Preservation of exceptional vertebrate assemblages in Middle Permian fluviolacustrine mudstones of Kotelʹnich, Russia: stratigraphy, sedimentology, and taphonomy

The Kotelʹnich locality in European Russia has long been a rich source of high-quality tetrapod fossils, including pareiasaurs, dicynodonts, gorgonopsians and theriodonts. The age of the Kotelʹnich locality has been debated, but it corresponds to early Severodvinian in the Russian stratigraphic scheme, equivalent to the late Capitanian (late Middle Permian) on the international time scale. Remarkably, the majority of specimens are complete, quite unlike those from most Russian Permo-Triassic red bed localities; commonest of all are 1–2-metre long pareiasaur skeletons of the genus Deltavjatia, preserved in hollows on top of a consolidated palaeosol horizon. Previous taphonomic scenarios in the Russian literature have included suggestions that the animals were overwhelmed beneath sand dunes, mired in soft fluviatile sediments, caught at the bottom of a deep lake, trapped in burrows, or dumped in fluviatile scours. It is probable that the pareiasaurs were searching for water in a time of catastrophic aridification, and died, weakened, in shallow hollows. In this case, we also emphasise the importance of floodplain microtopography in creating the sedimentary conditions necessary for the preservation of exceptional vertebrate assemblages in a slowly aggrading fluviolacustrine setting.