Revision ofNannopterygius(Ichthyosauria: Ophthalmosauridae): reappraisal of the ‘inaccessible’ holotype resolves a taxonomic tangle and reveals an obscure ophthalmosaurid lineage with a wide distribution

Abstract

The Late Jurassic ichthyosaur Nannopterygius is among the poorest known, with the only skeleton, NHMUK PV 46497, on display in the Natural History Museum, London and, therefore, difficult to access. This holotype specimen is here reassessed. The newly obtained data have enabled the identification of several additional specimens of Nannopterygius in museum collections across the UK. Furthermore, all the material of Russian ichthyosaurs previously referred to genera Paraophthalmosaurus and Yasykovia, and considered as junior synonyms of Ophthalmosaurus in the majority of subsequent works, are also reassessed. Both these genera are synonymized with Nannopterygius with preservation of the two from six originally erected species: Nannopterygius saveljeviensis comb. nov. and Nannopterygius yasykovi comb. nov. Additionally, a new species from the Berriasian of Arctic (Svalbard and Franz Josef Land) is proposed. To resolve the phylogenetic relations within Ophthalmosauria, a revised dataset, including 44 taxa and 134 characters, 20 of which are new, was compiled. The results of a phylogenetic analysis places Nannopterygius spp. as sister to Arthropterygius spp. within Ophthalmosaurinae. Thus, the lineage of Nannopterygius was among several ophthalmosaurine lineages that crossed the Jurassic–Cretaceous boundary and, similarly to Arthropterygius, survived the Jurassic–Cretaceous transition at high latitudes.

A new ophthalmosaurid ichthyosaur from the Upper Jurassic (Early Tithonian) Kimmeridge Clay of Dorset, UK, with implications for Late Jurassic ichthyosaur diversity

“…The main topology of the analysis shows Ophthalmosauridae diverges into three clades; Ophthalmosaurinae and Platypterygiinae, and a third, Nannopterygius clade including Nannopterygius + Thalassodraco and Gengasaurus. This clade has been recovered in previous cladistic analyses by Zverkov & Efimov [60], Zverkov & Prilepskaya [61], and Zverkov & Jacobs [49].…”

“…The phylogenetic analysis resulted in a strict consensus tree of 64 most parsimonious trees of 325 steps in length, with a consistency index (CI) of 0.397 and a retention index (RI) of 0.652 (Fig 18). The overall topology does not differ significantly from that of Zverkov & Jacobs [49].…”

“…For the phylogenetic analysis, we used the matrix of Zverkov & Jacobs [49]. The data matrix consisted of 34 taxa and 112 characters.…”

“…Despite falling in the same clade there are considerable differences between T. etchesi, Gengasaurus [62] and Nannopterygius [49]. Notably T. etchesi has a wide postorbital bar of the jugal, whereas in Nannopterygius, this element is thin and gracile (char.…”

“…T. etchesi possesses a elongate supranarial process of the premaxillae, contacting the external naris, which is a character shared with Brachypterygius [19], Caypullisaurus [67][68][69], Nannopterygius [49], Aegirosaurus [55,70] and but is not present in Sveltonectes [71]. In Ophthalmosaurus [18], Arthropterygius [61,72] and Undorosaurus [60,73] the supranarial process is reduced.…”