Paleoecology and paleobiogeography of fossil mollusks from Isla Isabela (Galápagos, Ecuador)

Sandy sediments outcropping near Villamil (Isla Isabela, Galapagos) have yielded a fossil molluscan assemblage represented by 42 taxa (23 bivalves and 19 gastropods) and dominated by the lucinids Divalinga eburea and Pegophysema spherica. The paleocommunity developed on a inner subtidal sandy bottom no deeper than 30-40 m and located near reef habitats; the clear predominance of suspension over deposit feeders is in accordance with the sandy substrate, limiting the organic matter preservation in the seafloor. Although the presence of the fossil bivalve P. spherica might indicate a Pliocene age, the modern structure of the fauna and scant radiometric ages proposed to date seem to suggest a Pleistocene age. A biogeographic analysis highlights a large predominance of the Panamic component; the remaining mollusks include a significant endemic component and negligible percentages of forms from neighboring bioprovinces. Other than some differences in percentages, a similar biogeographic pattern results for the other fossil molluscan communities and the modern one of the archipelago. From a taxonomic point of view, most of the fossil and modern assemblages show a high ratio of gastropods to bivalves, whereas in the fauna discussed herein, bivalves are slightly predominant over gastropods.