Crinoid biofacies in Upper Carboniferous cyclothems, midcontinent North America: faunal tracking and the role of regional processes in biofacies recurrence

Abstract Coordinated stasis is a pattern of long-term, multispecies evolutionary and ecological stability bounded by rapid turnover events. One class of models proposed to explain the paleoecological pattern of biofacies stability, typified by ecological locking, states that biotas within habitats become integrated into ecological units that, through population and community-level processes, rebound and persist through short-term disturbances. This high frequency community resilience allows the units to persist through geologic time, tracking their preferred habitats during transgressions and regressions. Because species are tightly woven into the fabric of the ecosystem, there is little opportunity for speciation; this also buffers them against extinction. This ecological locking by ecosystem homeostatic processes produces long intervals of regionally stable biofacies in the fossil record. In these models, the primary structuring mechanisms are at the local, community level, controlled by species autecology, species interactions, and local disturbance regime. An opposing class of models states that species are fundamentally independent and that species interactions are not strong nor spatio-temporally persistent enough to forge communities into integrated units with long-term persistence. Community recurrence would be expected as long as the various habitat types recur in time and space, species autecologies remain stable, the pool of available species remains intact and the recruitment of species into local habitats is not interrupted. The focus of these models is dominantly at the regional level, controlled by biogeographic history and dispersal among, as well as within, communities. The results of this study of crinoids from the Upper Carboniferous Lansing Group of midcontinent North America show that they are distributed among five recurrent paleocommunity types, or biofacies, which are generally arrayed along an onshore-offshore gradient. Two of these biofacies are restricted to the offshore, dysaerobic horizons of the most extensive cyclothem in the group. The other three biofacies recur consistently among the cycles. However, this repeated pattern includes a regionally distributed offshore biofacies, unique to late transgressive deposits, that is not found in analogous regressive deposits. While the distribution of the two shoreward biofacies is consistent with faunal tracking, the recurrence of a biofacies unique to late transgressive horizons is not consistent with an integrated paleocommunity that tracks shifting habitats through transgression and regression. These results favor the alternative models, namely that biofacies develop in place via dispersal from a persistent species pool in response to prevailing environmental conditions. The recurrence of apparently ephemeral biofacies through time underscores the importance of regional processes in controlling local paleocommunity composition. The establishment of refuges, or “source” populations, during environmental bottlenecks (i.e. lowstands) is critical to maintaining the regional species pool through time for continued recruitment into local “sink” paleocommunities during more amenable times (i.e. highstands). Metapopulation dynamics among habitats are equally important in maintaining species within a region through time. Conflicting observations of stochastic (individualistic) and deterministic (integrated) behaviors of the same ecological systems probably result from differences in the scales of analyses and therefore the processes being studied.