ИСТИНА

Pliosaurid plesiosaurians played important roles in marine food chains from the Middle Jurassic to the mid Cretaceous, frequently as apex predators. All hitherto described pliosaurid species of the Kimmeridgian to the Hauterivian possess trihedral and subtrihedral teeth with a smooth labial surface, while Aptian–Turonian pliosaurids have exclusively conical crowns with apicobasal ridges typically arranged around the entire circumference, suggesting, for each of these time intervals, a restricted range of tooth morphologies, and thus, possibly, of ecological niches. However, current knowledge of pliosaurids comes mostly from several restricted time intervals and areas; the Jurassic-Cretaceous transition remains a persistent gap in our knowledge of pliosaurid evolution. Consequently, the effect of the Jurassic–Cretaceous boundary faunal turnover on pliosaurids is poorly understood. To tackle this issue, we describe a series of pliosaurid teeth ranging from the upper Volgian (Tithonian, Upper Jurassic) of the Kheta river basin (Eastern Siberia, Russia), to the Berriasian and Valanginian (Lower Cretaceous) of the Volga region (European Russia) and analyse the evolution of pliosaurid teeth through time. Unexpectedly, these teeth belong to widely distinct morphotypes, including the first report of conical-toothed pliosaurids from the latest Jurassic–earliest Cretaceous interval. This challenges the hypothesis that only one lineage of pliosaurids crossed the Jurassic–Cretaceous boundary. It appears that conical-toothed pliosaurids coexisted with their trihedral-toothed relatives for at least 25 million years over the Jurassic-Cretaceous transition. In fact, our quantitative analyses indicate that pliosaurids reached their maximal dental disparity (in both size and shape) during this interval, showing little evidence of a turnover associated with the Jurassic–Cretaceous boundary. Instead, disparity decreased later in the Early Cretaceous, with the disappearance of trihedral-toothed forms in the Barremian. Pliosaurids thus decreased their dental disparity (and thus, possibly, their ecological diversity) long before their extinction in the early Late Cretaceous.