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

Abstract

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 the comparisons and in some cases affecting taxonomic decisions of 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 the coeval strata of Svalbard show 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 relations of other ophthalmosaurids, a new dataset 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 Undorosaurus spp. recovered deeply nested within Platypterygiinae as a sister group to derived platypterygiines.

Ontogeny of the braincase inStenopterygius(Reptilia, Ichthyosauria) from the Lower Jurassic of Germany

“…In articulated material, this curvature would create a distinct quadrate foramen (the posterior-medial opening between the quadrate and the quadratojugal; 'elliptical/oval foramen' of McGowan, 1973b), as seen in other Jurassic ichthyosaurs (e.g., Temnodontosaurus, Ichthyosaurus, Ophthalmosaurus, and Undorosaurus;McGowan, 1973b;Maisch and Matzke, 2000: fig. 13;Moon and Kirton, 2016;Zverkov and Efimov, 2019). In adults, the ovoid stapedial facet is located (postero)ventrally on the medial side of the quadrate, just dorsal to the articular condyle.…”

A prevalence ofArthropterygius(Ichthyosauria: Ophthalmosauridae) in the Late Jurassic—earliest Cretaceous of the Boreal Realm

“…We know this due to several Late Jurassic formations that yielded significant ichthyosaur materials. These are primarily Kimmeridge Clay Formation of England and France (Hulke, 1871; Mansell-Pleydell, 1890; Sauvage, 1911; Delair, 1960, 1986; McGowan, 1976, 1997; Grange et al, 1996; Etches & Clarke, 1999; Moon & Kirton, 2016), the Solnhofen Formation of Germany (Wagner, 1852, 1853; Meyer, 1864; Bauer, 1898; Bardet & Fernández, 2000), the Vaca Muerta Formation of Argentina (Fernández, 1997, 2000, 2007a, 2007b; Gasparini, Spalletti & De La Fuente, 1997, Gasparini et al, 2015), the Agardhfjellet Formation of Svalbard, Norway (Angst et al, 2010; Druckenmiller et al, 2012; Roberts et al, 2014; Delsett et al, 2016, 2017) and a number of formations of the Volgian (Tithonian) age in European Russia (Kabanov, 1959; Efimov, 1998, 1999a, 1999b; Arkhangelsky, 1997, 1998, 2000, 2001a, 2001b; Zverkov, Arkhangelsky & Stenshin, 2015; Zverkov et al, 2015; Zverkov & Efimov, 2019). Still our knowledge of the Late Jurassic ichthyosaurs is non-uniform: some taxa are quite well known owing to relatively complete and well-preserved specimens ( Ophthalmosaurus Seeley, 1874; Grendelius McGowan, 1976; Caypullisaurus Fernández, 1997; Aegirosaurus Bardet & Fernández, 2000; Undorosaurus Efimov, 1999b), whereas others are poorly known from only a small number of largely incomplete and/or poorly preserved specimens (e.g., Nannopterygius Huene, 1922, Brachypterygius Huene, 1922, and Arthropterygius Maxwell, 2010).…”

“…In recent years, the Slottsmøya Member of the Agardhfjellet Formation of Svalbard has yielded numerous marine reptile specimens including four monotypic ichthyosaur genera, for most of which only one specimen is known (Druckenmiller et al, 2012; Roberts et al, 2014; Delsett et al, 2017). Recently, it has been proposed and argued that one of the ichthyosaur genera from Svalbard, “ Cryopterygius ,” is a junior subjective synonym of Undorosaurus Efimov, 1999b (Zverkov & Efimov, 2019). The other three genera from Svalbard are discussed in the present contribution and are all considered as junior subjective synonyms of Arthropterygius .…”

“…At the same time, “ Macropterygius ” do not have a facet for an anterior accessory epipodial element, and, furthermore, it has slightly anteriorly deflected radial facet, which differs from distally faced radial facets in Arthropterygius , Ophthalmosaurus and some other opthalmosaurids. The variation of the humerus in ophthalmosaurids has not been previously assessed with implementation of morphometric techniques, although this skeletal element has a complex morphology that gives a number of phylogenetically informative characters (see characters in e.g., Fischer et al, 2012; Moon, 2019; Zverkov & Efimov, 2019). In order to highlight this, we gather a new dataset and run the principal component analysis (PCA) of ophthalmosaurid humeral morphology.…”

Late Jurassic – earliest Cretaceous prolonged shelf dysoxic–anoxic event and its possible causes

“…Remains of coleoid molluscs (i.e. Rogov & Bizikov, 2006) and marine reptiles, especially ichthyosaurs (Zverkov & Efimov, 2019), are also very typical here.…”