The Jurassic/Cretaceous boundary: a hidden mass extinction in tetrapods?

Abstract

Reconstructing deep time trends in biodiversity remains a central goal for palaeobiologists, but our understanding of the magnitude and tempo of extinctions and radiations is confounded by uneven sampling of the fossil record. In particular, the Jurassic/Cretaceous (J/K) boundary, 145 million years ago, remains poorly understood. By applying a range of techniques for assessing changes in diversity, I demonstrate that both marine and non-marine tetrapod faunas show evidence for a protracted period of regional and global ecological and taxonomic reorganisation across the J/K boundary. Although much of the signal is exclusively European, almost every higher tetrapod group was affected by a substantial decline across the boundary, culminating in the extinction of several important clades and the ecological release and radiation of numerous modern tetrapod groups, including amphibians, birds and sharks. Groups such as pterosaurs and sauropods began their decline before the J/K boundary, whereas others (including mammaliaforms and ornithischians) did not appear to be affected at the J/K boundary, but declined subsequently in the earliest Cretaceous. However, the majority of clades document their greatest magnitude of decline through the Jurassic–Cretaceous boundary, indicating that the overall extinction tempo was staggered and occurred in a ‘wave’ through the J/K transition. These major shifts in tetrapod diversity are shown to be independent of both global and regional sampling proxies, except for the North American record for which evidence of the common cause hypothesis is strong. Variation in eustatic sea level was the primary driver of these patterns, controlling biodiversity through availability of shallow marine environments and via allopatric speciation on land. I further investigated the systematics of Atoposauridae, poorly known group of highly-specialised crocodyliforms that appear to have crossed through the J/K boundary. A detailed revision of their taxonomy and systematics indicates that they went extinct at the J/K boundary.