SOME LIKE IT HOT: STRATEGIES FOR SURVIVAL IN EQUATORIAL GREENHOUSE SEAS OF THE EARLY TRIASSIC

Extreme warming in low-latitude, shallow marine settings in the early Triassic is thought to have contributed to the protracted biotic recovery from the end-Permian mass extinction, but the survival of bivalves, brachiopods, gastropods, and echinoids within these settings raise questions about the survival strategies they employed under hostile, high-temperature conditions. Oxygen isotope paleotemperature estimates from conodonts and brachiopods indicate mean equatorial early Triassic sea surface temperatures (SSTs) reached or exceeded 40°C. Discussion of survival strategies has often focused on shifts in biogeographic distribution, with species tending towards higher latitude. The ‘Lilliput effect’ would also have been adaptive, as smaller body sizes counter increased metabolic oxygen demand resulting from elevated temperatures, and increases the ratio of surface area to body volume, enhancing gas diffusion. Shifting environmental zonation to deeper, cooler waters would have been a possible survival strategy, but dysoxic ocean conditions below the surface mixed layer would have reduced its viability. The role of seasonal temperature variation, however, has received limited attention to date, in part due to the limited data available for it, although the existence of a significant Pangaean monsoon is widely accepted. Lingulid based paleotemperature estimates from Induan age ramp facies of the Dinwoody Formation support climate models for seasonal temperature ranges varying between 17°C and 39°C in low latitude shelf waters off the west coast of Pangaea. Modern analogues from the modern Gulf of California provide insight into a variety of survival strategies marine taxa employ under these extreme conditions. Modern aragonitic taxa appear to stop growing and shut down biomineralization processes at elevated temperatures, with an upper limit around 32C, but grow normally during the cooler months of spring and autumn. By contrast, phosphatic taxa appear better adapted to growth at temperatures between 32°C – 40°C, including lingulid brachiopods, conodonts and many vertebrates, and may have contributed to their relative success in low-latitude shallow water conditions.