PHYLOGENETIC DISTINCTIVENESS OF A THREATENED AQUATIC TURTLE (STERNOTHERUS-DEPRESSUS)

The musk turtle (Sternotherus minor) is common throughout the southeas tern United States. In 1955 a morphologically atypical form confined t o the Black Warrior River System in Alabama was discovered and accorde d full species status as S. depressus, the ''flattened musk turtle.'' Questions remain about the taxonomic status and evolutionary history o f the flattened musk turtle because (1) the geographic distribution of S. depressus is nested within the range of S. minor; (2) the flattene d shell might be a recently evolved anti-predator adaptation; and (3) reports exist of intergrades between S. depressus and S. minor. We gen erated and employed sequence data from mitochondrial DNA to address th e phylogenetic distinctiveness and phylogeographic position of S. depr essus relative to all other musk and mud turtles (Kinosternidae) in No rth America. The flattened musk turtle forms a well-supported monophyl etic group separate from S. minor. Genetic divergence observed between S. depressus and geographic populations of S. minor is no less than t hat distinguishing many kinosternid congeners from one another. These molecular genetic findings bolster rationale for the taxonomic recogni tion of S. depressus and, hence, for special efforts to protect this f ederally threatened species.