The early Toarcian (Early Jurassic) and the Cenomanian–Turonian (Late Cretaceous) mass extinctions: similarities and contrasts

The early Toarcian (eTo) and Cenomanian–Turonian (C–T) mass extinctions are comparable from a wide range of scales and perspectives. From a broad standpoint, their similarities include: virtually identical extinction intensity at the familial and generic levels, widespread basinal facies deposited during sea-level highstands, an overall greenhouse climate, and anoxia as an important causal mechanism. The high-resolution, macroinvertebrate data analyzed here, consisting of stratigraphic ranges, diversity and abundance, point to smaller-scale similarities. The two events resulted in significant ecological disruption and, in both cases, the biotic responses were very similar. Taxa inhabiting the upper water column were unaffected by anoxia and included ammonites and, in the eTo, belemnites. In addition, epifaunal taxa adapted to low-oxygen conditions, such as the buchiids, posidoniids and inoceramids, flourished in the post-extinction environment during the survival interval. As conditions ameliorated, the biota became more diverse and gradually began to resemble pre-extinction biotas. Furthermore, the δ13C curves predict the end of the survival interval and suggest that the period characterized by carbon isotope excursions represent disrupted environmental conditions. This points to the potential application of δ13C as a tool for determining the repopulation modes and timing for other mass extinctions.