MULTIPLE COMPETITIVE DISPLACEMENT EVENTS IN TWO SPECIES OF THE NEOGENE GASTROPOD PYRGULOPSIS

Two morphologically similar species of the snail genus Pyrgulopsis (P. kolobensis and P. transversa) occur in post-Bonneville stream deposits in the Salt Lake valley. Landmark-based morphometric analyses have revealed that in sedimentary horizons where only one species occurs, variations in shell morphology show no particular pattern. However, in horizons where both species occur, variations in shell morphology sort the two species out into discrete regions of morphospace. There are at least four different instances of such morphological displacement within the study area. The truly remarkable pattern in the data is that in each of the displacement events, the segregation of species in morphospace occurs along a different morphological axis.

We interpret these morphometric results, together with the tightly constrained stratigraphic placement of the study area as representing at least 4 separate encounters between P. kolobensis and P. transversa within the past 14,000 years. Each of these encounters resulted in interspecific competition and resource partitioning between the two species; however, during each encounter, ecological resources were partitioned differently between the two species.