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

Abstract

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 before 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 before 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 the 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 and the preceding reductions in mean shell size, because thermal tolerances in modern bivalves are among the highest within marine invertebrates.

Warm afterglow from the Toarcian Oceanic Anoxic Event drives the success of deep-adapted brachiopods

“…Brachiopod ranges. In the studied interval, profound changes in the brachiopod assemblage are recorded in both successions which are consistent with previous reports 21,27 (Fig. 2).…”

“…In Fonte Coberta/Rabaçal the rhynchonellid genera Cirpa and Nannirhynchia are observed in the Tenuicostatum zone, together with common terebratulids, the last spiriferinids, and very rare finds of Gibbirhynchia, Soaresirhynchia, and lingulid brachiopods 27,28 (Fig. 2).…”

“…Sea-level reconstructions neither suggest entirely parallel evolution in Portugal and Spain, nor a marked shallowing confined to the upper tenuicostatum and lower serpentinum zones. A gradual shallowing of a thermally stratified water body is also contradicted by the depositional environment of the studied sections 21,27 . Water mass stratification would likely have induced persistent bottom water anoxia for which there is no lithological or faunal indication in either of the studied successions.…”

Addressing priority questions of conservation science with palaeontological data

“…Research increasingly reveals that such crises are less abrupt than previously thought. At least two deep-time extinction pulses, associated with wholesale losses of tropical reef systems (the end-Permian and the Toarcian crises) were preceded by community changes, minor extinction pulses and declining body sizes of marine organisms [33][34][35][36]. Improved constraints on the patterns that signal early warning of ecosystem collapse is thus an obvious route to address this question.…”

Resilience of marine invertebrate communities during the early Cenozoic hyperthermals

“…This conclusion does not rule out transient changes in other ecological attributes. An increase in temperature, for example, may reduce body-size, promote parasitism, and/or alter habitat preferences, as demonstrated by work on modern systems 32 and Mesozoic hyperthermal events 33 . Nonetheless, if any ecological changes did occur during the hyperthermal events in the Gulf Coastal Plain they must have been reversible and short-lived.…”