Turtles in the Kinosternidae are well-known to have unique shell shapes and shell constructions, including hinged plastrons in many species. Members of this group also live in a wide range of habitats. Despite this variability, and the likely importance of shell shape in determining fitness across this clade's range, no studies have comprehensively surveyed the variation in shell shape in this clade, nor the allometric or ecomorphological patterns possible among these species. This study used geometric morphometric methods to quantify shape in the shells of 25 of the 26 species in Kinosternidae. We assessed carapace strength using finite element modeling and calculated several morphometric measures that are correlated with the ability of turtles to right themselves when overturned. These morphological and functional data were used to describe patterns of morphological and functional evolution, and to test hypotheses about the ecological or functional morphology within this group. No patterns in ecomorphology were identified. A relationship between posterior plastral kinesis and shell morphology and strength was found, however, indicating that different mechanisms of defense exert a stronger influence on shell shape than environmental pressures. Such mechanisms are likely important for turtles that cannot rely solely on shell strength for defense as a result of their small size. In addition, morphological proxies for righting performance indicate that kinosternids are unexceptional among turtles regarding this function. This is the first large-scale study of functional and ecological morphology in this clade, but much opportunity remains for discovering finer scale patterns of variation and unique functional characteristics associated with the unique shell shapes of this group.