Nannopterygius yasykovi (Efimov, 1999) 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 YASYKOVI (EFIMOV, 1999A)

[ v. 1997 Jasykovia jasykovi V. Efimov: 98, fig. 7.6.] [pars.] .

[v. 1997 Jasykovia sumini V. Efimov : 107, fig. 7.9 в–ж.].

v* 1999a Yasykovia yasykovi Efimov, 1999: 94 , fig. 1 [pars.].

v. 1999 a Yasykovia sumini Efimov, 1999: 94 , figs 4в, г, 6а, в.

2000 Paraophthalmosaurus yasykovi ( Efimov, 1999a) – Storrs et al.: 2000: 200 [pars.].

2000 Ophthalmosaurus yasykovi (Efimov, 1999) comb. nov. – Maisch, Matzke: 78 [pars.].

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

2004 Jasykovia sumini V. Efimov, 1999 [sic.] – Efimov: 134, fig. 1b [pars.].

2008 Paraophthalmosaurus saveljeviensis Arkhangelsky, 1997 – Arkhangelsky: 249 [pars.].

2010 Ophthalmosaurus yasykovi (Efimov, 1999) – Maisch: 166 [pars.].

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

Holotype: UPM EP-II-7 (1235), anterior portion of skeleton embedded in matrix ( Fig. 17).

R e f e r r e d s p e c i m e n: U P M I I - 1 1(3-M), p a r t i a l disarticulated skeleton, holotype of Yasykovia sumini ( Figs. 17–20, 21) .

Remarks: The clarification of specimen composition of the presented hypodigm is provided in the Discussion.

Diagnosis

Nannopterygius yasykovi differs from other species of Nannopterygius in the following combination of features, including two autapomorphies (marked with ‘*’): medial articulation of parietals strongly dorsoventrally thickened (unlike in any other species), anteroposteriorly long and bipartite* (short in N. saveljeviensis ); slender supratemporal process of the parietal, as in N. saveljeviensis (more robust and bearing a rugose dorsal ridge in the type species); prominent anteromedial protrusion of the quadrate (present, but less pronounced, in N. enthekiodon and N. saveljeviensis ); coracoids with spatulate posterior portions as in the type species (not as wide as in N. borealis ; strongly mediolaterally constricted and tapered in N. saveljeviensis ); intercoracoidal facet trapezoidal in outline with straight ventral margin bearing protrusions posteriorly and anteriorly*; relatively small scapular facet of the coracoid (proportionally larger in all other species of the genus); more pronounced curvature of the scapular shaft; comparable in size coracoidal facet and glenoid contribution of the scapula; radius comparable in size to ulna (radius markedly smaller than ulna in N. borealis ); ulna with concave posterior margin (unlike convex in N. borealis ); intermedium not wedging between radius and ulna, and having a short contact with ulna compared to that with radius as in N. yasykovi (wedging between the two elements and equally contacting them in N. enthekiodon and N.borealis ); limb elements polygonal and tightly packed (rounded and loosely arranged in N. enthekiodon and N. borealis ).

Occurrence: Middle to Upper Volgian ( Epivirgatites nikitini and Kachpurites fuigens ammonite biozones), Upper Jurassic of European Russia.

Description

The skull is incomplete and partially disarticulated in the holotype and is represented by several isolated elements in the referred specimen. In general, the skull is similar to that of Nannopterygius saveljeviensis , although some differences do exist.

Premaxilla ( Fig. 17): The partial premaxilla preserved in the holotype UPM EP-II-7 (1235) has two elongated processes posteriorly ( Fig. 17). The process supranarialis is well pronounced and slender, the process subnarialis extends posteriorly and contacts the jugal on the level of the posterior margin of the external naris ( Fig. 17D) .

femur of Nannopterygius cf. saveljeviensis PRM 2836 in distal (J), proximal (K), anterior (L), dorsal (M) and ventral (N) views. Abbreviations: dp, dorsal process; faae, facet for the preaxial accessory epipodial element; ffi, facet for the fibula; fti, facet for the tibia; obtf, obturator foramen vp, ventral process. Scale bar = 3 cm.

Nasal ( Fig. 17): Based on the exposed part, the nasal has no significant differences from that of the type species and is similar to that of N. saveljevensis . The morphology of the narial process cannot be assessed.

Frontal ( Fig. 17): The frontal is also similar to that of N. saveljevensis with the medial facet for the counterpart located in the anterior half and the elongated margin of the parietal foramen.

Parietal ( Figs. 17, 18A – F): The parietal is similar to that of the type species in a moderately anteroposteriorly long interparietal articulation, unlike that shortened symphysis in N.saveljeviensis . The interparietal facet is massive, triangular in outline and has a rugose surface ( Fig. 18E). There is a rudimentary notch posterior to the main interparietal facet, but posterior to this notch the medial edge of the parietal forms an additional contact with the counterpart. A low ridge along the posterior margin of the parietal marks a border of a posterior parietal shelf ( Fig. 18A, D). In lateral view, the dorsal surface of the parietal is straight along the midline with no sagittal eminence ( Fig. 18A, F). The supratemporal process is slender, unlike that of the type species, and similar to that of N. saveljeviensis . The impression of the cerebral hemisphere forms a deep, circular cup on the anterior part of the ventral parietal ( Fig. 18C). The impression of the optic lobe is extensive and circular in outline, occupying a posterior part of the parietal ventral surface ( Fig. 18B).

Prefrontal ( Fig. 17): The prefrontal is similar to that of the other species, in having a well-developed dorsomedial expansion and contributing to the external naris. The lateral curcumorbital crest is also well developed.

Lacrimal ( Fig. 17): The lacrimal is similar to that of the type species. The dorsal process of the lacrimal contacts the narial process of the prefrontal in a comparatively simple, sinusoidal suture with no marked interdigitation. The anteroventral tip of the lacrimal is in contact with the premaxilla. The posteroventral process of the lacrimal follows the dorsal edge of the jugal and shapes the anteroventral margin of the orbit. Laterally, along the orbital margin, the lacrimal develops a ridge that is continued around the orbit by other elements.

Postfrontal ( Fig. 15G–I): The postfrontal has no marked difference from that of the type species and N. saveljeviensis . It is curved and widest anteriorly, grading into a narrower and mediolaterally facing posterior strut.

Supratemporal ( Fig. 17): The supratemporal is too poorly preserved for detailed observations.

Squamosal and quadratojugal: At present, these elements are unknown for the species.

Jugal ( Fig. 17): The jugal is similar to that of other species: it is a gracile J-shaped element markedly bowed ventrally. It has a lender posterior process and thin suborbital bar laterally bearing a ventral continuation of the circumorbital crest.

Pterygoid ( Fig. 18O–Q): The preserved posterior portion of the pterygoid with lateral, dorsal and medial lamellae is gracile.

Quadrate ( Fig. 19Q–T): The quadrate of UPM II- 11(3-M) has a well-developed angular protrusion. The articular condyle is similar to those of other species, with saddle-shaped articular surface and rounded ventral edge of the articular boss ( Fig. 19Q, T). The stapedial facet is oval in outline and surrounded by a raised peripheral area.

Basioccipital ( Fig. 19F–J): The floor of the foramen magnum of the basioccipital is anteriorly expanded and bilobed ( Fig. 19H). The condyle is circular in outline, slightly wider than high, with the posterior notochordal pit located in its dorsal half ( Fig. 19G). The extracondylar area is smooth and lacks peripheral excavation and a ventral crest, present in other species of the genus ( Fig. 19H, I). The facets for opisthotic and stapes both shifted anteriorly and nearly equal in dorsoventral height in lateral view ( Fig. 19I). The exoccipital facets have rounded posterior borders. The anterior protrusion in the middle of the anterior surface is pronounced and with a deep vertically oriented medial groove in its center ( Fig. 19J).

Parabasisphenoid ( Fig. 19A–F): The parabasisphenoid is square in ventral view due to an extremely reduced basipterygoid processes. In lateral view, it is irregularly pentagonal due to a dorsally raised basioccipital facet and an additional free posterior surface ventral to it, similar to the condition observed in some specimens of Arthropterygius ( Zverkov & Prilepskaya, 2019) . The anterior vertical wall is high, being only slightly less than the anderoposterior length of the element. The posterior foramen for the internal carotid arteries is located in the posterior half of the ventral surface, close to its middle, and is continued posteriorly by a groove. The anterior impessions of a cartilaginous continuation of the cristae trabecularis are pronounced, forming a curved smooth surface ventral to the anterior foramen of the internal carotid arteries canal.

Opisthotic ( Fig. 19K–O): The opisthotic is similar to that of N. saveljeviensis in every aspect, although it is slightly less massive.

Supraoccipital ( Fig. 19P): Only the exoccipital processes of the supraoccipital are preserved and are less massive than in N. saveljeviensis . The impression of the otic capsule is a deep L-shape curve.

Hyoid apparatus ( Fig. 17A, B): A hyoid element is short and bowed. It is 87 mm in maximum length and shows no differences from that of N. saveljeviensis .

Mandible ( Figs. 17, 18J–W): The morphology of the mandibular elements is typical of the genus. The dentary is slender; it terminates approximately at the middle of the orbit. The surangular demonstrates a typical curvature in its posterior part.The paracoronoid process is poorly developed, whereas the process for M. adductor mandibulae externus group is a large ridge ( Fig. 18J–L). The articular is similar to those of other Nannopterygius species in general outline, differing in that it is more isometric (H/L = 0.86) and narrowed posteriorly ( Fig. 18R–W). The muscular knob on the medial surface is poorly pronounced as in the type species, and unlike that of N. saveljeviensis .

Vertebral column ( Fig. 20): The atlas–axis complex and two anterior presacral vertebrae are preserved in UPM II- 11(3-M); they are similar to those of N. saveljeviensis . The atlas and axis are fused with a marked suture and lack a ventral keel ( Fig. 20A, B). The anterior presacral vertebral centra are rounded in articular view and slightly flattened at the dorsal margin ( Fig. 20C). Ventrally, they bear a rudimentary keel ( Fig. 20D).