ASPECTS OF JURASSIC BIVALVE PALEOBIOLOGY

Marine Jurassic bivalves, the dominant invertebrate macrofauna, underwent a steady recovery of diversity after the end-Triassic mass extinction, with a major setback caused by the early Toarcian mass extinction, before stabilising in the Middle Jurassic. The level of endemism was reduced especially by migration into Europe from the eastern Pacific through the Hispanic Corridor. There is a clear inverse relationship between sea level and regional endemism. Ecologically there is an overwhelming numerical dominance of pteriomorph epifauna, suggesting that Vermeij's Mesozoic marine revolution was primarily accomplished in the Cretaceous. With regard to mode of speciation, a broad survey of European taxa suggests that it was overwhelmingly dominated by punctuated equilibria, a result confirmed by detailed taxonomic analyses of pectinids and trigoniids. The most convincing case of phyletic gradualism is Lower Jurassic Gryphaea. Phyletic size increase is common, and is not matched by corresponding trends towards size decrease. Any size reduction during speciation must have been comparatively sudden, suggesting a fundamental asymmetry through time. The European survey indicates a strongly positively skewed distribution of species duration, with a mode of 7-8 million years.