PALEOPHYSIOLOGY, THERMAL TOLERANCE, AND EXTINCTION OF THE PERMIAN GLACIAL BIVALVE EURYDESMA

Temperature exerts a fundamental control on organism distributions and survival, but to what extent are physiological impacts of temperature itself, rather than indirect effects like temperature-mediated biotic interactions, directly responsible? The distinctive Permian bivalve Eurydesma, which flourished in south polar regions during the late Paleozoic ice age before becoming extinct during the icehouse-greenhouse transition, is an ideal model organism for testing the direct role of temperature on survival during warming. Bivalve shell accretion is punctuated by dark increments formed during slower growth, providing clear markers for the physiological condition of the organism. We used stable isotope sclerochronology to constrain the seasonal timing of growth cessation in Eurydesma specimens spanning 2000 km of the eastern Australia paleo-margin from Tasmania to Queensland. Higher-latitude specimens from Tasmania and the Sydney basin, living during the peak of the icehouse, primarily exhibit dark growth increments that formed during the coldest part of the year. In contrast, growth cessation occurred during warm months in slightly younger specimens from lower-latitude sites in Queensland. Annual growth banding may reflect energy allocation from growth to reproduction or a response to stresses other than temperature (such as salinity), but the shift in the timing of growth cessation with paleolatitude is best explained by the metabolic effects of temperature. Lower-latitude populations of Eurydesma were therefore at their thermal tolerance limits during the warmest months of the year, even early in the icehouse-greenhouse transition, implying that further warming would have been a plausible driver of range contraction and extinction. Although these findings do not exclude additional indirect effects, the importance of temperature is also consistent with the geographic restriction of Eurydesma to eastern Australia and with its retreat to deeper-water, and therefore cooler, settings just prior to its extinction.