HABITAT HETEROGENEITY, PALEOECOLOGY, AND LOCAL BIODIVERSITY OF AN UPPER ORDOVICIAN MOUNDED HARDGROUND:IMPLICATIONS FOR BIODIVERSIFICATION

Substrate type is a critical factor in determining the autecologies possible in benthic marine environments. Consequently, various substrates often host unique communities of organisms, and substrate type can be determined by the inhabitants and their traces. A mounded hardground horizon in the Upper Bobcaygeon (Upper Kirkfield) Formation (Upper Ordovician, Mohawkian) of southern Ontario, which hosts a high diversity of organisms (>150 species), is unique in showing contemporaneous hard- and soft-substrate assemblages. The heterogeneous topography of this hardground allowed lower portions to be buried while higher promontories of the hard substrate remained exposed to colonization. We examined how this habitat heterogeneity, partitioning, and exposure to occasional disturbance affected local diversity and ecological tiering patterns. The preservation of these hardgrounds has a large impact on the taphonomy of unique inhabitant communities. In situ preservation demonstrates the microhabitat affinities of specific taxa, including enigmatic forms, e.g. edrioblastoids. We calculated the total biomass and number of individuals in each ecological tier of organisms (mainly pelmatozoan echinoderms) to test the idea that sedentary suspension feeders in hard substrate communities partition themselves at different levels in the water column to avoid competition for resources. Comparison of the Kirkfield hardground biota with those of other hardgrounds of comparable age in Ohio, Kentucky, Tennessee, and Oklahoma demonstrates strong similarities as well as local variations in biodiversity related to hardground morphology. Mounded hardgrounds increase local biodiversity by partitioning the substrate, creating a suite of microhabitats (mound crests, margins, overhanging ledges, lower flat hardground) and allowing the hardground to withstand several episodes of disturbance and burial. Finally, we examine the effect of this novel hard substrate community on gamma diversity and the major biodiversification of the Ordovician and pinpoint the factors that led to a proliferation of mounded hardgrounds during this time. Upper Ordovician hardgrounds hosted some of the oldest and most diverse hard substrate communities and their heterogeneity may have contributed to the GOBE.