An Early Pleistocene molluscan assemblage from Fiji: gastropod faunal composition, paleoecology and biogeography

Ecologically important in many modern marine environments as both predators and prey, gastropod molluscs also offer keys to understanding the organization of past biotas. We analyzed Early Pleistocene gastropod shells from the Nasaki Beds in Viti Levu (∼1.8 Myr B.P.) to describe and interpret diversity at the species and higher levels, taxonomic and trophic aspects of community composition, and the taxonomic and biogeographic relations of past and present faunas. The assemblage studied probably represents a paleocommunity type that occurred in fine sand sediment in depths of 5–50 m inshore. Gastropods outnumbered bivalves 4:1. At least 320 gastropod species from at least 52 families and 146 genera were present. Neogastropods accounted for 51% of the species and two-thirds of individuals. Numerically the family-groups Nassariinae, Turridae, Olividae and Naticidae dominated. Turridae, Conidae and Naticidae had the most species. Most gastropod species and individuals were probably primary carnivores. About 15% were secondary predators, but these caused significant mortality of other gastropods present. Nearly half of all gastropod species suffered drilling predation, mainly by naticids, and these attacks were nearly always successful. Crabs and other durophagous predators probably killed only about 4% as many gastropods as drillers; a high frequency of shell repairs also suggests that they were less important mortality factors. The Lyellian percentage of the gastropod assemblage exceeds 80%. Two-thirds of the 212 extant species presently occur in Fijian waters; a similar modern fauna occurs in Laucala Bay in 8–25 m. Almost all of the locally extinct but globally extant species whose southern hemisphere distributions are known reach the eastern limits of their ranges west of Fiji, occurring as far east as Indonesia (99%), New Guinea (60%) and Vanuatu (15%). Pleistocene sea level fluctuations likely extensively altered the geographic distributions of gastropods inhabiting tropical subtidal nearshore soft sediments, as previously documented for reef-associated corals and bivalves.