Revision of Undorosaurus, a mysterious Late Jurassic ichthyosaur of the Boreal Realm

Recent study of ophthalmosaurid ichthyosaurs has brought us a number of new taxa; however, the validity of several ophthalmosaurid taxa from the Volgian (Tithonian) of European Russia still remains unclear, complicating comparisons and in some cases affecting taxonomic decisions about new contributions. A revision of the type series of all three species of Undorosaurus, erected by Efimov in 1999, reveals the potential validity of two of them. This contradicts previous research, which concluded that only the type species, U. gorodischensis, is valid. Furthermore, examination of the holotype of Cryopterygius kristiansenae from coeval strata of Svalbard shows that it is synonymous with Undorosaurus gorodischensis, sharing all diagnostic features of the species, especially those related to forelimb morphology: humerus with extensive anteroposteriorly elongate proximal end, poorly pronounced trochanter dorsalis and reduced deltopectoral crest; and ulna proximodistally elongate and not involved in perichondral ossification on its whole posterior edge. This supports the idea of intensive exchange of ichthyosaurs between the Middle Russian Sea and other Boreal seas in the Late Jurassic. In order to resolve the phylogenetic position of Undorosaurus within Ophthalmosauridae as well as relationships of other ophthalmosaurids, a new data set including 33 taxa and 106 characters, 23 of which are new, was compiled. The results of this analysis challenge all previous phylogenetic hypotheses for Ophthalmosauridae in a number of aspects, including the fact that Undorosaurus spp. was recovered deeply nested within Platypterygiinae as a sister group to derived platypterygiines.