The importance of geochronology for interpreting non-marine recovery from the end-Permian mass extinction

Abstract

Over the past two decades, a critical question has been the nature and timing of ecological recovery from the end-Permian mass extinction. With the advent of highly accurate and precise CA-ID-TIMS U-Pb dating of interbedded volcanic zircons, key Early-Middle Triassic sedimentary archives of marine fossil assemblages are now well-constrained by absolute ages. These data demonstrate that the Early Triassic was relatively short (~4.7 Ma), and that ultimate recovery of most marine ecosystems did not occur until the beginning of the Middle Triassic, some five million years after the extinction event. Recent work in fossil non-marine ecosystems suggests that recovery in these ecosystems was also delayed until the Middle Triassic, with elevated endemicity in these early Middle Triassic recovery assemblages. However, proper interpretation of these non-marine fossil patterns is severely limited by the fact that most key fossiliferous strata have been dated using long-distance vertebrate biostratigraphy, and remain largely unconstrained by precise and accurate absolute ages. Thus, it is largely unknown whether ?Early Triassic? and ?Middle Triassic? non-marine strata are actually assignable to these chronostratigraphic units (whose boundaries are defined in marine sections), and the timing of extinction recovery is unclear. New U-Pb ages from multiple basins in Argentina suggest several iconic ?Middle Triassic? assemblages are actually Late Triassic (Carnian) in age. Preliminary age data from the supposedly Anisian upper Moenkopi Formation of the western United States suggest these strata could be as young as upper Ladinian. The iconic and intensely studied fossil records of southern and eastern Africa remain unconstrained by absolute ages, so any age model applied to these data is largely heuristic. The unknown or Late Triassic ages of many ?Middle Triassic? assemblages means they cannot speak to ecological recovery from the end-Permian mass extinction. Suggestions of Middle Triassic endemicity among non-marine ecosystems may instead reflect differing ages among the different assemblages sampled. As such, these new radioisotopic ages demonstrate that we know relatively little about the non-marine recovery from the mass extinction, and more broadly, the importance of accurate and precise biostratigraphically-independent age constraints when interrogating the fossil record with macroevolutionary and paleoecological questions.