Coral diversity, ecology, and provincial structure during a time of crisis; the latest Ordovician to earliest Silurian Edgewood Province in Laurentia

Abstract

The latest Ordovician to earliest Silurian Edgewood Province in the cratonic interior of Laurentia contained a post-extinction coral fauna. These corals inhabited a broad range of shallow-marine, carbonate environments. Diversity generally increased along a gradient from restricted, low-energy, relatively turbid conditions, to open, high-energy, low-turbidity conditions. Changes in diversity involved progressive addition or subtraction of stenotopic species within a single association dominated by opportunistic, ecologic generalists. Patterns and trends in diversity, abundance, and distribution reflected locally variable environmental parameters and an overall gradient from the continental interior toward the open ocean. Structural characteristics of the Edgewood Province differed from those of Late Ordovician pre-extinction and Early Silurian recovery faunas in the Laurentian cratonic interior. These included the low total diversity, the abundance and strong dominance of a single, solitary rugosan species, and the lack of distinct, specialized, recurring species-associations. The origin, overall structure, and fate of the province were determined by factors operating on a broad scale. Nutrient enrichment and related environmental destabilization in the Laurentian epeiric sea during the end-Richmondian regression were probably significant factors in extinctions among Late Ordovician "perched faunas," including corals. This would have occurred as nutrients provided by runoff from expanding terrestrial areas became concentrated in the shrinking sea. During the Gamachian and early Rhuddanian, elevated nutrient levels and environmental instability within the small sea were likely instrumental in maintaining the structure of the Edgewood Province. By the late Rhuddanian, transgression had expanded the sea and reduced the effects of runoff sufficiently to permit the Silurian recovery of corals.