AN UNUSUAL PATTERN OF DIVERGENCE BETWEEN 2 FOSSIL GASTROPODS - ECOPHENOTYPE, DIMORPHISM, OR HYBRIDIZATION

The temporal dimension of fossil sequences provides a critical component to the study of intraspecific dynamics and species formation. Here I report on the branching and subsequent morphological evolution of two gastropods (Melanopsis fossilis and M. vindobonensis) from the Late Miocene of the Pannonian basin in eastern and central Europe. Although morphological divergence between species is rapid, intermediates between the two species co-occur with typical individuals for approximately 1 m.y. and then disappear. The long-term persistence of intermediates followed by their ultimate disappearance is a pattern that, to my knowledge, has not been previously observed. Distinguishing genetic from ecophenotypic influences on shell form in freshwater prosobranchs is difficult. Nevertheless, consideration of the temporal, geographic, lithologic, and paleoecologic patterns of this sequence suggests that the morphologic differences between M. fossilis and M. vindobonensis had some genetic basis. Whether these forms were initially morphs of a single species or two species with some hybridization between them is impossible to determine. In either case, the morphological changes that resulted in M. vindobonensis were rapid, but the attainment of complete isolation between M. fossilis and M. vindobonensis apparently did not occur until approximately 1 m.y. later.