Evolutionary models in the Early Triassic marine realm

Abstract

The relative influences of extrinsic compared to intrinsic drivers of evolutionary change have long been theorized and debated in the fossil record. Ecological recoveries from mass extinction events present records in which to examine these contrasts. Competition in a low diversity world, reproductive strategy, reconstruction of trophic systems and ecological specialization represent possible intrinsic controls on diversification. Feedback between diversity and abundance shifts of marine organisms with biogeochemical cycling and environmental conditions act as extrinsic controls on recovery process and rate. Disentangling these evolutionary pressures is a major challenge for understanding evolutionary recovery from mass extinction.The end-Permian mass extinction (251.88Ma) represents the largest mass extinction in Earth history and led to an extended recovery interval which lasted the duration of the Early Triassic (~4.8Myr) and beyond. Recent analyses suggest that the survivors of the mass extinction were biased toward organisms with higher metabolic rates that were more resilient against the volatile environmental changes that pervaded the Early Triassic including extreme temperature events, low pH, and low oxygen conditions. We use the Early Triassic recovery of gastropods, echinoids, and ammonoids to examine the processes of taxonomic and ecological evolution in response to, or in spite of, extrinsic environmental perturbations.The case studies of benthic gastropods and echinoids, when compared to pelagic ammonoids reflect similarities and differences in recovery following the end-Permian mass extinction. Gastropods and echinoids exhibit evidence of strong extrinsic environmental limitations which implicate the availability of refugia as a control on recovery. Low initial taxonomic diversity of survivors may have also limited the evolutionary recovery of both clades. Abundant and diverse microgastropod assemblages are interpreted as an adaptation to extreme environmental conditions. The morphological diversity of disarticulated echinoid spines and plates described in the southwestern United States, and examination of phylogenetic ghost lineages hints at a significant “hidden diversity” of Early Triassic echinoids. Ammonoids experienced taxonomic resets but are shown to be resilient to repeated environmental perturbations in the Boreal Ocean over the duration of the Early Triassic. Ammonoids may have adapted to persistent latitudinal temperature gradients and oxygen minimum zones that developed in the Early Triassic ocean basins.