Nannopterygius borealis, Zverkov & Jacobs, 2021

Zverkov, Nikolay G. & Jacobs, Megan L., 2021, Revision of Nannopterygius (Ichthyosauria: Ophthalmosauridae): reappraisal of the ‘ inaccessible’ holotype resolves a taxonomic tangle and reveals an obscure ophthalmosaurid lineage with a wide distribution, Zoological Journal of the Linnean Society 191 : -

NANNOPTERYGIUS BOREALIS SP. NOV.

lsid:zoobank.org:act: F494C45A-A54E-4244-B815- 09B6D2D3F077

v. 2018 Ophthalmosauridae indet. – Delsett et al.: 34, figs 15, 16.

Holotype: PMO 222.658, partial nasal and several fragments of cranial bones; several vertebrae, coracoids, distal part of the scapula, incomplete left forelimb and proximal portion of the right humerus ( Fig. 22A–L).

Paratype: CCMGE 45–46 /13328, right humerus and caudal vertebra ( Fig. 22M–Q) .

Diagnosis: Nannopterygius borealis differs from other species of Nannopterygius in the following combination of features:coracoids with relatively wide rounded posterior portions (wider than in the type species and N. yasykovi but still not as wide as in other ophthalmosaurids); intercoracoidal facet lenticular in outline and dorsoventrally thin (like that of N. enthekiodon and unlike those with complex outlines in N. saveljeviensis and N. yasykovi ); humerus with accessory anterodistal facet (variable in N. enthekiodon , but also present in N. savelyeviensis and N. yasykovi ); humeral ulnar facet larger than the radial facet in both anteroposterior and dorsoventral width (non-unique autapomorpy; nearly equal in all other species); radius markedly smaller than ulna (unambiguous autapomorpy; comparable in size to ulna in other species); ulna with convex posterior margin lacking perichondral ossification (non-unique autapomorpy; concave and completely ossified in other species); intermedium wedging between radius and ulna, and equally contacting them as in N. enthekiodon (not wedging and having a short contact with ulna compared to that with radius in N. saveljeviensis and N. yasykovi ); limb elements rounded and loosely arranged in N. enthekiodon (polygonal and tightly packed in N. saveljeviensis and N. yasykovi ).

Occurrence: Late Volgian Jurassic–Cretaceous transitional interval (latest Tithonian or earliest Berriasian) of Svalbard; early Berriasian of Franz Josef Land.

Remarks: The specimen described by Delsett et al. (2018) can be referred to Nannopterygius based on its modest size (coracoid anteroposterior length = 155 mm, humerus proximodistal length = 103 mm); elongated coracoids with intercoracoid facet shifted far anteriorly relative to scapular facet, primarily occupying the anteromedial process. Other features observed in PMO 222.658 are also well consistent with Nannopterygius : narrow anterior notch of the coacoid; large scapular facet clearly demarcated from the glenoid contribution; wide and mediolaterally flattened (strap-like) dorsal ramus of the scapula; three distal articular facets of the humerus and well-pronounced deltopectoral crest.

The description of PMO 222.658 is provided by Delsett et al. (2018) and we do not consider it necessary to expand that description. The humerus CCMGE 45/13328 ( Fig. 22M–Q) is well consistent with that of PMO 222.658.

PHYLOGENETIC ANALYSIS

Our ‘unordered’ parsimony analysis resulted in 112 most parsimonious trees of 416 steps in length with the consistency index (CI) = 0.373 and retention index (RI) = 0.665. The strict consensus (length of 435 steps; CI = 0.356; RI = 0.640) is poorly resolved ( Fig. 22A). The recovered topology differs from the results of Zverkov & Efimov (2019) and Zverkov & Prilepskaya (2019), and, to a greter degree, from those of Delsett et al. (2019) and Campos et al. (2020). Our results support the division of Ophthalmosauridae (Ophthalmosauria) into two clades: Ophthalmosaurinae and Platypterygiinae sensu Fischer et al. (2012) or, alternatively, interpreted as two families Ophthalmosauridae and Undorosauridae (see discussion in: Zverkov & Efimov, 2019).

A clade that includes all species of Nannopterygius is no further positioned at the base of Ophthalmosauridae (Ophthalmosauria) as was recovered in the works of Zverkov & Efimov (2019) and Zverkov & Prilepskaya (2019). This clade is now recovered as a sister of Arthropterygius spp. within Ophthalmosaurinae. Contra to previous results, Gengasaurus is not in the Nannopterygius clade. This taxon is recovered in a polytomy with the other two main clades of ophthalmosaurines (ophthalmosaurids). The recovery of Arthropterygius within Ophthalmosaurinae ( Ophthalmosauridae ) supports the result of the pruned analysis of Zverkov & Prilepskaya (2019) and Barrientos-Lara & Alvarado-Ortega (2020).

The Nannopterygius and Arthropterygius clades share seven synapomorphies: appearance of jugal/ premaxilla contact (23.0/23.1; not unique); posterior margin of the jugal laterally excluded from contact with the quadratojugal by the postorbital (24.0/24.1; not unique); extremely reduced quadratojugal nearly unseen laterally (41.1/41.2; not unique); a weak condyle of the quadrate (45.0/45.1; not unique); dorsally shifted basioccipital facet of the basisphenoid (49.0/49.1; the only unique synapomorphy of this clade); articular that is longer than high (77.0/77.1; not unique); and a clavicle that is plate-like and high medially (105.0/105.1; not unique).

The Nannopterygius clade is supported by 19 autapomorphies, including two unique characters: bulbous roots (4.0/4.1) and anteromedial process of coracoid strongly protruding anteriorly along with the intercoracoidal facet (92.1/92.2).

The Arthropterygius clade is now supported by only four autapomorphies [cf. nine in Zverkov & Prilepskaya (2019)], only one of which is unambiguous, a marked angle between the articulated coracoids (95.0/95.2; unique); the other three related to the reduction of the interparietal symphysis (36.0/36.1), position of the posterior foramen of the internal carotid arteries (48.1/48.2) and separation of scapular and glenoid facets of the coracoid (96.0/96.1) are not unique. All species of Arthropterygius , excepting A. volgensis (Kasansky, 1903) , share a marked development of the occipital lamella of quadrate (41.1/41.0; not a unique autapomorphy). Arthropterygius chrisorum and A. hoybergeti ( Druckenmiller et al., 2012) further share relatively enlarged teeth with well-pronounced striations (1.1/1.0; 7.1/7.0; both not unique) and central position of the posterior notochordal pit on the basioccipital (60.1/60.0; not unique). Arthropterygius thalassonotus is characterized by two non-unique autapomorphies: contribution of the maxilla to the external naris in lateral view (12.1/12.0) and nasomaxillary pillar dividing the naris (14.0/14.1). This result supports the suggestion of Campos et al. (2020) that completely divided external nares evolved independently in this taxon.

Although the relations of platypterygiines is not the focus of the current paper, some interesting results are recovered for this clade. Adding Brachypterygius extremus , coded solely on the holotype (NHMUK R 3177), resulted in its recovery as a sister-taxon to Aegirosaurus at the base of Platypterygiinae. This supports the previous arguments of Zverkov et al. (2015b) contra to its synonymy with Grendelius . All species of Grendelius are found in a polytomy sister to Undorosaurus . Undorosaurus kielanae is recovered as sister to other species of Undorosaurus , which is congruent with its stratigraphic position (see: Zverkov & Efimov, 2019). The clade of Undorosaurus has a relatively high Bremer support (4; Fig. 22). This clade is supported by reduction of the supranarial process (8.0/8.1); elongation of the subnarial process (9.0/9.1); compression of the intercoracoidal symphysis (94.1/94.0); reduction of the acromial process (99.1/99.0); and compression of the scapular shaft (101.1/101.0). None of these synapomorphies is unique but, in combination, implies a peculiar reversal to the basal state in the pectoral girdle, contrasting with the derived morphology of the skull. Furthermore, the topology recovered for platypterygiines (undorosaurids) is poorly congruent with the fossil record, implying the appearence of numerous (seven) lineages in the Jurassic, for which representatives are known only from the Cretaceous. It is thus possible that further discoveries and additional data on the included taxa will challenge the results of the present analysis. The relations of the most derived Cretaceous platypterygiines are still poorly resolved and Platypterygius is found to be polyphyletic, similarly to results of other recent contributions (e.g. Fischer et al., 2016; Barrientos-Lara & Alvarado-Ortega, 2020; Campos et al., 2020).

Our analysis of the dataset with some multistate characters set as ordered, resulted in 56 most parsimonious trees of 419 steps long (CI = 0.370; RI = 0.669). The strict consensus (length of 438 steps; CI = 0.354; RI = 0.645) is not much better resolved than in the previous analysis and the recovered topology remains similar ( Fig. 23B). The difference from the results of previous analysis is that Gengasaurus is recovered as sister to Arthropterygyus and Nannopterygius and that the Bremer support values for Ophthalmosauridae (Ophthalmosauria) , Ophthalmosaurinae ( Ophthalmosauridae ) and Nannopterygius spp. are slightly higher ( Fig. 23B).