RETHINKING THE MASS EXTINCTION AT THE TRIASSIC-JURASSIC BOUNDARY

Many paleontologists have long described the end-Triassic extinction as one of the five largest extinctions in Earth history. Estimates of the severity vary, but extinction of ~76% of species and ~20% of families is commonly cited. Some recent analyses posit a sudden, synchronous extinction on both land and sea attributed to a catastrophic cause, such as bolide impact or widespread volcanism. However, the apparent suddenness of this extinction is largely an artifact of poor stratigraphic resolution and thus an excellent example of the compiled correlation effect. According to most workers, the most severely affected groups in the marine realm were the ammonoids, bivalves, articulate brachiopods, conodonts, and marine reptiles, and plants and tetrapod vertebrates on land. A careful reading of the stratigraphic distribution of fossils across the Triassic-Jurassic boundary indicates a prolonged turnover of detectable duration, not a single mass extinction. Most taxa that supposedly became extinct at the end of the Triassic actually disappeared by the beginning of the Rhaetian, and further extinctions occurred stepwise during the Rhaetian. The apparent suddenness of the extinction of ammonoids, bivalves, brachiopods and conodonts disappears upon careful examination of detailed stratigraphic data. Extinction of marine reptiles was most substantial at the Ladinian-Carnian boundary, not at the end Triassic. Extinction of megafossil plant species at the end of the Triassic appears to be mainly regional, and the supposed dramatic palynological extinction in the Newark Supergroup is both less sudden than generally assumed and at most a local event. Tetrapod extinctions on land are difficult to document because the extensive tetrapod fossil record jumps from Norian to Sinemurian with little between. Furthermore, tetrapod footprints do not document a mass extinction, only modest turnover. Indeed, only radiolarians appear to show a sudden, massive evolutionary turnover at the end of the Triassic-200 years of fossil collecting have failed to document a sudden mass extinction of any other major taxon at the end of the Triassic.