PALEOECOLOGICAL CONTROLS ON THE DIVERSIFICATION OF MIDDLE ORDOVICIAN GASTROPODS IN LAURENTIA

The Ordovician Radiation was one of the most dynamic periods of life on Earth. Overall marine diversity increased three-fold, at an unprecedented and since unparalleled rate, only to then be devastated by Earth’s second-largest extinction event at the end of the period. Though gastropods experienced their first radiation and subsequent extinction during the Ordovician, this time period remains understudied and previous studies have necessarily focused on taxonomy and diversity in order to determine first order diversity patterns. Gastropod diversity more than tripled during the Ordovician radiation and during this period of rapid evolution, and the clade expanded into a larger range of environments and adapted to new lifestyles.

The classic Lower to Middle Ordovician strata cropping out in the Ibex region of western Utah provides an excellent opportunity to resolve the paleoecologcial signature of the clade. This region was a large equatorial carbonate platform during the Ordovician and strata representing deposition in a range of environments (nearshore lagoon to deep water shelf) are preserved. Throughout the Early and Middle Ordovician, Gastropods were present in nearly every type of depositional environment but appear to have flourished in fine-grained carbonate muds, regardless of water depth. Gastropods are both more abundant and more diverse in mudstones and dominate these assemblages, indicating that environment had a strong effect on Ordovician gastropod radiation. Dominant genera in these mudstones include Clathospira, Liospira and Murchisonia. In other facies, typically dominated by brachiopods, ostracods and trilobites, gastropods are rare, with Maclurites, Malayaspira and Barnesella typically occurring as minor components within a shellbed. The bellerophontid gastropod Groomodiscus is rare and most commonly preserved in carbonate concretions in shale horizons. These data suggest that environment had a significant role in the diversification of gastropods.