk glands. The head is larger in adults than in juveniles, and in some mollusk-eating populations it may be quite enlarged. The snout projects; the upper jaw is only slightly hooked. The triturating surfaces are broad, strong and ridgeless. A partial secondary palate is present. The maxilla touches the quadratojugal, but no contact occurs between the squamosal and the postorbital. The temporal region is strongly emarginate. Barbels are present on the chin, and also on the throat of S. odoratus. All toes are webbed. Sternotherus ranges from New England and southern Ontario west to Kansas and south to Florida and Texas.
Zug (1966) found that in Sternotherus the structure of the glans penis is of two general types: a minor-depressus-odoratus type and a carinatus type, but traditionally S. minor has been considered less closely related to S. odoratus than S. carinatus. This was supported by electrophoretic studies by Seidel et al. (1981) that showed S. odoratus to be most closely related to S. carinatus; however, both the magnitude and general pattern of mtDNA phylogeography in S. odoratus are remarkably similar to those of S. minor (Walker et al., 1997).
Some turtle taxonomists would include these species in the genus Kinosternon (Seidel et al., 1986; Iverson, 1991b, 1992), but see Ernst et al. (1994) and the clades in Iverson (1992) for reasons to recognize two genera. Zug (1971b) and Smith and Smith (1980) have discussed the past use of the name Sternothaerus for the American musk turtles and have presented evidence that Sternotherus is the valid spelling.
Jump to the key: Page 385: Genus Sternotherus