ANIMAL ABUNDANCE IN LATE ORDOVICIAN REEF ENVIRONMENTS: NEW INSIGHTS FROM THE UPPER ORDOVICIAN LOURDES FORMATION, WESTERN NEWFOUNDLAND

The evolution of reefs from the Neorproterozoic through the early Paleozoic sees a transition from microbial communities as the primary framework to reefs constructed by skeletal animals. By the Late Ordovician, tabulate corals and stromatoporoids among other skeletal organisms have emerged as dominant reef builders. In the Neoproterozoic and early Cambrian, skeletal organisms make up a proportionally small amount of the total carbonate in reef settings, but it has remained unclear how and when skeletal abundance and diversity increased in these settings. Here we report new skeletal abundance data from within and outside small coral-sponge bioherms of the Upper Ordovician Lourdes Formation of the Long Point Group exposed on Long Point on the Port au Port Peninsula, western Newfoundland. The bioherms grew and thrived in shallow, carbonate-rich environments. We examined 24 samples from within the reefs and 52 samples from surrounding settings; these samples were point-counted for skeletal abundance and diversity. The coral bioherms and grainstone channels associated with the reefs contain the most fossil material (average 37% and 31% of all points counted for reefs and grainstone channels, respectively). Total skeletal material is lower in the overlying and surrounding stratigraphy (average of 18% of all points counted). Tabulate corals account for the majority of skeletal material within the reefs whereas crinoids, bryozoans, and brachiopods dominate samples from the peripheral environments. This work shows that skeletal abundance and diversity increased in reef settings from the Cambrian to the Late Ordovician, likely related to the substantial increase in skeletal organisms across the Great Ordovician Biodiversification Event. The abundance and diversity of fossil material is additionally dependent on setting; in the Upper Ordovician Lourdes Formation, reef environments offer the most favorable conditions for benthic skeletal organisms of the Ordovician.