American mud and musk turtles
This subfamily contains 2 genera and 20 species. The genus Kinosternon ranges from New England to South America, while the genus Sternotherus is restricted to North America. These are small to medium-sized turtles, usually with well-developed, large plastra having 10 or 11 scutes, and no entoplastron. The carapace may lack a keel, or have one or three; five to seven neural bones are present. The pubic and ischiadic symphyses are in contact. Kinosternon and Sternotherus have temperature sex determination.
Electrophoretic data gathered by Seidel et al. (1986) indicate that the subfamily Kinosterninae is monophyletic and divergent from the subfamily Staurotypinae.
Morphological studies by Lamb (1983) and those on the karyotype by Sites et al. (1979a) and the biochemistry by Frair (1972) and Seidel et al. (1986) indicate that Kinosternon baurii and K. subrubrum of the United States are more closely related to the sympatric species of Sternotherus than to the other more tropical species of Kinosternon. It appears that the genus Kinosternon is a paraphyletic group, and that K. baurii and K. subrubrum share a more immediate ancestor with the four species of Sternotherus. Separation of Sternotherus from Kinosternon on the basis of shell hinges is dubious, in view of the studies of Bramble et al. (1984) showing that K. herrerai exhibits no more structural evidence of posterior plastral lobe movement than does Sternotherus. Seidel et al. (1986) placed Sternotherus in the synonymy of Kinosternon but this is generally not accepted today (see Ernst et al., 1994).