Thalattosuchian crocodylomorphs from European Russia, and new insights into metriorhynchid tooth serration evolution and their palaeolatitudinal distribution

Abstract

From the Middle Jurassic to the Early Cretaceous, metriorhynchid crocodylomorphs inhabited marine ecosystems across the European archipelago. Unfortunately, European metriorhynchids are only well known from Germany, France, and the UK, with the Eastern European fossil record being especially poor. This hinders our understanding of metriorhynchid biodiversity across these continuous seaways, and our ability to investigate provincialism. Here we describe eleven isolated tooth crowns and six vertebrae referable to Metriorhynchidae from the Callovian, Oxfordian, Volgian (Tithonian), and Ryazanian (Berriasian) or Valanginian of European Russia. We also describe an indeterminate thalattosuchian tooth from the lower Bajocian of the Volgograd Oblast, the first discovery of a marine reptile from the Bajocian strata of European Russia. These rare fossils, along with previous reports of Russian thalattosuchians, indicate that thalattosuchians have been common in the Middle Russian Sea since it was formed. Palaeolatitude calculations for worldwide metriorhynchid-bearing localities demonstrate that the occurrences in European Russia are the most northern, located mainly between 44–50 degrees north. However, metriorhynchids appear to be rare at these palaeolatitudes, and are absent from palaeolatitudes higher than 50°. These observations support the hypothesis that metriorhynchids evolved an elevated metabolism but were not endo-homeothermic, especially as endo-homeothermic marine reptiles (ichthyosaurs and plesiosaurs) remained abundant at much higher palaeolatitudes.

Callovian Marine Reptiles of European Russia

“…Among metriorhynchids, Young et al [40] reported several indeterminate metryorhynchid teeth, cf. Thalattosuchus, and two morphotypes of Geosaurini indet.…”

“…Thalattosuchus and Tyrannoneustes sp. [40], all of which have no marked differences from their contemporary congeners of Western Europe. This is not surprising, as the invertebrate faunas of the middle and late Callovian are also highly similar in these basins [91,144].…”

“…Recently Young et al [40] described isolated thalattosuchian teeth from the Callovian of European Russia, therefore, we refer the reader to that contribution for details. Young et al [40] identified three teeth from the lower Callovian of Unzha Village as Metriorhynchidae indet., and one more crown, from nearby Mikhalenino Village, as Geosaurini indet. (NB localities "Unzha" and "Mikhalenino" may represent different records of one natural outcrop near the villages Unzha, Mikalenino, and Popovo).…”

“…Two crowns from the middle Callovian of Gzhel and Mikhaylovcement were referred to emphTyrannoneustes sp. [40]. In autumn 2023, a cervical vertebra (SGM 2007-02) from the Callovian of Mikhaylovcement Quarry was donated to SGM by Konstantin Volkov.…”

“…More recent works on the Callovian marine reptiles from Russia include (in addition to the aforementioned contribution of Benson et al [36]) a short report on a finding of the right maxilla of Simolestes from the lower Callovian of the Kostroma Region [37], a report on a finding of a partial ophthalmosaurid skeleton from the lower Callovian of the Republic of Mordovia [38], and an account of Jurassic marine reptiles from Moscow and adjacent territories [39] that provided brief descriptions and figures of pliosaurid and thalattosuchian tooth crowns, as well as several plesiosaurian and ichthyosaurian bones from the middle and upper Callovian of the Moscow and Ryazan regions. Finally, Young et al [40] recently examined all the available thalattosuchian specimens from Russia and described, among others, several metriorhynchid tooth crowns from the Callovian.…”