DECLINES IN THE BODY SIZE OF BENTHIC MACROINVERTEBRATES AS A HARBINGER OF THE EARLY TOARCIAN (EARLY JURASSIC) EXTINCTION EVENT IN THE LUSITANIAN BASIN, PORTUGAL

We tested whether the body size distribution within populations and communities of benthic marine macroinvertebrates changed in relation to temperature-related stresses (i.e. warming, hypoxia, ocean acidification) associated with the Early Toarcian warming/extinction event. We expect a decrease in body size as a response to environmental stress, with large species being more affected than small species because of their higher metabolic demands. We studied in detail the Early Toarcian succession in the Lusitanian Basin, Portugal and herein focus on body size trends of brachiopods and bivalves across a 7.30 m thick pre-extinction interval.

Linear regression of body size (i.e. the log geometric mean of shell length and height) against sampling level shows that the mean body size of faunal samples decreases significantly. Such a decrease is also observed when the trends for brachiopods and bivalves are analysed separately. Only in brachiopods the trend towards smaller sizes is statistically significant and body size trajectories of brachiopods and bivalves are not correlated, suggesting a decoupled response to environmental stress. The size of relatively larger-sized brachiopod species decreases significantly, whereas no distinct trends occur in smaller-sized brachiopods nor in the smaller- and the larger-sized bivalves. Larger-sized brachiopod species also become less abundant over time. Thus, the significant body size decrease across the pre-extinction interval can be related to the larger-sized brachiopod taxa becoming both smaller and less abundant while approaching the crisis interval.

Experimental work suggests that bivalves are among the first to be affected by ocean acidification whereas brachiopods are hardly affected. A lowering of oceanic pH as a main cause of extinction therefore seems incompatible with the observed body size trends. As hypoxic conditions are also an unlikely extinction cause (the sedimentary rocks of the crisis interval are strongly bioturbated) we tentatively suggest a role of heat stress. Our results indicate long-term decreases in body size that occur well before the onset of faunal extinctions and local extirpations. This suggests that reductions in body size may serve as an early warning signal of imminent turnover at the community level and diversity decline.