INFLUENCE OF PALEOECOLOGY, PALEOCLIMATE, AND SEA LEVEL ON LATE PALEOZOIC TRILOBITE EVOLUTIONARY PATTERNS

Four stages characterize trilobite evolution during the Late Paleozoic. Stage 1 spans the Tournaisian to early Viséan and consists of high origination rates, high generic richness, and short generic durations. Diversification was confined to the family Phillipsiidae, whose species moved into and occupied all available environmental niches that resulted from sea level rise following the end-Devonian glacial episodes and extinction events. A mid-Mississippian crisis in trilobites brought an end to Stage 1 by reducing generic diversity by more than 50% and alpha diversity by 75%. This crisis resulted from sea level drop and oceanic cooling coincident an episode of Gondwana glaciation. Crisis survivors were thermally-tolerant and were restricted to the pre-crisis deep-water environments or occurred at high latitude. Stage 2 included the Viséan and Serpukovian, displays a modest diversity increase, and correlates with an interval of global sea level rise related to late Viséan global warming. The end of Stage 2 coincides with sea level drop at the mid-Carboniferous boundary. Stage 3 spans much of the Pennsylvanian and Early Permian and consists of very low origin rates, low extinction rates, and very long generic durations. Ecological strategies of trilobites species of Stage 3 are characterized by ecologic generalists that survived in the rapidly changing environments associated with eustatic fluctuations and compressed climate zones during the ice house climates of the Pennsylvanian and early Permian. Stage 4 occurs in the Late Permian, and is represented by elevated origin rates, short generic durations, and high extinction rates. Trilobite distribution in Stage 4 appears to center around the existence of widespread reefs that developed during the global warming that followed the Late Paleozoic Ice Age. Only a few eurytopic genera survived the elevated extinctions levels of the late Capitanian, and were able to persist to the end of the Permian.