A gradual morphologic transition during a rapid speciation event in marginellid gastropods (Neogene: Dominican Republic)

We document a speciation event between two species of Prunum (Marginellidae: Gastropoda) in Pliocene strata of the northern Dominican Republic. The ancestral species, P. coniforme, is widely distributed in the Mio-Pliocene of the Dominican Republic and Jamaica, and has a range of at least 11 m.y. The descendant species, P. christineladdae, is endemic to the northern Dominican Republic. The ancestral species persists after its descendant arises. The transition between species is marked by stratigraphic and morphologic intermediates, and occurs during an interval estimated to be between 73,000 and 275,000 years (representing 0.6-2.5 percent of the duration of the ancestral species). Although the transition takes more than a geologic instant, the overall pattern of morphologic change is best described by the model of punctuated equilibrium. The change between species apparently involved a habitat shift into deeper water, and occurred during accelerated deepening. All of the Neogene Dominican species of Prunum appear to have had depth-defined ranges.