American mud and musk turtles
Kinosternidae is a New World family of small to medium-sized semi-aquatic mud or musk turtles ranging from Canada to South America. These turtles apparently evolved in the Americas, as their fossils have been found only there. Their center of diversification is Mexico, but the oldest known fossil kinosternid is Xenochelys formosa from Chadronian Oligocene deposits in South Dakota. This species is closely related to the Recent genus Staurotypus, but also shares some characters with the Dermatemydidae. Of the modern genera, the oldest fossils are Kinosternon flavescens and K. subrubrum from the Pliocene (Hemphilliand) and Sternotherus odoratus from the Pliocene (Blancan) deposits (Ernst et al., 1994). Today 4 genera and 23 species are recognized.
The elongated skull of this group has the temporal region only moderately emarginated. Frontal bones are reduced and do not enter the orbits. There is maxilla-quadratojugal contact, and the triturating surface lacks a ridge. The squamosal is separated from the postorbital, and the quadrate bone does not enclose the stapes. A secondary palate is present. The carapace may be flattened or slightly domed with one to three longitudinal keels. The nuchal bone has riblike lateral processes which extend below the marginals. There are 10 pairs of peripheral bones and 23 marginal scutes (including the cervical). Neural bones range from five to seven. The 10th dorsal vertebra lacks ribs, and there is only one biconvex cervical vertebra; caudal vertebrae are procoelous. The plastron is variable in shape: small and cross shaped in some, large and well-developed in others. One or two hinges may be present on the plastron, and the number of plastral scutes varies from 7 or 8 to 10 or 11. An entoplastron is not present in all species. Limbs are developed for bottom crawling, but always have some toe webbing; the phalanges have condyles. Musk glands, associated with the bridge, exude malodorous secretions when the turtle is disturbed. The family is divided into two subfamilies; Staurotypinae and Kinosterninae.
Hutchison and Bramble (1981) analyzed the scutes of fossil and living specimens of the Kinosternidae and related Dermatemydidae, and compared these scutes to those of testudinoid turtles. They found that the gular scute of kinosternids is really a new scale of different origin, which they term an intergular. They also found that the humeral scute of extant kinosternids is divided into two scutes by the development of the anterior plastral hinge between the epiplastron and hyoplastron; thus, kinosternids are unique in not having true pectoral or abdominal scutes.