SUFFERING A SEA CHANGE: THE EVOLUTION AND EXTINCTION OF TRIASSIC MARINE REPTILES

Secondarily marine reptiles occupied the top trophic levels of many Mesozoic marine ecosystems. The first appearance of marine reptiles in the Spathian was synchronous with the stabilization of carbon isotopes and the end of the Early Triassic delayed recovery interval following the Permo-Triassic mass extinction. Marine reptiles diversified rapidly during the Middle Triassic and exhibited high morphological disparity in terms of dentition and body plan indicative of substantial ecological diversity. Overall marine reptile taxonomic diversity declined in Late Triassic, apparently well-before the terminal Triassic mass-extinction of other marine and terrestrial taxa. However, the mechanisms behind this decline have not been scrutinized.

We compiled species level taxonomic records at the stage level to explore patterns in marine reptile diversity through time. Overall patterns of diversification were compared to published trends of Triassic seawater chemistry based on measurements from marine invertebrate fossils to explore possible links between changes in the Triassic oceans and marine reptile evolution. We also grouped marine reptiles according to tooth shape, limb shape and tail shape to test for ecologically selective extinction based on habitat or dietary preferences.

Peak taxonomic and ecological diversity of marine reptiles in the Middle Triassic corresponds to an interval characterized by relatively stable δ13C and δ18O values, as well as less radiogenic 87Sr/86Sr values, consistent with reduced sedimentary input, higher sea level and well oxygenated, temperate ocean conditions. The Late Triassic is characterized by pronounced δ13C and δ18O fluctuations and increasingly radiogenic 87Sr/86Sr consistent with falling sea levels and anoxic pulses. The step-wise extinction of marine reptile clades adapted to near-shore/benthic environments may be linked to these changes. Only fully pelagic forms survived into the Jurassic. Other marine organisms continued to thrive until the terminal Triassic extinction event suggesting that marine reptiles may have been especially sensitive to fluctuations in sea-level and reduction in nearshore habitat.