Simbirskiasaurus and Pervushovisaurus reassessed: implications for the taxonomy and cranial osteology of Cretaceous platypterygiine ichthyosaurs

The ichthyosaur fossil record is interspersed by several hiatuses, notably during the Cretaceous. This hampers our understanding of the evolution and extinction of this group of marine reptiles during the last 50 million years of its history. Several Cretaceous ichthyosaur taxa named in the past have subsequently been dismissed and re- ferred to the highly inclusive taxon Platypterygius, a trend that has created the impression of low Cretaceous ichthyosaur diversity. Here, we describe the cranial osteology, reassess the stratigraphic age, and evaluate the taxonomy and phylogenetic relationships of two Cretaceous ichthyosaurs from western Russia: Simbirskiasaurus birjukovi from the early Barremian and Pervushovisaurus bannovkensis from the middle Cenomanian, both for- merly regarded as nomina dubia, and allocated to Platypterygius sp. and Platypterygius campylodon, respectively. We show that Simbirskiasaurus birjukovi and Pervushovisaurus bannovkensis are valid platypterygiine ophthalmosaurids, notably characterized by a peculiar narial aperture. The cranial anatomy and phylogenetic relationships of these taxa illuminate the evolution of narial aperture anatomy in Cretaceous ichthyosaurs, clarify the phylogenetic re- lationships among platypterygiines, and provide further arguments for a thorough revision of Platypterygius.