EXPLORING THE ECOLOGICAL IMPACT OF THE END-EARLY CAMBRIAN MASS EXTINCTION

The Early Cambrian is well known for the metazoan radiation, however the mass extinction that occurred a few million years later is less understood. Although absolute taxonomic loss is low (related to low diversity), percentage loss of genera is comparable to that of the Permo-Triassic extinction. This study presents the first quantitative analysis of ecological impact of the end-Early Cambrian (Botomian) extinction. The combination of high rates of origination and extinction with a mass extinction during a major radiation provides a unique opportunity to examine the ecological signature of extinction and to explore the ecological dynamics of early metazoan communities. Using Landing’s 1988 dataset (J Paleontol), we examine changes in community structure of small shelly fauna from the Tommotian, Atdabanian, and Botomian (Cambrian Stages 2, 3, and 4).

A polar ordination using Sorenson distance reveals Botomian communities group separately from other Early Cambrian communities, which is to be expected due to the faunal change (loss) that characterizes extinction events. Community structure was quantified using kurtosis of rank-abundance curves (RACs). Changes in kurtosis measure community restructuring as a proxy for changes in resource partitioning. RAC shape change has been linked to the recency and severity of stress. Stressed communities have peaked RACs (high kurtosis) in contrast to more stable communities with flatter RACs (low kurtosis). Botomian communities had significantly higher kurtosis values (p<0.001, one tailed t-test).

The relatively peaked RACs of Botomian communities suggest an extended interval of stress impacted Early Cambrian communities. Although all taxonomic groups were affected by the extinction, archaeocyathans were by far the most impacted; the Landing dataset does not include any archaeocyathans. Therefore, our interpretation that Botomian communities were stressed is a conservative estimate of stress levels. These findings suggest the evolution of metazoan communities was shaped not only by rapid taxonomic turnover, but also by significant intervals of community stress and extinction events. This initial study of the Botomian extinction shows the promise of further work on the ecological dynamics of early metazoan extinctions.