Undorosaurus

Undorosaurus was named by Vladimir M. Efimov in 1999 and the type species is Undorosaurus gorodischensis. The specific name is named after Gorodische, the type locality of this taxon.[4] U. trautscholdi is named in honor of the geologist H. Trautschold who collected and made the first description of the fossils of the holotype of the species.[2]

Undorosaurus was first known from the holotype UPM EP-II-20 (527), a partial three-dimensionally preserved skeleton which preserved partial skull. It was collected near the Volga river at Gorodische from the Epivirgatites nikitini ammonoid zone, dating to the Late Jurassic.[4] A second species, U. trautscholdi was described by M.S. Arkhangelsky and N.G. Zverkov in 2014 from a partial left forefin found in the locality of Mnevniki, in the Moscow Oblast.[2]

Maisch and Matzke (2000) regarded Undorosaurus to be a species of Ophthalmosaurus.[5] However, Storrs et al. 2000 rejected this synonymy based on the tooth morphology of the specimen.[6] Chris McGowan and Ryosuke Motani (2003) pointed out two noteworthy differences to Ophthalmosaurus, an incompletely fused ischiopubis and a remarkably strong dentition, and considered Undorosaurus to be a valid genus of ophthalmosaurid.[7] Undorosaurus's validity is now accepted by most authors, even by Maisch (2010) who originally proposed the synonymy.[8][9][10]

Zverkov & Efimov (2019) considered the genus Cryopterygius to be a junior synonym of the genus Undorosaurus. The authors considered the type species of the former genus, C. kristiansenae, to be synonymous with Undorosaurus gorodischensis; second species of Cryopterygius, C. kielanae, was tentatively maintained by the authors as a distinct species within the genus Undorosaurus.[11]

The following cladogram shows a possible phylogenetic position of Undorosaurus in Ophthalmosauridae according to the analysis performed by Zverkov and Jacobs (2020).[12]

• List of ichthyosaurs
• Timeline of ichthyosaur research