Omphalosauridae (Ichthyosauriformes): enigmatic marine reptiles in the global recovery from the end-Permian...

Omphalosauridae are an under-appreciated part of the biotic recovery from the end-Permian Mass Extinction. This clade, including the genera Omphalosaurus, Sclerocormus and Cartorhynchus, is an enigmatic radiation of secondarily marine reptiles related to ichthyosaurs (Ichthyopterygia). Found across the Northern Hemisphere, omphalosaurid fossils invariably originate from open marine rocks of Olenekian (Early Triassic, 250-247 Ma) to earliest Ladinian (Middle Triassic, 237 Ma) age. The most distinctive feature (and the most common kind of fossil) of Omphalosauridae is its peculiar maxillary and dentary tooth batteries, presumably adapted for grinding up shelled cephalopods (ammonoids). Here, we report the first three-dimensionally preserved omphalosaurid skull and associated neck. The fossil originates from the Olenekian Vikinghøgda Formation of the Arctic archipelago of Svalbard and is assigned to a new genus and species, Ammonitritor scientiacivium. Phylogenetic analysis recovers a well-supported Omphalosauridae as the sister group to a clade comprising the Chinese Chaohusaurus and Ichthyopterygia. The most basal member of the clade is Cartorhynchus, followed by Ammonitritor. The clade Nasorostra, previously proposed for the Chinese omphalosaurids Cartorhynchus and Sclerocormus, was not recovered. Olenekian omphalosaurids likely attained maximum body lengths exceeding 6 m, while Middle Triassic forms reached well over 10 m. Thus, some of the oldest marine reptiles from Panthalassa were already large-bodied at their first appearance, approximately 2 million years after the end-Permian extinction. Together with other Ichthyosauriformes, omphalosaurids represent the only marine reptile clade generally present in Panthalassa during the Early and Middle Triassic.