A SERIES OF BIOTIC CRISES MARK THE MID-PHANEROZOIC TRANSITION FROM PALEOZOIC TO MODERN MARINE FAUNAS

With the early analyses of the Sepkoski marine Phanerozoic diversity curve in the 1980's and the development of an understanding of the “Big 5” mass extinctions, it has become deeply embedded in our thinking that a single event, the end-Permian mass extinction, the biggest of the “Big 5”, was the cause of the change from Paleozoic to Modern marine evolutionary faunas. Subsequent studies of this time interval have involved intensive global documentation of the fossil content, sedimentology, and chemostratigraphy of key stratigraphic sections. And, as our understanding of this interval in time has become more detailed, we have been able to recognize a finer structure to this greatest of Phanerozoic marine faunal transitions. Rather than being just one single event, it comprises a long period of environmental stress, beginning at the end of the Middle Permian with the end-Guadalupian (~260 Ma) biotic crisis. This is followed by what is indeed the biggest of mass extinctions at the end of the Permian (~252 Ma). Until the last decade or so it was not widely recognized that the following Early Triassic period was a time of continuing environmental stress, and it now appears that this 4-5 million-year time interval was punctuated by several intervals of global biotic crisis. This occurrence of repeated global biotic crises in a period of ~12 million years provided the evolutionary and environmental stage that initiated the largest known marine faunal transition in Phanerozoic animal evolution. A related mass extinction at the Triassic-Jurassic (~200 Ma) boundary ~50 million years later helped push marine faunas further into the Modern mode. This ~60 million-year long period of punctuated environmental stress, related to the broad atmospheric and oceanographic effects of flood-basalt volcanism and the break-up of Pangea, provides an intriguing deep time laboratory to understand the response of Earth's marine biota to the overall consequences of significant changes in greenhouse gasses.