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 stan dpoint, their similarities include: virtually identical extinction intensit y at the familial and generic levels, widespread basinal facies deposited d uring sea-level highstands, an overall greenhouse climate, and anoxia as an important causal mechanism. The high-resolution, macroinvertebrate data an alyzed here, consisting of stratigraphic ranges, diversity and abundance, p oint to smaller-scale similarities. The two events resulted in significant ecological disruption and, in both cases, the biotic responses were very si milar. Taxa inhabiting the upper water column were unaffected by anoxia and included ammonites and, in the eTo, belemnites. In addition, epifaunal tax a adapted to low-oxygen conditions, such as the buchiids, posidoniids and i noceramids, flourished in the post-extinction environment during the surviv al interval. As conditions ameliorated, the biota became more diverse and g radually began to resemble pre-extinction biotas. Furthermore, the delta(13 )C curves predict the end of the survival interval and suggest that the per iod characterized by carbon isotope excursions represent disrupted environm ental conditions. This points to the potential application of delta(13)C as a tool for determining the repopulation modes and timing for other mass ex tinctions. (C) 1999 Elsevier Science B.V. All rights reserved.