Data from: Reductions in body size of benthic macroinvertebrates as a precursor of the Early Toarcian (Early Jurassic) extinction event in the Lusitanian Basin, Portugal

Reduction of body size is a common response of organisms to environmental
stress. Studying the Early Toarcian succession in the Lusitanian Basin of
Portugal, we tested whether the shell size of benthic marine communities
of bivalves and brachiopods changed at and prior to the global,
warming-related Toarcian Oceanic Anoxic Event (T-OAE). Statistical
analyses of shell size over time show that the mean shell size of
communities decreased significantly prior to the T-OAE. This trend is
distinct in brachiopods and is caused by larger-sized species becoming
less abundant over time, whereas it is not significant in bivalves,
suggesting a decoupled response to environmental stress. Reductions in
shell size precede the decline in standardized sample-level species
richness associated with the Early Toarcian extinction event. Such
decreases in the shell size of marine invertebrates, well before the onset
of biodiversity change, suggest that reductions in body size more
generally may be a precursor of a subsequent loss of species and turnover
at the community-level caused by climate change. Sedimentological evidence
is against hypoxia as a driver of extinction and preceding size decrease
in the brachiopod fauna in the studied succession, although low oxygen
levels are widely held responsible for elevated Early Toarcian extinction
rates globally. Reduction of mean shell size in brachiopods but stasis in
bivalves is difficult to explain with ocean acidification because
experimental work shows that brachiopods can be resilient to lowered pH,
albeit long-term metabolic costs and potential evolutionary adaptations
are unknown. Rising Early Toarcian temperatures in the Lusitanian Basin
seem to be a plausible factor in both diversity decline associated with
the T-OAE as well as the preceding reductions in mean shell size because
thermal tolerances in modern bivalves are among the highest within marine
invertebrates.