Abstract
The Toarcian Oceanic Anoxic Event (TOAE, Early Jurassic, ~182 Ma ago) was characterised by severe environmental perturbations which led to habitat degradation and extinction of marine species. Warming-induced anoxia is usually identified as main driver, but because marine life was also affected in oxygenated environments the role of raised temperature and its effects on marine life need to be addressed. Body size is a fundamental characteristic of organisms and is expected to decrease as a response to heat stress. We present quantitative size data of bivalves and brachiopods across the TOAE from oxygenated habitats in the Iberian Basin, integrated with geochemical proxy data (δ13C and δ18O), to investigate the relationship between changes in temperature and body size. We find a strong negative correlation between the mean shell size of bivalve communities and isotope-derived temperature estimates, suggesting heat stress as a main cause of body size reduction. While within-species size changes were minor, we identify changes in the abundance of differently sized species as the dominant mechanism of reduced community shell size during the TOAE. Brachiopods experienced a wholesale turnover across the early warming phase and were replaced by a virtually monotypic assemblage of a smaller-sized, opportunistic species.
Highlights
IntroductionBody size of organisms, is affected by both biotic and abiotic factors
Growth, and body size of organisms, is affected by both biotic and abiotic factors
We focus on changes in shell size of marine benthic macroinvertebrates across the Early Toarcian Oceanic Anoxic Event (TOAE; Early Jurassic, ~182 Ma)[10]
Summary
Body size of organisms, is affected by both biotic and abiotic factors. We focus on changes in shell size of marine benthic macroinvertebrates across the Early Toarcian Oceanic Anoxic Event (TOAE; Early Jurassic, ~182 Ma)[10]. We quantitatively evaluate the role of temperature for changes in body size of bivalves and brachiopods from an oxygenated setting in the Iberian Range of Spain. Changes in shell size were tested for correlation with the δ18O time series, our geochemical proxy for ambient water temperatures, derived from rhynchonellid brachiopods and oyster shells from the same levels as the faunal data. 28-m-thick sedimentary succession at Barranco de la Cañada (40°23′53.4′′N 1°30′07.4′′W) near Albarracín, Spain (Fig. 1), where the Toarcian Turmiel Formation[27] is characterised by rhythmic alternations of marlstones and partly argillaceous limestones The latter primarily comprise mudstones, wackestones, and floatstones indicative of low-energy conditions below storm wave base at an estimated water depth of 40–70 m28–30. The TOAE spans the uppermost Tenuicostatum Zone up to the lower Serpentinum Zone
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