Ecology of the Mollusks of the Bowden Formation, Jamaica

Abstract

The Bowden formation of Jamaica, of late middle Miocene age, carries a tropical fauna of about 600 species of mollusks. The presence of a few brackish-water genera (Neritina, Mytilopsis, and an oyster similar to the modern mangrove oyster) is attributed to unusual events, perhaps unusually heavy floods. Most of the Bowden mollusks lived on a bottom of sand and mixtures of mud, sand, and gravel. Rock-clingers, rock-borers, and mud-burrowers are rare or absent. Modern representatives of two genera (Tralia and Planawis) live between high- and low-water marks or even above high-water mark. The sparsely represented rock-clingers (Chlorostoma and Thais) and some of the abundant sand-burrowers (Oliva, Olivella, and Natica) live in the intertidal zone and also below low-water mark. It is apparent that most of the Bowden mollusks represent the neritic zone. According to frequency graphs based on dredgings in the West Indian region, several genera (Seguenzia, Cocculina, Fissurisepta, and Tindaria) indicate a probable depth of more than 100 fathoms. The relatively large number of pelagic mollusks and the large proportion of carnivores among the gastropods may have the same depth significance. It is suggested that the Bowden formation was deposited on a narrow coastal shelf, and that at times the sediments at the edge of the shelf and the shells buried in them were washed down the slope and came to rest at a greater depth, where they were mixed with autochthonous deep-water material. One genus of predaceous carnivores (Nassarius) seems to be responsible for most of the neatly bored holes seen in many shells.