A. V. Nikiforov’s scientific contributions

Since the discovery of the holotype of Polycotylus sopozkoi in the Southern Urals, several skeletons referable to Polycotylus were collected from the upper Santonian – lower Campanian bone bed at its type locality (Izhberda quarry, Orenburg Region, Russia). Here we describe these new specimens, which together with the holotype constitute an ontogenetic series from small juveniles with the propodial length of 27 cm to a large adult individual with the propodial length exceeding 47 cm and traces of senile pathologies of limb elements. The new specimens significantly expand the knowledge of the morphology and variation of the postcranial skeleton in Polycotylus. They also reveal additional diagnostic features of Polycotylus sopozkoi (28 cervical vertebrae, one of the highest counts for polycotylines, and strongly tapered anterodistal processes of cervical ribs), and shared traits with P. latipinnis (divergent anteromedial processes and pronounced anterior shelf of the coracoid), which were previously unnoticed in the known specimens and supplement the generic diagnosis. The observed individual variation in Polycotylus sopozkoi has implications for phylogenetic and ontogenetic studies of plesiosaurs, and for assessment of the taxonomic value of some characters in polycotylids.

Изучение новых находок мозазаврид из Ижбердинского местонахождения (Южный Урал, Оренбург- ская обл.) позволило впервые установить для верхнего мела Оренбуржья присутствие мозазавров из подсемейств Mosasaurinae, Tylosaurinae и Plioplatecarpinae, в т.ч. представителей родов Mosasaurus, Prognathodon и Clidastes, известных из верхнего мела Северной Америки и Западной Европы. Интерес представляет обнаружение тилозаврина Taniwhasaurus, прежде известного из Новой Зеландии, Ан- тарктики, Южной Африки и Японии. Таким образом, кампанская фауна мозазаврид Южного Урала является смешанной и включает в свой состав североамериканско–европейские и азиатско–тихооке- анские таксоны. Однако все находки мозазавров из Ижбердинского местонахождения можно опре- делить лишь в открытой номенклатуре, что затрудняет подробные сравнения фаун. Ревизия типовой серии мозазавра Liodon rhipaeus Bogolubov, 1910 из Южного Урала показала, что в нее были также ошибочно включены кости плезиозавра. Liodon rhipaeus является nomen dubium. Типовой материал по этому виду определен как Tylosaurinae indet.

The study of new finds of mosasaurids from the Izhberda locality (Southern Urals, Orenburg Region) has made it possible to record for the first time the presence of mosasaurs from the subfamilies Mosasaurinae, Tylosaurinae, and Plioplatecarpinae from the Upper Cretaceous of the Orenburg Region, including representatives of the genera Mosasaurus, Prognathodon, and Clidastes, which are known from the Upper Cretaceous of North America and Western Europe. Of interest is the discovery of the tylosaurine Taniwhasaurus, previously known from New Zealand, Antarctica, South Africa, and Japan. Therefore, the Campanian mosasaurid fauna of the Southern Urals is intermediate and includes North American–European and Asia–Pacific taxa. However, all the finds of mosasaurs from the Izhberda locality can only be identified in open nomenclature, which makes detailed comparisons of the faunas difficult. Revision of the type series of mosasaur Liodon rhipaeus Bogolyubov, 1910 from the Southern Urals has allowed us to conclude that, in addition to the three mosasaurian vertebrae, it includes the ischium and posterior fragment of the mandible of a plesiosaur. Liodon rhipaeus is clearly a nomen dubium and the vertebrae of its type series cannot be iden- tified more precisely than Tylosaurinae indet.

Polycotylids are among the most common plesiosaurians of the Late Cretaceous, however, in Eurasia their findings are rare and fragmentary. In 2016, a partial polycotylid skeleton from the Upper Cretaceous of the Izhberda quarry in the Southern Urals region was described by Efimov et al. as a new species, Polycotylus sopozkoi. Here we revise this holotype specimen and show that many characters initially proposed to distinguish the species are the result of misinterpretations. However, P. sopozkoi is indeed referable to Polycotylus and is highly similar to its type species, P. latipinnis. Although only one distinctive trait of the species noted by Efimov et al., the protruding basioccipital tubera with deep carotid canals on their anterodorsal surface, is confirmed here, new observations revealed additional features that allow us to substantiate the validity of P. sopozkoi. The presence of Polycotylus in the Upper Cretaceous of North America and Eastern Europe highlights a wide distribution of some plesiosaurian genera and suggests caution in assumptions of ‘endemic’ plesiosaurian taxa in particular regions of the world.