THE RISE OF SKELETONS PRECEDING THE GREAT ORDOVICIAN BIODIVERSIFICATION EVENT: EVIDENCE FROM WESTERN NEWFOUNDLAND AND WESTERN UTAH

The Great Ordovician Biodiversification Event (GOBE) is known for the significant increase in the diversity of marine invertebrates. Skeletal abundance of animals also increases during this time, but what is less well understood is the environmental/ecological conditions that surround this rise in abundance leading up the GOBE. The Lower Ordovician Catoche Formation from western Newfoundland, Canada, and Lower Ordovician Fillmore Formation from western Utah, USA, capture the interval of time leading up to the GOBE and provide an opportunity to examine skeletal abundances during this time. Through petrographic analyses and point counts, we determined that the Catoche Formation contains low skeletal abundance throughout 125 m of carbonate, with fossils making up only 1.7% of total thin section point counts. Fossil diversity consisted of trilobites, cephalopods, brachiopods, echinoderms, and gastropods in declining order of abundance. The upper Fillmore Formation had different results, where fossil content made up 17.0% of total thin section counts throughout 63 m. Fossil point counts were composed of echinoderms, brachiopods, sponges, and trilobites in declining order of abundance. Petrographic analyses were accompanied by field observations that suggest shallow marine paleoenvironments for both formations with slight variations. The Catoche Formation mostly represents a storm-dominated subtidal environment, whereas the upper Fillmore Formation consists mostly of a high-energy subtidal environment adjacent to sponge reefs. We suggest that environmental differences between the two depositional settings had strong impacts on the skeletal abundance and diversity in these Lower Ordovician carbonates and that the initial stages of the rise in skeletal abundance that occurred through the GOBE most likely happened in shallow subtidal environments, like those preserved in the uppermost Fillmore Formation.