SEQUENCE STRATIGRAPHY, PALEOECOLOGY, AND EVOLUTION - BIOTIC CLUES ANDRESPONSES TO SEA-LEVEL FLUCTUATIONS

Paleoecology has a dual relationship with sequence stratigraphy. On on e hand, body and trace fossils, together with their taphonomy, may pro vide sensitive indicators of environmental parameters, including depth , substrate consistency, sedimentation rate/turbidity, and benthic oxy genation, which are critical in recognizing and interpreting paraseque nces and sequences. Fossils may provide some of the best guides to ide ntifying hey surfaces and inferring sedimentation dynamics within sequ ences. Conversely, the sequence stratigraphic paradigm and its corolla ries provide a predictive framework within which to examine biotic cha nges and interpret their probable causes. Such changes include ecologi cal epiboles (short-term, widespread proliferation of normally rare sp ecies), outages (absence of normally common species), ecophenotypic ch anges, and longterm (tens to hundreds of Ka) community replacement. Co mmunity replacement should be carefully distinguished from short-term (10 to a few hundred years) ecological succession, rarely resolvable a t the scale of single beds, although replacement series through shallo wing-to-deepening cycles may display some features that parallel true succession. Replacement in marine communities may he relatively chaoti c, but, more commonly in offshore settings, it appears to involve late ral, facies-related shifting of broad biofacies belts, or habitat trac king. Tracking patterns may be nearly symmetrical in areas of low sedi ment input. However replacement cycles are commonly asymmetrical. The asymmetries involve both apparent and real effects; deletion of portio ns of facies transitions at sequence boundaries or condensed sections leads to artifactual asymmetries. Alternatively, in areas proximal to siliciclastic sources, tracking asymmetries arise from the markedly hi gher sedimentation rates during regressive (late highstand) than trans gressive phases. Replacements may also involve immigration of species into a sedimentary basin, either as short-lived events (incursion epib oles) or as wholesale faunal immigrations. The latter will typically f ollow intervals of extinction/emigration of the indigenous faunas. Bot h large and small immigration events appear most commonly during highs tands (transgressive peaks), which may be associated with altered wate r-mass properties, and may open migration pathways for nekton and plan ktonic Larvae. At least in isolated basins, allopatric speciation may also occur during fragmentation of habitats associated with regression s. Finally, there are predicted and empirical correlations between seq uence-producing sea-level fluctuations and macroevolution. Major extin ctions may be associated with habitat reduction during major regressio ns (lowstands), or with anoxic events during major transgressions, Gen erally, rising sea level may be correlated with evolutionary radiation s. Hence, some ecological-evolutionary unit boundaries may correlate e ither with sequence boundaries or maximum flooding surfaces. However i n other cases, no correlation has been found between macroevolutionary patterns and sequence stratigraphy. The situation is obviously comple x, but sequence stratigraphy at least provides a heuristic framework f or developing and testing models of macroevolutionary process.