PALEOECOLOGY AND STRATIGRAPHY OF AN UPPER ORDOVICIAN (KATIAN) HARDGROUND

Extraordinarily preserved Upper Ordovician (Katian) hardgrounds in the Kirkfield Member of the Bobcaygeon Formation, Simcoe Group, exposed in the Lake Simcoe area of southern Ontario, Canada offer an unprecedented opportunity for a high-resolution paleoecological study of in situ fossil organisms in the context of their nearly half-billion year old, subtropical environment. The hardgrounds exhibit heterogeneous topographies with 1 to 15 cm relief, mounded features up to several meters long, sheltered overhangs, small caverns, and a hummocky lower surface. This heterogeneous seafloor formed a variety of microhabitats inhabited by a diverse echinoderm- and bryozoan-dominated community that is preserved in life position owing to mud burial events.

These circumstances permit us to determine paleoecological parameters typically inaccessible to paleontologists, such as community succession, specific affinities for a spread of microhabitats, diversity comparisons between variably-sized mound “islands,” and the modes of life of several enigmatic and rare echinoderms representing at least eight classes. The hardground is characterized using an archaeological-style approach by stripping the site layer-by-layer. The hardground surface is photographed in high resolution and reconstructed in 3D using structure-from-motion and multi-view-stereo algorithms. This approach, never before used in paleoecological modelling, permits precise analysis of spatial distributions of fossil taxa and characterization of the hardground surface topography.

We also describe the origin of the unique mounded hardground and place the surface into a stratigraphic context, allowing for an examination of this community from a global perspective at a time when the biosphere was undergoing an unprecedented diversification and the Earth's carbon cycle was undergoing drastic changes. Carbon isotope profiles and sequence stratigraphic correlation allow us to refine formational contacts and correlate these strata with equivalent units on the Cincinnati Arch and Nashville Dome, which feature similar assemblages. Finally, we compare these ancient hardground assemblages with hard substrate biotas of different geologic ages to examine the evolution of this type of well constrained ecosystem.