The Subfamily Staurotypinae
The turtles in the subfamily Staurotypinae are all moderately large musk turtles. All three turtles have three keels running the length of the carapace. Their plastron is small and narrow, cruciform
in shape, with seven or eight scutes. Some researchers regard the Staurotypinae as distinct enough from the Kinosterninae to warrant elevation to full family status, as the family Staurotypidae (Bickham et al., 1983). They base this elevation on chromosomal
variations, but a number of other studies on morphological variation within the Kinosternidae support the current system.
The genus Claudius contains only one turtle, Claudius angustatus. The plastron
of the Narrow-bridged Musk Turtle has seven bones, unlike any other turtle. Another unique feature is the presence of two cusps on the turtle's upper jaw.
Two species, Staurotypus salvinii and Staurotypus
triporcatus, make up the genus Staurotypus. These turtles are the largest in the family; S. salvinii (the Pacific Coast Giant Musk Turtle) can reach a length of 25 cm, while S. triporcatus (the Mexican
Giant Musk Turtle) grows to almost 40 cm. They can be distinguished from any other member of the Kinosternidae by their size.
The Subfamily Kinosterninae
The subfamily Kinosterninae is a much larger and more
varied group than the Staurotypinae. Turtles in the Kinosterninae subfamily can be distinguished from Staurotypinae chelonians by the number of plastral scutes. Turtles in the Kinosterninae have ten or eleven scutes compared to the seven or eight in Staurotypinae.
Another diagnostic feature of the subfamily is the absence of the entoplastral bone present in the Staurotypinae, although this characteristic is less useful when comparing living turtles.
Two genera are present in the subfamily, the
genus Kinosternon (mud turtles), with at least fifteen species and a number of subspecies, and the genus Sternotherus (musk turtles), with four species, one of which has two subspecies. Musk turtles are native to the United
States, with one species (S. odoratus) ranging up into Canada. Mud turtles are more widely dispersed, with species found from Connecticut (K. subrubrum) south through the southern and central portions of the United States, through Mexico
and Central America, and entering South America as far as northern Argentina (K. scorpioides).
In recent years there has been a large amount of scientific debate over the degree of separation within the Kinosterninae. Some
researchers believe that the two genera should be combined into one genus, Kinosternon. Others favor maintaining the split. The most recent controversy arose in 1986 when Seidel et al. published a report on a detailed comparison of the variation
in thirteen protein complexes in 18 species within the family Kinosternidae. The variation among the thirteen proteins was not great, and analysis of the data supported many of the conclusions of other researchers based on morphological data (such as Bramble
et al., 1984) and karyotyping (Sites et al., 1979). However, based on their data, Seidel et al. argued that two members of Kinosternon, K. baurii and K. subrubrum, were more similar to members of Sternotherus than
to species of Kinosternon located in Central and South America. Sites et al. had noted a similar result based solely on karyotypes of the various species, but did not suggest any revisions in taxonomy. Based on this relationship, Seidel
et al. relegated Sternotherus to subgeneric status (i.e. Sternotherus odoratus becomes Kinosternon odoratum, etc.). A few years later, John Iverson published a work using a very large data set of variable
morphological features (Iverson, 1991) to resolve phylogenies within Kinosternon and the possibly suspect Sternotherus. As he could find no unique character diagnostic for Sternotherus turtles, Iverson followed
the taxonomy suggested by Seidel et al. Some authors also adopted the revised Kinosternon genus; others have remained unconvinced. In Ernst and Barbour's 1989 Turtles of the World, they included Sternotherus in Kinosternon.
However, with the publication of the 1994 volume Turtles of the United States and Canada, Ernst, Lovich and Barbour question the split, wondering "why not relegate K. baurii and K. subrubrum to the genus Sternotherus rather
than the four Sternotherus species to Kinosternon?" (pg 138). Obviously, the jury is still out on the status of Sternotherus.