ANIMALS OR THE ENVIRONMENT? BIOTURBATION AS A KEY CONTROL ON EARLY MESOZOIC BIODIVERSITY

The fossil record provides an archive of a series of global-scale, natural experiments documenting the responses of marine ecosystems to past episodes of climatic and environmental change. Unfortunately, the archive is not perfectly preserved and although it remains adequate for addressing many key questions, accurate interrogation of the record requires a quantitative, multidisciplinary and multivariate approach. The late Palaeozoic and early Mesozoic interval witnessed the appearance and early diversification of a number of marine invertebrate groups that occupy key roles in modern marine ecosystems. It is also characterized by several episodes of global warming, each of which is associated with extinctions and a biotic crisis in marine ecosystems. Recently, major advances have been made in collecting the type of high-resolution, quantitative relative abundance data needed to determine the patterns and nature of ecological response, especially of benthic marine ecosystems. Changes in biodiversity in the aftermath of these biotic crises, during ecosystem recovery and re-establishment, is typically interpreted as being the result of changes in environmental factors such as hypoxia, temperature and sedimentation rate. In modern marine ecosystems, however, the burrowing activities of key bioturbating macroinvertebrates are critical in enhancing nutrient cycling and productivity, and play a key role in maintaining healthy ecosystem function. It seems likely that the influence of biotic factors such as bioturbation on the recovery of local biodiversity in deep time may have been overlooked. Analyses of the Triassic-Jurassic and Permian-Triassic fossil records show that parameters such as burrow depth and diameter often co-vary with key palaeoecological diversity metrics based on analysis of the body fossil record. The re-appearance of a functionally diverse ecosystem with high tier suspension feeders in the late Hettangian of southwest UK is closely linked to a relatively rapid switch to deeper burrowing by larger bioturbators. On current evidence, this seems to occur in the absence of significant environmental change, perhaps implying a key biotic control on the recovery of biodiversity at that time.