CHEILOSTOME BRYOZOAN FAUNAL TURNOVER IN THE TROPICAL WESTERN ATLANTIC

Timing and rates of faunal turnover over the past 10 Ma varied greatly among major taxa. Origination generally preceded extinction but peaks in extinction ranged from 4-3 Ma for bivalve mollusks to 2-1 Ma for gastropods and reef corals. Cheilostome assemblages also changed dramatically, especially in terms of the relative abundance and diversity of erect, encrusting, and free-living taxa. However, problems of cryptic species have obscured the timing of the rise and fall of diversity of the different growth forms and thus the environmental correlations of their diversification and extinction. Our taxonomic revision of the abundant free-living genus Cupuladria provides an opportunity to compare patterns of origination and extinction in Cupuladria with those for predominantly erect Metrarabdotos and encrusting Stylopoma studied previously. As expected, 15 of the 17 Cupuladria species were undescribed taxonomically compared to 23 out of 34 new species in the other two genera. Relative timing of maximum diversity, origination, and extinction differed greatly. Origination of Cupuladria peaked in the middle Pliocene and diversity increased until a strong peak in extinction in the Early Pleistocene halved the number of species. In contrast, origination of Metrarabdotos peaked in the Late Miocene and extinction peaked in the Early Pliocene, and Stylopoma origination peaked twice in the Late Miocene and Pleistocene while only 1 of 19 species became extinct during the entire 10 Ma. Thus, of the 50 species in these genera, the percentage of erect species declined from 42 to 7% (1 of the 3 recent Metrarabdotos species is encrusting) while that of encrusting species increased from 32 to 66% and free-living species from 26% to 43% before falling back to 28%. Data on skeletal abundance suggests the ecological footprint of these trends was even greater since free-living species are the most abundant bryozoans by weight in Late Pliocene to recent samples whereas erect species dominated in the Late Miocene. With the exception of massive forms like Celleporaria, encrusting species have been a minor ecological component of the fauna despite their great increase in diversity. Thus, the major ecological shift was from dominance by erect to free-living species that appears to have been driven by the collapse in productivity in the Late Pliocene as the Panamanian portal closed.