A second species of Pyrgulopsis (Hydrobiidae) from the Missouri River basin, with molecular evidence supporting faunal origin through Pliocene stream capture across the northern continental divide

The large, predominantly western North American hydrobiid gastropod genus Pyrgulopsis includes a single species from the upper Missouri River basin (MRB), P. bedfordensis, which is thought to have originated through a late Neogene transfer of drainage from the eastern Snake River Plain across the northern continental divide. Here we describe a second, morphologically distinctive congener living in the MRB, P. blainica new species; investigate the phylogenetic relationships of these two snails rela- tive to other regional congeners using cytochrome c oxidase subunit I (COI) sequences; and evaluate whether our findings are consistent with vicariance resulting from prior stream capture across the northern continental divide. A Bayesian analysis of 39 COI sequences delineated the two MRB species as a well supported, terminal clade that is most closely related to a congener from the southern Bonneville Basin (P. anguina). Application of an available molecular clock for Pyrgulopsis suggests that the MRB clade diverged from its sister taxon 3.64 • 2.53 Ma (late Pliocene). Although these results are not consistent with the hypothesized origin of this fauna through a late Miocene drainage transfer from the upper Snake River basin, they conform to a more recent stream capture event involving truncation of southward drainage from the MRB that was previously postulated on the basis of molluscan distributions. This study provides the first molecular phylogenetic evidence that bears upon the freshwater molluscan biogeography across the northern continental divide.