COORDINATED STASIS - AN OVERVIEW

Coordinated stasis, as defined herein, represents an empirical pattern , common in the fossil record, wherein groups of coexisting species li neages display concurrent stability over extended intervals of geologi c time separated by episodes of relatively abrupt change. In marine be nthic fossil assemblages, where the pattern was first recognized, the majority of species lineages (60 to more than 80%) are present in thei r respective biofacies throughout timespans of 3-7 million years. Most lineages display morphological stasis or only very minor, typically n on-directional, anagenetic change in a few characters throughout a pro longed time interval; evidence for successful speciation (cladogenesis ) is rare, few lineages (<10%) become extinct, and very few new immigr ant taxa become established within a region or province during such in tervals. Moreover, species associations (biofacies) are nearly constan t during an interval of stability, showing very similar taxonomic memb ership, species richnesses, dominance-diversity patterns and guild str ucture throughout. Conversely, during the intervening episodes of rapi d change, many species (generally 70% or more) become extinct, at leas t locally, some lineages undergo rapid speciation and/or anagenetic ch ange, and new immigrant taxa become successfully (semi-permanently) es tablished. All (or most) biofacies arrayed across an environmental gra dient display rapid and nearly synchronous changes in various aspects, including species composition, richness, dominance and guild structur e. These intervals of abrupt evolutionary and ecological change typica lly represent only a small fraction (<10%) of the duration of the stab le units. The resulting stable blocks of species separated by turnover events comprise ''ecological-evolutionary sub-units'' in the Appalach ian Basin type example, and are considered to be components of the lon ger, more generalized ecological evolutionary units (EEUs) recognized by Boucot, Sheehan, and others. Causes of coordinated stasis and of re gional ecological crisis/reorganization remain poorly understood. Trac king of spatially shifting environments appears to be the rule, rather than adaptation to local change. Incumbent species appear to have a v ery strong advantage and may exclude potential immigrants, as evidence d by temporary incursions of exotic taxa (''incursion epiboles''); thi s suggests a role for ecological and biogeographic factors in maintain ing paleoecological stability. Stabilizing selection may be critical f or producing morphological stability in individual lineages. Episodic crises appear to involve environmental perturbations that were too per vasive and/or abrupt to permit local tracking of environment to contin ue. Some faunal turnovers associated with unconformities may be partia lly an artifact of stratigraphic incompleteness. Others, however, seem to occur within conformable successions and were evidently rapid. Wid espread anoxia, changes in current patterns, and/or climatic change as sociated with major marine transgression are common correlates of faun al turnovers in marine habitats in the Appalachian Basin. The phenomen on 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 assembla ges in order to discern the relative frequency of coordinated stasis i n the rock record, to evaluate the mechanisms by which such apparent e volutionary and ecological stability might be produced, and to seek cl ues (e.g., paleobiological and stratigraphic patterns, geochemical ano malies) as to causes of abrupt pulses of faunal change.