Coordinated stasis, as defined herein, represents an empirical pattern, common in the fossil record, wherein groups of coexisting species lineages display concurrent stability over extended intervals of geologic time separated by episodes of relatively abrupt change. In marine benthic fossil assemblages, where the pattern was first recognized, the majority of species lineages (60 to more than 80%) are present in their respective biofacies throughout timespans of 3-7 million years. Most lineages display morphological stasis or only very minor, typically non-directional, anagenetic change in a few characters throughout a prolonged time interval; evidence for successful speciation (cladogenesis) is rare, few lineages (< 10%) become extinct, and very few new immigrant taxa become established within a region or province during such intervals. Moreover, species associations (biofacies) are nearly constant during an interval of stability, showing very similar taxonomic membership, species richnesses, dominance- diversity patterns and guild structure throughout. Conversely, during the intervening episodes of rapid change, many species (generally 70% or more) become extinct, at least locally, some lineages undergo rapid speciation and/or anagenetic change, and new immigrant taxa become successfully (semi- permanently) established. All (or most) biofacies arrayed across an environmental gradient display rapid and nearly synchronous changes in various aspects, including species composition, richness, dominance and guild structure. These intervals of abrupt evolutionary and ecological change typically represent only a small fraction (< 10%) of the duration of the stable units. The resulting stable blocks of species separated by turnover events comprise 'ecological-evolutionary sub-units' in the Appalachian Basin type example, and are considered to be components of the longer, more generalized ecological evolutionary units (EEUs) recognized by Boucot, Sheehan, and others. Causes of coordinated stasis and of regional ecological crisis/reorganization remain poorly understood. Tracking of spatially shifting environments appears to be the rule, rather than adaptation to local change. Incumbent species appear to have a very strong advantage and may exclude potential immigrants, as evidenced by temporary incursions of exotic taxa ('incursion epiboles'); this suggests a role for ecological and biogeographic factors in maintaining paleoecological stability. Stabilizing selection may be critical for producing morphological stability in individual lineages. Episodic crises appear to involve environmental perturbations that were too pervasive and/or abrupt to permit local tracking of environment to continue. Some faunal turnovers associated with unconformities may be partially an artifact of stratigraphic incompleteness. Others, however, seem to occur within conformable successions and were evidently rapid. Widespread anoxia, changes in current patterns, and/or climatic change associated with major marine transgression are common correlates of faunal turnovers in marine habitats in the Appalachian Basin. The phenomenon of coordinated stasis has been noted, albeit not fully documented, in a number of ancient marine and terrestrial ecosystems. An important goal for evolutionary paleoecology should be to document the patterns of stability and change in common and rare members of fossil assemblages in order to discern the relative frequency of coordinated stasis in the rock record, to evaluate the mechanisms by which such apparent evolutionary and ecological stability might be produced, and to seek clues (e.g., paleobiological and stratigraphic patterns, geochemical anomalies) as to causes of abrupt pulses of faunal change.
- coordinated stasis
ASJC Scopus subject areas
- Ecology, Evolution, Behavior and Systematics
- Earth-Surface Processes