EXTREME FAUNAL MAKEOVER: ECOLOGICAL AND TAXONOMIC REORGANIZATION OF TROPICAL AMERICAN SCALLOPS

Scallop genera exhibit very low rates of origination and extinction typical of other bivalves but scallop species are evolutionarily much more volatile. Diversity in the Southwest Caribbean increased from a low of 12 species 10 Ma to a maximum of 38 species between 4-3 Ma and subsequently declined to 22 species today. Both origination and extinction rates peaked between 4-3 Ma although extinction rates remained high until 2-1 Ma, which resulted in a 95 % turnover of species between 3.5 and 2 Ma. All living species originated within the last 4 Ma as reflected in the geologically sudden increase in Lyellian percentages of scallop species from nearly zero to as high as 100% 3 Ma. However, faunules with Lyellian percentages near zero persisted until 1.8 Ma, and faunules with Lyellian percentages ranging from 0 to 100% occurred within the same stratigraphic horizons less that 100-200 km apart. This extraordinary heterogeneity in the nearby geographic distributions of predominantly ancient and modern faunas is reminiscent of what Petuch termed geographical heterochrony, but on a much finer geographic scale and without any obvious differences in environments such as upwelling versus non-upwelling regions. There were also striking differences in comparative diversity and abundance among major ecological groups of scallops. Free-swimming taxa (Euvola sensu latu) were among the most diverse scallops throughout the last 10 Ma and were always moderately to very abundant. Leptopecten and Argopecten were also among the most diverse and abundant Late Miocene and Early Pliocene scallops but declined to very few species from the Early Pliocene to recent. In contrast, byssally-attaching scallops increased in both diversity and abundance since their first appearance in our samples 8-9 Ma, and are the most diverse group today, especially in association with coral reef environments. Maximum rates of evolution and ecological replacement occurred between 4-3 Ma during the interval of most rapid environmental change associated with the isolation of the Caribbean from the Pacific and the increased development of shallow water seagrass meadows and coral reefs. Scallop evolution is a textbook example of evolutionary turnover driven by environmental change.