ECOLOGICAL DRIVERS OF BODY SIZE EVOLUTION IN BRACHIOPODS AND TRILOBITES ACROSS THE LATE ORDOVICIAN MASS EXTINCTION

The Late Ordovician mass extinction was first of the “Big Five” mass extinctions. Biodiversity losses were likely driven by a number of coincident factors, including cooling, glaciation, eustatic sea level fall, and changes in ocean chemistry. Previous studies of the Late Ordovician mass extinction have found evidence for selective extinction of brachiopods across bathymetric gradients and of trilobites by larval type (i.e., benthic vs. pelagic). Further, there is some evidence for a Late Ordovician “Lilliput Effect,” a general reduction in the body size of organisms after a mass extinction due to environmental stressors and/or selective extinction of larger taxa. This has been observed over the extinction in several major groups of marine invertebrates, including brachiopods, trilobites, and crinoids. However, the timing, geographic extent, and taxonomic scope of these changes in body size are not well-understood, and their potential relationship with heterogeneity in extinction intensity has not been previously explored.

Here, we evaluated whether previously observed differences in extinction intensity based on ecological factors (i.e., depth preference in brachiopods and larval ecology in trilobites) also correspond to changes in body size over the Ordovician extinction. Body size was estimated using log area calculated from length and width measurements for Middle Ordovician to middle Silurian orthids, strophomenids, and trilobites using a combination of museum collections and published literature. Based on existing compendiums, depth preferences in terms of benthic assemblage zones were assigned to brachiopod genera, and pelagic vs. benthic larval types were assigned to trilobite genera. The resulting dataset includes taxa from all major paleogeographic regions to capture global trends in body size across the Ordovician extinction. A series of statistical analyses were conducted to identify trends in body size for 1) strophomenid and orthid brachiopods with regards to water depth preference and 2) trilobites with regards to larval ecology. Results provide insight into whether ecological controls of extinction intensity also play a role in size reduction trends, as well as the timing and magnitude of size decrease in several major invertebrate clades over the Late Ordovician mass extinction.