TRILOBITE SURVIVORSHIP DURING THE LATE ORDOVICIAN MASS EXTINCTION IS ASSOCIATED WITH GEOGRAPHIC, ENVIRONMENTAL, AND ECOLOGICAL BREADTH

Because trilobites were one of the most affected taxa during the Late Ordovician mass extinction—nearly equal numbers of lineages went extinct as survived—trilobites offer an ideal test case to understand the “rules” governing extinction risk in general. Although studies of taxa during other extinctions have demonstrated that geographic breadth of higher taxa is often linked to survivorship, such analyses, among others, have not been conducted for trilobites. Here we use a database of worldwide trilobite genus occurrences to analyze how a variety of geographic and environmental factors, body size, and functional ecology traits (related to how animals feed, move, and inhabit their environment) affected trilobite survivorship during the Late Ordovician mass extinction. Results demonstrate the absence of individual environmental or geographic "refugia." Instead, surviving genera preferentially inhabited a wider range of paleolatitudes, depositional environments, and lithologies than did victims. Surviving genera also had greater numbers of occurrences. Analysis of trilobite functional ecology—analyzed using a quantitative ecospace framework of trilobite feeding, mobility, and microhabitat preferences, among other life-habit traits—demonstrated that surviving genera, as a group, also exhibited a greater breadth of life-habit disparity than did victims. In contrast, victims and survivors did not differ in terms of body size. Taken together, these results suggest that the most important factor for survivorship of trilobite genera during the Late Ordovician mass extinction was inhabiting a range of geographic and environmental venues and ecological life habits. Given that the Late Ordovician extinction was likely caused by "normal" terrestrial, climate-induced glacioeustatic changes—compared to the extraterrestrial trigger for the more widely benchmarked Late Cretaceous extinction—these patterns further bolster the generality of survivorship rules during mass extinctions.