Drivers of elasmobranch diversity in deep time

The current biodiversity on our planet is experiencing an anthropogenically driven extinction event, very similar to past mass extinctions, with global extinction rates being elevated up to a thousand times higher than in former extinction events as documented by the fossil record. Despite being well-adapted predators, this new mass extinction event doesn’t spare sharks, rays, and skates, which face many threats today, with an estimated one third of all species being threatened according to the IUCN Red List criteria. However, it is mandatory to study biodiversity dynamics of elasmobranchs (sharks, rays, skates) in deep time to better understand the current effects of environmental changes on and the future for these fishes.
Isolated teeth predominantly characterize the fossil record of elasmobranchs, which renders recognizing the timing of their origin difficult. The morphological variety of teeth and other elasmobranch remains from the Triassic and Early Jurassic implies that elasmobranchs including both stem and crown members were already very diverse and might have been far more diverse than currently known. In the Late Triassic, a major radiation and diversification of elasmobranchs, which probably all represent stem members, is supposed to have occurred in relation to the Rhaetian transgression, which created an extensive shallow epicontinental sea over most of western Europe providing suitable environments for elasmobranchs. The status of all pre-Jurassic elasmobranchs including synechodontiforms, however, needs to be re-evaluated. After the Late Triassic mass extinction, a rapid diversification and expansion of crown elasmobranchs can be observed in the Early Jurassic, with the first substantial divergence into the two major shark clades Galeomorphii and Squalomorphii, as well as the first fossil appearance of batoids in the Toarcian. So far, crown elasmobranchs thus only are known to extend back into the earliest Jurassic.
In the Late Jurassic, batoids became more diverse and abundant and, except for squaliforms and pristiophoriforms, all major clades of elasmobranchs already occurred in marine habitats, due to a great radiation and by the Cretaceous, also freshwater habitats were inhabited. After a further diversification event during the latest Jurassic and the earliest Cretaceous, the first shark faunas with truly modern appearance are supposed to have been emerged during the Early Cretaceous. The earliest squaliforms as well as various basal lamniforms are known from this time, while pristiophoriform sharks were the last modern sharks that probably evolved around the Early/Late Cretaceous boundary. Although the beginning of the Cretaceous thus seems to be an important time interval in the evolution of modern elasmobranchs, assemblages from this time nevertheless are scarce and insufficiently described.
At the end of the Mesozoic, elasmobranch fishes were already very abundant and diverse, with more than 200 known fossil species. During the K/Pg-boundary extinction event, elasmobranchs suffered significant losses at all systematic levels, while new taxa replaced those going extinct in the Palaeogene. The severity of this mass extinction is also reflected by the ecological impact to the ecosystem structure, which led to a faunal turnover in lamniform sharks from big oceanic apex predators (pre-extinction event) to small fish eaters (post extinction event; Lilliput effect). The Neogene, finally witnessed the major diversification events and the appearance of modern taxa contributing to the modern elasmobranch diversity.
The ultimate goals is to analyse possible biotic and abiotic drivers and environmental perturbations of major transitions in crown elasmobranch evolution to better understand their diversity and diversification patterns in deep time. Relationships between morphology, biological behaviour and environmental context in the fossil record will be established and the observed adaptations in the past will be compared to today’s conditions for better understanding possible responses to external influences. Also of importance will be the resilience of individual groups in terms of diversity.