Did the “Recurrent Hamilton Fauna” Recur or Persist? Developing a Framework for Interpreting Faunal Turnovers

Intervals of paleoecological stability and the extinction events that separate them involve the interplay of ecological, evolutionary, and biogeographic factors. A majority of studies have focused on these patterns, and their underlying processes, intra-basinally at the level of individual species or groups of species with similar environmental tolerances (biofacies). Here, we investigate similar ecological-evolutionary processes in the context of regional turnovers intra- and extra-basinally at the broader, faunal level (unique suites of biofacies related spatially and temporally). Large portions of faunas may be lost when environmental conditions change basin-wide, characterized by the emigration and replacement of most taxa from the spectrum of biofacies for a substantial period of time (at least several kyr). Subsequent immigration and re-establishment of a fauna from extra-basinal refugia when appropriate environmental conditions return is herein termed “faunal recurrence.” On a more confined environmental scale, facies-based outages may occur when a portion of the basin becomes uninhabitable to a previously occupying fauna. These local faunal outages may represent the regional manifestations of global extinction, loss via emigration, or intra-basinal habitat shifting; the latter is termed “faunal persistence”.

Using a high-resolution stratigraphic framework superimposed on an onshore-offshore gradient, we document these different patterns of faunal turnover during the Late Middle Devonian Taghanic Biocrisis in its type area of the New York Appalachian Basin. These include: 1) the loss of the long-ranging Hamilton Fauna and its temporary replacement by the Tully Fauna; 2) the subsequent loss of the Tully Fauna and recurrence of the Hamilton Fauna; and 3) the persistence in nearshore settings of a subset of Hamilton Fauna biofacies while experiencing outage offshore. Recognition of these turnover types suggests that habitat tracking, a process typically attributed to organism lineages, and therefore biofacies, may also operate extra-basinally at the biofacies level, and, moreover, at the broader, faunal level.