Impact of early Toarcian climatic changes on marine reptiles: Extinction and recovery

Abstract

Environmental changes governed the diversity of marine ecosystems and the evolution of marine reptiles during the Jurassic. Abrupt climatic changes, mainly cooling, produced crises in marine ecosystems including marine reptiles, but global warming events at the Pliensbachian/Toarcian boundary and the early Toarcian Jenkyns Event led to a second order mass extinction. The Jenkyns Event coincides with exceptional preservation of marine reptiles in black shales, so widespread extinctions are masked to some extent by increases in specimens and diversity in the Lagerstätten. Marine reptile diversity responded to this biotic crisis; in the early Toarcian, stenopterygid ichthyosaurs and marine crocodylomorphs (thalattosuchians) diversified whereas some groups that survived the Jenkyns Event disappeared during the hyperthermal conditions of the middle and late Toarcian, including ichthyosaurs (leptonectids, temnodontosaurids, baso-parvipelvians), and sauropterygians (plesiosaurids, microcleidids).

After the crisis, Aalenian climates were cold and the sea-level low with a poor record of marine reptiles. The diversity of marine ecosystems increased from the early Bajocian with a turnover in ichthyosaurs (Early Jurassic ichthyosaurs were replaced by ophthalmosaurids), sauropterygians diversified and increased in size (mainly pliosaurids and cryptocleidids), and pelagosaurid thalattosuchians disappeared at the same time that machimosaurids diversified and the Metriorhynchidae appeared. Marine reptiles reached a new maximum of diversity during the Callovian, but the Callovian/Oxfordian transition was a time of climatic cooling and sea-level fall that negatively impacted on marine ecosystems, including the extinction of rhomaleosaurids. From the middle Oxfordian, the development of large epeiric platforms and increase of temperature favoured the diversification of marine ecosystems, included marine reptiles. During the Late Jurassic, ophthalmosaurids, pliosaurids, cryptocleidids, and metriorhynchids dominated, and marine turtles diversified in coastal environments (eurysternids) and open carbonate platforms (plesiochelyids and thalassemydids).