Nannopterygius, VON HUENE, 1922

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 VON HUENE, 1922

1871 Ichthyosaurus – Hulke: 440 [pars.].

1889 Ichthyosaurus – Lydekker: 32 [pars.].

1922 Nannopterygius von Huene , 91.

1960 Nannopterygius – Delair: 73.

1976 Nannopterygius – McGowan: 671.

[1983 Nannopterygius – Kirton: 122.]

1986 Ophthalmosaurus – Delair: 133 [pars.].

1992 Nannopterygius – Bardet: 654.

1997 Paraophthalmosaurus Arkhangelsky : 87. [1997 Jasykovia Efimov, 97.]

1998 Paraophthalmosaurus –Arkhangelsky:87 [pars.]. 1999a Yasykovi a Efimov: 93.

1999 Nannopterygius von Huene 1922 – Motani: 484. 1999 Paraophthalmosaurus Arkhangelsky, 1997 – Motani: 485.

2000 Paraophthalmosaurus Arkhangel’skii, 1997 – Storrs, Arkhangel’skii & Efimov: 200.

2000 Ophthalmosauru s Seeley, 1874 – Maisch & Matzke: 78 [pars.].

2000 Nannopterygius von Huene – Maisch & Matzke: 81.

2003 Ophthalmosaurus Seeley, 1874 – McGowan & Motani: 113 [pars.].

2003 Nannopterygius Huene, 1922 – McGowan & Motani: 109.

2004 Jasykovia [sic.] Efimov, 1999 – Efimov: 134.

2008 Paraophthalmosaurus Arkhangelsky, 1997 – Arkhangelsky: 249.

2009 Jasykovia [sic.] – Efimov: 54.

2010 Ophthalmosaurus Seeley, 1874 – Maisch: 166 [pars.].

2010 Nannopterygius von Huene, 1922 – Maisch:167.

2014 Paraophthalmosaurus Arkhangelsky, 1997 – Arkhangelsky & Zverkov.

2016 Ophthalmosaurus Seeley, 1874 – Moon & Kirton: 13 [pars.].

2017 Paraophthalmosaurus Arkhangelsky, 1997 – Zverkov, Shmakov, Arkhangelsky: 248.

2018 Nannopterygius Huene – Moon & Kirton: 110.

2018 Macropterygius Huene – Moon & Kirton: 117 [pars.].

2018 Paraophthalmosaurus Arkhangelsky, 1997 – Moon & Kirton: 142.

Type species: Ichthyosaurus enthekiodon Hulke, 1871 .

Referred species: Nannopterygius saveljeviensis ( Arkhangelsky, 1997) comb. nov.; Nannopterygius yasykovi ( Efimov, 1999 a) comb. nov.; N a n n o p t e ry g i u s b o re a l i s s p. n o v. from the lowermost Cretaceous (Berriasian) of Arctic Islands ( Svalbard and Franz Josef Land).

Emended diagnosis: Medium-sized ophthalmosaurid (up to 3.5 m in maximum estimated length) characterized by the following autapomorphies: teeth with bulbous roots and slender crowns (width of the root nearly twice exceeds maximum diameter of the crown); marked curvature of the posterior mandible similar to that of Hauffiopteryx and unlike in any ophthalmosaurid; well-pronounced Musculus adductor mandibulae externus process (unique, although similar condition present in Grendelius mordax , pers. obs); coracoids markedly elongate (anteroposterior length to mediolateral width ratio = 1.7–1.4), with divergent posterior ends and large, square anteromedial processes; intercoracoidal facet shifted anteriorly and occupying anteromedial process.

Nannopterygius is also characterized by the following combination of features: gracile and elongated rostrum as in Aegirosaurus and Sveltonectes (less robust than in Arthropterygius , Caypullisaurus , Grendelius , Ophthalmosaurus and Undorosaurus ); snout ratio of c. 0.64, orbital ratio 0.25–0.28; supranarial process of premaxilla well developed and projecting into the external naris (supranarial process reduced in Arthropterygius, Opthalmosaurus and Undorosaurus ); subnarial process contacts the jugal (as in Grendelius and Undorosaurus ); narial process of nasal present and, in the type species, similar in shape to that of Acamptonectes , Ophthalmosaurus and Undorosaurus , although in N. saveljeviensis narial process is more columnar as in Sveltonectes ; prefrontal contributing to the external naris as in Sveltonectes ; prefrontal forming anteromedial expansion as in Caypullisaurus , Leninia , Simbirskiasaurus and Sveltonectes ; narrow supratemporal anteromedial tongue protruding far anteriorly and covering the postfrontal as in Ophthalmosaurus and unlike that wide of Athabascasaurus and Arthropterygius ; jugal bowed ventrally similarly to that of Arthropterygius ; jugal posterior process anteroposteriorly narrow (unlike that of Caypullisaurus , Grendelius , Platypterygius and Undorosaurus ); extremely narrowed postorbital bar due to extreme reduction of quadratojugal lateral exposure (as in Arthropterygius and Ophthalmosaurus ); squamosal is present and triangular in shape as in Ophthalmosaurus (less dorsoventrally narrow than in Arthropterygius and Undorosaurus ); parietal lacking sagittal eminence and having a slender supratemporal process (as in Arthropterygius and Ophthalmosaurus ); basioccipital with reduced extracondylar area (as in Acamptonectes , Ophthalmosaurus and Undorosaurus , but to a lesser degree than in Grendelius and Platypterygius ); quadrate with reduced occipital lamella and pronounced angular process (unlike in Arthropterygius ); expanded and bilobed anterior portion of the foramen magnum floor (uniquely shared with Acamptonectes ); posteriorly rounded edges of the exoccipital facets of the basioccipital (unlike in Acamptonectes , Sveltonectes and Undorosaurus ); basisphenoid with extremely reduced basipterygoid processes (width to length ratio 1.2); basioccipital

Region); 3, bank of the Volga near Gorodischi village, Slantsevy Rudnik village and ‘Detskiy sanatorium’ (Ulyanovsk Region); 4, Kashpir (Samara Region); 4, Gorny (Krasnopartisansky District, Saratov Region); 6, Kutseba (Perelyub District, Saratov Region). Zonal correlation of the Volgian regional stage of the European part of Russia, Svalbard, Franz Joseph Land and England. Distribution of Nannopterygius spp. is shown in grey. Correlation of ammonite zones after Casey (1973), Rogov & Zakharov (2009), Rogov (2010a, b, 2017) and Kiselev et al. (2018).

facet of the basisphenoid facing posterodorsally, occupying in dorsal view area nearly equal to that of dorsal plateau (uniquely shared with Arthropterygius ); stapes with moderately stout shaft (like that of Ophthalmosaurus and Undorosaurus ); short and robust paroccipital process of the opisthotic, poorly demarcated from the main body of the element (unlike that of Ophthalmosaurus and Acamptonectes ); pronounced stapedial curvature as in basal parvipelvians Ichthyosaurus and Hauffiopteryx and, among ophthalmosaurids, uniquely shared with Arthropterygius ; articular markedly anteroposteriorly longer than dorsoventrally high in all species excepting N. yasykovi (height to length ratio less than 0.8 as in Arthropterygius and unlike in Grendelius , Mollesaurus and Ophthalmosaurus ); pronounced bony boss on the articular medial surface (as in Undorosaurus nessovi Efimov, 1999 ); teeth comparatively small with crowns either lacking ornamentation, or bearing rare and fine striations (as in Ophthalmosaurus natans (Marsh, 1879) , Arthropterygius lundi , Acamptonectes , Athabascasaurus , Sveltonectes and Muiscasaurus ); 45 presacral vertebrae [42 in Ophthalmosaurus ; 47 in Platypterygius australis (McCoy, 1867) , 52 in Undorosaurus and? Aegirosaurus ]; angle between the dorsal surfaces of articulated coracoids is nearly straight (180–170º); scapular and glenoid facets of coracoid clearly demarcated and comparable in size similarly to those of Sveltonectes ; well-developed acromial process of scapula unlike that of Undorosaurus ; in some specimens, acromial process and anteromedial process of the coracoid could be in contact as observed in some Stenopterygius and Leptonectes specimens; mediolaterally compressed scapular shaft, oval in cross-section (as in Acamptonectes , Arthropterygius , Ophthalmosaurus and Undorosaurus , and distinct from the thick and rod-like cross-section in Grendelius and Platypterygius ); scapular notch present as in Sveltonectes , Grendelius pseudoscythicus , G. zhuravlevi and, as a rare variation, in Ophthalmosaurus icenicus ; scapular glenoid contribution reduced compared to coracoid facet as in Sveltonectes and Ophthalmosaurus natans ; dorsoventrally high and relatively robust clavicles (as in Arthropterygius ); interclavicle with narrow posterior median stem as in Arthropterygius and Ophthalmosaurus (mediolaterally wide in Grendelius and Undorosaurus ); interclavicle ventral knob present as in Undorosaurus and Grendelius ; two or three distal humeral facets: for radius and ulna, and, in some species, for an anterior accessory epipodial element; poorly developed dorsal process of the humerus; deltopectoral crest commonly better developed than the dorsal process; metacarpal five contacting ulnare posterodistally (i.e. ‘longipinnate’ condition); intermedium bearing extensive distal facet for distal carpal three and anteriorly contacting distal carpal two (as in Undorosaurus and unlike in Aegirosaurus , Arthropterygius , Brachypterygius and Ophthalmosaurus ); ischiopubis slender and rod-like with small obturator foramen; two or three distal femoral facets; ventral process of the femur is more developed than the dorsal process, although the latter is also well pronounced.

Occurrence: Upper Kimmeridgian to lower Volgian (Lower Tithonian, Upper Kimmeridge Clay) of the UK; Middle to Upper Volgian (Tithonian to Lower Berriasian) of European Russia and Upper Volgian to Ryazanian (Berriasian) of the Arctic.

Remarks: In 1870, Hulke described fragmentary jaws with associated teeth from the Kimmeridge Clay Formation of Kimmeridge Bay. Based on peculiar teeth with ‘great development of the cementum, which gives the fang the appearance of being inserted in a bulbous sheath’ ( Hulke, 1870: 172), he proposed the provisional name ‘ Enthekiodon ’. Later he used this as a specific name for an ichthyosaur skeleton newly discovered in the same locality and horizon, and having teeth that ‘agreed so closely with those of Enthekiodon as to leave no reasonable doubt of their identity’ ( Hulke, 1871: 440). The skeleton was designated as the holotype of Ichthyosaurus enthekiodon . The location of the first described ‘ Enthekiodon ’ material was considered unknown ( Moon & Kirton, 2018). Some authors suggested that the specimen was ‘destroyed through an action of pyrites’ ( Delair, 1960: 74). However, during the examination of the NHMUK collection in April 2019, NGZ observed an uncatalogued specimen from the Kimmeridge Clay Formation ( Fig. 2) that according to the label is part of Hulke’s collection and perfectly agrees with the description of Hulke (1870). Thus, it is likely that this is the ‘lost’ specimen of ‘ Enthekiodon ’. Indeed, the preserved portions of rostrum indicate that it was slender. Teeth are small, not exceeding 13 mm in their apicobasal length (including root). The crowns are slender and poorly ornamented; some of them lack the ornamentation entirely ( Fig. 2B, C), whereas others are ornamented by slight, rare striations ( Fig. 2D, E). The base of the crown is 2.4 mm in diameter and the root is 4.6 mm in maximum width, thus nearly twice exceeding the maximum diameter of the crown. This agrees well with the morphology of teeth observed in other specimens of all species of Nannopterygius . In this regard, the species-level identification of this material is impossible, but its affinity to Nannopterygius is highly plausible.

The concept of the genus composition performed in the current contribution is based primarily on peculiar morphology of the pectoral girdle found in all referred specimens, where preserved, but not seen in any other ophthalmosaurid. Other osteological traits and results of the phylogenetic analysis (see below), in our opinion, are substantial support for the presented taxonomic decision.