MORPHOLOGICAL VARIATION AND TAXONOMIC REVISION OF LATE MIOCENE NEOGASTROPODS (AMERICOLIVA, OLIVELLA) FROM THE DOMINICAN REPUBLIC

The Neogene marine sequences of the Cibao Valley in the Dominican Republic contain well-preserved fossiliferous deposits, ideal for the study of species diversity and evolution through time. These deposits record tropical American fauna prior to and during transitions of environment and biodiversity associated with the closure of the Panama Seaway (~3.1 mya). Here we examine the occurrence and morphology of four shallow-water oliviform gastropods from Nanoplankton Zone 11 of the Late Miocene Cercado Formation. Landmark morphometric analyses show that individuals of a previously described species, Americoliva cylindrica, are morphologically distinct across three stratigraphically consecutive samples that represent a relatively narrow interval of time, suggesting response to dynamic environmental changes. The same phenomenon is exhibited by other contemporaneous oliviform gastropods, formerly grouped as a single taxon or incorrectly assigned to morphologically similar species. Qualitative and morphometric assessments of conchological characters lead to the recognition of two new species, Olivella tangae and Olivella vespanatis which, despite significant intraspecific stratigraphic variation, are nonetheless consistently distinguishable from each other. Both species are very small (~ 6mm and 8mm, respectively), as is typical of this molluscan fauna, suggesting that the Cercado Formation’s richness is likely underestimated.

Post-larval shell growth of O. vespanatis is consistently isometric, as is the case for O. tangae and A. cylindrica in two of the three beds examined. However, the latter two species exhibit significant allometry in a third bed, which also contains a substantial increase in the relative abundance of naticid gastropods. Ongoing work seeks to establish whether this might be a response to increased naticid predation of oliviform taxa, or an indirect correspondence caused by the response of both clades to other environmental factors.