Stasis and faunal overturn in ancient marine communities: punctuation and evolution on the periphery

Abstract Punctuated equilibria argue for intervals of long-term net stasis and comparatively abrupt change in the morphology of individual species lineages resulting from the process of allopatric speciation as recorded in the stratigraphic and fossil record. The concept of coordinated stasis extends punctuated equilibria to posit that not only individual species, but groups of coexisting lineages within a basin, display concurrent morphological and ecological stability over the same extended intervals of geologic time (10 5 to 10 6 yr). These blocks of stability termed ecological–evolutionary subunits (EESUs) are separated by shorter-lived (on the order of 10 3 to 10 4 yr) episodes of change characterized by varying combinations of speciation, extinction, immigration, and emigration. The result is a pattern of evolutionary and ecological stasis and change that is coincident and highly punctuational. Here, we assess the connections among environment, evolution, and ecology by documenting patterns of stability, geographic extent, and synchronous turnover during medium-scale bioevents in the Middle Devonian of the eastern United States, and we briefly compare these with patterns of EESUs across the Late Ordovician mass extinction (LOME) based on ongoing work. We quantify the geographic extent and stability of faunas originally documented in the Appalachian Basin and identify their likely places of origin and refugia during turnovers. Faunas are geographically widespread during times of stability and border comparably stable faunas in adjacent provinces. During geologically brief intervals, assemblages display near-synchronous shifts involving local extirpation/extinction and coordinated migration of biogeographic boundaries over very long distances. Allopatric speciation in small, locally isolated populations along the edges of basins during brief windows of dramatically altered environmental conditions is more consistent with the geological record, emphasizes the role of environment and biogeography in driving evolutionary change, and confirms the prevalence of punctuated equilibria.