Biogeography of recent marine bivalve molluscs and its implications for paleobiogeography and the geography of extinction: A progress report

The geographic distribution of Recent taxa is important in interpreting paleobiogeography and the fossil record of extinction. We examine the latitudinal diversity gradient in Recent marine bivalve molluscs and the effects of bivalve distributional patterns on the severity of model extinctions using a global biogeographic database for genera of Recent shallow (<200 m) marine bivalve molluscs. To date, we have collected information on genera and subgenera from 14 superfamilies: Arcoidea, Limopsoidea, Mytiloidea, Pteriodea, Pinnoidea, Pectinoidea, Trigonoidea, Carditoidea, Cardioidea, Tridacnoidea, Solenoidea, Tellinoidea, Arcticoidea and Veneroidea. Although these superfamilies represent only 30% of all marine superfamilies, they include approximately 49% of the extant marine genera. We used recent monographs to disentangle many generic assignments, thus attaining uniformity of treatment at the generic level among 115 sites or regions. The expected latitudinal diversity gradient is clearly seen in our data. Although high latitude faunas are consistently low in generic diversity, tropical bivalve faunas exhibit a wide range of diversities, mainly because of the relative impoverishment of Atlantic faunas and the low diversity of tropical island faunas relative to their continental shelf counterparts. Island faunas are dominated by cosmopolitans, underscoring the importance of dispersal in maintaining bivalve populations on oceanic islands. We simulated extinction of bivalve faunas by “eliminating” genera restricted to particular latitudinal zones, oceans, continental shelves or oceanic islands. Because endemism at the generic level is low (despite today’s high global thermal gradient and wide dispersal of continents), extinctions comparable to the major mass extinctions of the past can be accomplished only through the annihilation of faunas in several latitudinal zones or oceans. Either today’s fauna is more cosmopolitan than those of the past (which is unlikely), or major mass extinctions required truly pervasive and profound environmental upheavals.