RESPONSE OF SHALLOW MARINE BIOTAS TO SEA-LEVEL FLUCTUATIONS: A REVIEW OF FAUNAL REPLACEMENT AND THE PROCESS OF HABITAT TRACKING

Associations of fossil genera and species commonly display repeated and predictable patterns of change in stratigraphic sections. These changes exhibit some analogies with the phenomenon of ecological succession but are longer-term allogenic temporal changes, occurring over time scales of tens to hundreds of thousands of years, that should be referred to as biotic (faunal-floral) replacement. Habitat tracking is one of a suite of possible mechanisms of biotic replacement, but one that may be important in certain marine settings. Evidence of tracking includes (1) recurrence of similar replacement series of differing age; (2) mirroring of vertical replacement of faunas by lateral gradients of species associations along single time planes; (3) the occurrence of similar gradient transects at different time planes; (4) the correlatability of highs and lows of quantitative faunal curves (e.g., variations of detrended correspondence analysis scores) at different localities despite offsets of absolute scores; and (5) the high fidelity recurrence of stenotopic species in particular associations representing narrowly defined environments. The degree of ecological fidelity (e.g., similarity of species richness, guild structure) that is maintained during tracking is variable. Recurrent assemblages in different cycles, separated by as much as several million years, can be very similar in terms of species composition and trophic structure. Common species, however, may show significant differences in rank and relative abundances. This evidence indicates individualistic tracking of preferred habitat by various species. In shallow-shelf and ramp settings, sea-level fluctuations may produce approximately symmetrical patterns of biotic replacement where biofacies are arrayed typically in elongate belts parallel to depositional strike. Asymmetries, however, are common and may result from variations in sediment supply during sea-level fluctuation. Hence, the low siliciclastic input typical of transgressions predictably favors those organisms that require lower sedimentation or turbidity and perhaps firmgrounds to hardgrounds, whereas the regressive half cycle at analogous depths favors more eurytopic organisms that tolerate or prefer higher sedimentation or turbidity. The phenomenon of tracking may be of considerable importance in evolutionary paleoecology. Tracking implies that species commonly respond to long-term physical changes, not by adaptation, but primarily by migrati