Paleoenvironmental patterns in the evolution of postPaleozoic benthic marine invertebrates

The ecological context of large-scale evolutionary patterns has been neglected. Several workers have recently reported a bathymetric bias in the evolution of benthic marine communities, such that nearshore assemblages tend to contain advanced taxa and community structures and offshore assemblages contain more archaic features. A clade-by-clade analysis is the most powerful approach for assessing the generality of the pattern and testing hypotheses on underlying mechanisms. We develop a paleoenvironmental framework for constructing time-environment histories for higher taxa that should be of general comparative utility, based on five environmental categories recognized by physical sedimentary and biostratinomic criteria. The robustness of negative data (absence of a given taxon from a particular time and environment) is evaluated using taphonomic control groupshigher taxa with ecological, morphological, and mineralogical properties similar to those of the group under study. Within this framework we have constructed three detailed time-environment histories from the primary literature, for the crinoid Order Isocrinida (based on 99 early Triassic-Recent occurrences), the bryozoan Order Cheilostomata (67 late Jurassic-late Eocene occurrences), and the bivalve Superfamily Tellinacea (70 late Triassic-Miocene occurrences). These three time-environment diagrams as well as numerous anecdotal reports suggest that the onshore-offshore trends previously reported for communities are actually underlain by individualistic clade histories that only appear to act in concert when viewed on a coarse time scale. For these three particular higher taxa differences in rates and timing of movement through environments also falsify causal hypotheses that invoke sea-level fluctuations or mass extinctions. We outline remaining viable hypotheses for driving mechanisms and suggest further tests; at present the data are consistent with a broad array of intrinsic biological mechanisms. These pervasive shifts through time of environmental preferences severely undermine approaches to paleoenvironmental reconstruction based on interpreting fossil content in the light of present-day faunal distributions. Production of detailed timeenvironment histories for additional higher taxa will permit paleoenvironmental analyses based on overlapping environmental range zones, in which particular co-occurrences are diagnostic of different habitats at different times.