Nannopterygius enthekiodon (Hulke, 1871)

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 ENTHEKIODON ( HULKE, 1871)

v? 1870 Enthekiodon Hulke : 174

v*1871 Ichthyosaurus enthekiodon Hulke : 440, pl. 17.

v 1889 Ichthyosaurus entheciodon Hulke – Lydekker: 32, fig. 16.

v 1922b Nannopterygius entheciodon (Hulke) – von Huene: 91, 98, pl. 12, fig. 2.

1923 Nannopterygius euthecodon [sic.] (Hulke) – von Huene: 467.

1960 Nannopterygius enthekiodon . (Hulke) – Delair: 74.

1976 Nannopterygius enthekiodon (Hulke) – McGowan: 671.

[v 1983 Nannopterygius enthekiodon (Hulke) – Kirton: 122–128, fig. 39, pl. 5.]

1992 Nannopterygius entheckiodon [sic.] (Hulke) – Bardet: 654.

1999b Nannopterygius enthekiodon (Hulke) – Motani: 484.

2000 Nannopterygius enthekiodon (Hulke) – Maisch & Matzke: 81.

v 2003 Nannopterygius enthekiodon (Hulke) – McGowan & Motani: 109, fig. 91.

2010 Nannopterygius enthekiodon (Hulke) – Maisch: 167.

v 2018 Nannopterygius enthekiodon (Hulke) – Moon & Kirton: 110, pl. 39, figs 1–5; text-figs 43, 44.

Holotype: NHMUK PV 46497 , a largely complete embedded skeleton (see Supporting Information, Table S 1 and Fig. S1).

Referred specimens: NHMUK PV 46497a, partial right hindlimb; MJML K 1776, several slabs with disarticulated skull, vertebra, ribs, pectoral girdle and a forelimb; MJML K 2010, scapulae, clavicles and forelimbs; CAMSM J 29421 and J 29422 , scapulae ( Dakosaurus in Seeley, 1869: 93); OUMNH J 10346 , incomplete left and right forelimbs; OUMNH J 10360 , right scapula; OUMNH J 10574 /1–19, basioccipital, articulated parietals, nasals, quadrate, articular, surangular and eleven vertebrae; see Supporting Information, Table S1 for details.

Remarks: The basis for referral of additional specimens (excepting the isolated hindlimb NHMUK PV 46497a) is primarily the morphology of the pectoral girdle and forelimbs. All the referred specimens with preserved scapulae, including isolated scapulae (CAMSM J 29421 , J 29422 and OUMNH J 10360 ) have a peculiarly expanded and anteriorly rounded coracoidal facet, autapomorphic of N. enthekiodon . The forelimbs of MJML K 1776 and OUMNH J 10346 lack a contact of the anterior accessory epipodial element and humerus similarly to the holotype and unlike materials referred to other species of Nannopterygius , in which the contact is clearly present. The referred specimen, MJML K 1776, provides additional information on the morphology of cranial elements of N. enthekiodon ; in particular, it has a parietal with a moderately slender supratemporal process bearing a well-pronounced and somewhat serrated dorsal ridge (autapomorphy), basioccipital with anteriorly bilobed floor of the foramen magnum and articular that is anteroposteriorly longer than dorsoventrally high. This allows the robust referral to N. enthekiodon of a partial skull, OUMNH J 10574 /1–19, that is well consistent with MJML K 1776 in these overlapping elements.

Occurrence: Kimmeridge Clay Formation, Upper Kimmeridgian to Lower Tithonian (Volgian), Upper Jurassic of southern England, UK.

Revised diagnosis: Nannopterygius enthekiodon c a n b e d i a g n o s e d r e l a t i v e t o o t h e r s p e c i e s o f Nannopterygius by the following combination of characters: pronounced but not columnar processus narialis of the nasal (elongated and somewhat hook-like in N. saveljeviensis ); moderately long medial articulation of parietals (shortened in N. saveljeviensis ); absence of posterior medial notch of parietals (present in reduced form in N. yasykovi , extensive in N. saveljeviensis ); supratemporal process of the parietal not as slender as in other species and bearing a well-pronounced and somewhat serrated dorsal ridge (autapomorphy); scapula with coracoidal facet extensive and rounded anteriorly, consistent in the dorsoventral width throughout much of its length (autapomorphy; coracoidal facet is triangular, markedly decreasing in dorsoventral width anteriorly, in N. saveljeviensis and N. yasykovi , as well as in other ophthalmosaurians); coracoids with spatulate posterior portions (not as wide as in N. borealis ; tapered in N. saveljeviensis ); intercoracoidal facet lenticular in outline as in N. b o r e a l i s (d i s t i n c t a n d c o m p l e x o u t l i n e s i n N. saveljeviensis and N. yasykovi ); lack of direct contact of anterior accessory epipodial element and humerus (present in all other species); reduced dorsal trochanter and relatively poorly developed deltopectoral crest (large plate-like deltopectoral crest in N. saveljeviensis ); radius roughly trapezoidal in dorsal outline (pentagonal in N. saveljeviensis and N. yasykovi ) and comparable in size to ulna (markedly smaller than ulna in N. borealis ); ulna with concave posterior margin (unlike convex in N. borealis ); intermedium deeply wedging between radius and ulna and nearly reaching humerus in some specimens (similar condition in N. borealis , but not N. saveljeviensis and N. yasykovi ); limb elements rounded and more loosely packed than in N. saveljeviensis and N. yasykovi ; two demarcated distal femoral facets: preaxial accessory facet could be present but not clearly separated from the tibial facet (all three facets are demarcated in N. cf. saveljeviensis PRM 2836 ).

Description

The skull of the holotype specimen is poorly preserved and partially disarticulated ( Fig. 3A, B). In its orbital region, the postorbital, supratemporal, squamosal, lacrimal and jugal can be distinguished, although their preservation is too poor for a detailed description. Additional data are available from the referred specimens MJML K 1776 and OUMNH J 10574 .

Premaxilla ( Fig. 3A, B): The premaxilla is partially preserved in the holotype ( Fig. 3A, B). It is elongate and slender, bearing a longitudinal groove along much of the lateral surface.

Nasal ( Figs. 3, 5M, N): In general morphology, the nasal is similar to that of Ophthalmosaurus icenicus ( Moon & Kirton, 2016) . The ridge bordering the dorsal excavation is well pronounced ( Fig. 5M). The descending process of the nasal on the dorsal border of the external naris is present and the lateral ‘wing’ overhanging it posterodorsally ( Fig. 5N).

Lacrimal ( Fig. 3A, B): The lacrimal is similar to that of Arthropterygius and Ophthalmosaurus ( Moon & Kirton, 2016; Zverkov & Prilepskaya, 2019). It is L-shaped in lateral view and participates in the posterior border of the external naris, forming an extensive and shallow posterior margin of the narial opening (see Supporting Information, Table S4, character 20, state 0; Fig. 3A, B). The posteroventral process of the lacrimal is elongated; it follows the dorsal surface of the jugal and forms the anteroventral margin of the orbit ( Fig. 3B). Laterally, along the orbital margin, the lacrimal develops a high ridge (part of the circumorbital crest) that is continued around the orbit by other elements.

Prefrontal ( Fig. 3A, B): The prefrontal forms the anterodorsal margin of the orbit. Its preservation is too poor for a detailed description. It is possible that anteroventrally, the prefrontal contributed to the external naris ( Fig. 3B).

Parietal ( Figs. 4A, B, 5A–E): Both the parietals are preserved in articulation in OUMNH J 10574 ( Fig. 5A–E). The interparietal suture is moderately long anteroposteriorly ( Fig. 5B); it is not as long as in Ophthalmosaurus icenicus ( Moon & Kirton, 2016) and not as short as in Arthropterygius ( Zverkov & Prilepskaya, 2019) . The dorsal surface of the parietal is slightly concave along the midline in lateral view with no sagittal eminence ( Fig. 5D). The supratemporal process is slender ( Fig. 5B–E), similar to that of Ophthalmosaurus icenicus ( Moon & Kirton, 2016) and Arthropterygius spp. ( Zverkov & Prilepskaya, 2019). The posterodorsal surface of the supratemporal processes bears an irregular ridge that borders the supratemporal facet anteriorly ( Figs. 4A, 5B, D, E). The anterior border of the parietal bears two clearly demarcated facets for the frontal and postfrontal ( Fig. 5A, B). The frontal facet is faced anteromedially and reaches the interparietal symphysis, thus the parietal unlikely contributed to the posterior border of the parietal foramen. The impression of the cerebral hemisphere forms a deep and extensive cup in the anterior half of the ventral surface of the parietal ( Fig. 5A, C); posterior to it is the impression of the optic lobe, which is approximately equal in anteroposterior length and is roughly circular in outline.

Supratemporal ( Figs. 3A–D, 4A, B): The supratemporal forms the posterodorsal skull roof. In dorsal view, it articulates with the parietal posteromedially and with the postfrontal anteromedially; in lateral view, it articulates with the postfrontal anteriorly and with the squamosal and postorbital ventrally. In general morphology, it has no marked differences from Ophthalmosaurus icenicus ( Moon & Kirton, 2016) . The disarticulated supratemporal of MJML K 1776 demonstrates a long medial lamina of the ventral ramus ( Fig. 4A, B). It is possible that when articulated, this ramus was in contact with the stapes as in Ophthalmosaurus ( Moon & Kirton, 2016) .

Squamosal ( Fig. 3A, B): The squamosal is large compared to other ophthalmosaurids and is most similar to that of Stenopterygius (e.g. Godefroit, 1993; McGowan & Motani, 2003; Motani, 2005). It is a thin plate-like element triangular in outline and well exposed in lateral view. An extensive facet of the supratemporal for the squamosal could be observed in MJML K 1776 ( Fig. 4A, B)

Postorbital ( Fig. 3A, B): The postorbital is lunate in lateral view; it forms much of the posterior margin of the orbit. Its preservation in the holotype is too poor for further observations.

Jugal ( Fig. 3A, B): The jugal is a gracile J-shaped element with slender mediolaterally compressed posterior process and thin suborbital bar. In the holotype, the orbital region is disarticulated and the jugal has its anterior end rotated dorsally ( Fig. 3B).

Pterygoid ( Figs. 3C, D, 4F): Both pterygoids are nearly completely preserved in MJML K 1776, lacking only the anteriormost portions. The lateral margin of the anterior ramus, which contacted the palatine, is nearly straight with no evidence of a process postpalatinus ( Figs. 3C, D, 4F). The quadrate ramus of the pterygoid is slender, forming three wing-like flanges for the basisphenoid and quadrate. The medial flanges are elongate and were possibly in articulation, covering the basisphenoid ventrally ( Fig. 4F), but this posteromedial contact could be a taphonomic artefact. The anterior socket for the basipterygoid process of the basisphenoid is a small pit, indicating a poor development of the basipterygoid process of basisphenoid. The dorsal and lateral flanges of the quadrate ramus are short and weak, forming a concave lateral surface for articulation with the quadrate.

Quadrate ( Fig 5F–I): The fragmental right quadrate is preserved in OUMNH 10574 ( Fig. 5F–I). It has a gracile articular condyle with nearly equal in size bosses: the articular boss is slightly more shifted ventrally than the surangular boss ( Fig. 5G). The stapedial facet is dorsoventrally elongate ( Fig. 5F). The anteromedial protrusion is pronounced ( Fig. 5F, I), unlike that of Arthropterygius chrisorum ( Zverkov & Prilepskaya, 2019) .

Basioccipital ( Figs. 4D, E, 5R–U): The basioccipital is preserved in MJML K 1776 and OUMNH 10574. The element is similar to that of the Cretaceous Acamptonectes densus , including the feature that has previously been considered as an autapomorphy of Acamptonectes – an anteriorly bilobed floor of the foramen magnum ( Fischer et al., 2012). The condyle is oval in outline, although it is likely due to a diagenetic compression. The vertical incision of the posterior notochordal pit is raised close to the dorsal edge of the condyle, right under the floor of the foramen magnum ( Fig. 5T). The condyle is slightly deflected peripherally by an excavate extracondylar area. The extracondylar area is reduced, but can be observed in posterior view both laterally and, in a lesser degree, ventrally ( Fig.5T); it lacks a ventral notch. The excavate peripheral ring of the extracondylar area is incomplete, being separated ventrally by a crest in OUMNH 10574 ( Fig. 5S), but it is continuous in MJML K 1776 ( Fig. 4D). The anterior margin of the extracondylar area is obliquely S-curved in lateral view ( Fig. 5U). The opisthotic and stapedial facets are semi-oval in shape and occupy nearly equal height in lateral view. On the dorsal surface, there are exoccipital facets oval in outline. The posterior borders of the exoccipital facets are rounded, unlike those tapered in Undorosaurus ( Zverkov & Efimov, 2019) . The facets are medially separated by a wide floor of the foramen magnum, which is bilobed anteriorly ( Figs. 4E, 5R). The anterior surface of the basioccipital is irregularly pitted forming the basisphenoid facet.

Hyoid apparatus ( Fig. 3A, B): The paired hyoid elements are partially exposed in the holotype and one element can be observed in MJML K 1776 . The element is a short and strongly bowed rod. The exposed anterior end is compessed and expanded ( Fig. 3B) .

Mandible ( Figs. 3, 4G, 5O–Q): The mandible is nearly complete, but disarticulated, in the holotype. In this regard, the previously reported mandibular length of 60 cm (e.g, Moon & Kirton, 2016) is likely overestimated. Additional data on the morphology of surangular and articular are available from MJML K 1776 and OUMNH J 10574 .

Dentary ( Fig. 3A, B): The dentary is slender and bears a longitudinal groove on its lateral surface.

Splenial ( Fig. 3A, B): The splenials are partially exposed in the holotype and demonstrate a typical anterior bifurcation with dorsal and ventral rami being nearly equal in length and slender.

Angular ( Fig. 3C, D): Only a posterior fragment of the left angular is preserved in MJML K 1776. Based on this fragment it could be said that the posterior portion of the angular is expanded and covered the surangular externally, thus giving the angular a pronounced lateral exposure.

Surangular ( Figs. 3, 4G, 5O–Q): The surangular is generally similar to that of Ophthalmosaurus icenicus ( Moon & Kirton, 2016) , although more gracile and strongly mediolaterally compressed, resembling the surangulars of juvenile specimens of O. icenicus (NGZ pers. obs. on NHMUK specimens, April 2019). Among the principal differences from O. icenicus are a pronounced curvature of the surangular posterior part, which is uncommon for ophthalmosaurids but occurs in some basal thunnosaurians (e.g. Hauffiopteryx and some specimens of Ichthyosaurus ; McGowan, 1973; Marek et al., 2015), and a markedly better pronounced and more horizontally oriented process ( Fig. 2N, O), which is commonly interpreted as a point of attachment of M. adductor mandibulae externus group (e.g. Moon & Kirton, 2016). This process is well visible in dorsal view as in Grendelius mordax (NGZ pers. obs. on the holotype CAMSM J68516 , December 2018) and unlike in Ophthalmosaurus , Undorosaurus and Arthropterygius ( Moon & Kirton, 2016; Zverkov & Efimov, 2019; Zverkov & Prilepskaya, 2019).

Articular ( Figs. 4C, 5J–L): The articular is preserved in MJML K 1776 ( Fig. 4C) and OUMNH J. 10574 ( Fig. 5J–L). It is a small and isometric element with a saddle-shaped medial surface and flattened lateral surface. The anteroposterior length exceeds the dorsoventral heigth of the element with H/L = 0.70– 0.78. The posterior margin is convex, as well as dorsal and ventral margins, which are nearly parallel ( Figs. 4C, 5K). There is a small emerging bony bulge in the dorsal half of the medial surface. The anterior surface for the articulation with the quadrate is concave and lenticular in outline ( Fig. 5J).

Dentition. It was impossible to assess the teeth of the holotype during its distant examination; and in the referred specimens, the teeth are not preserved, exept for one partial tooth in MJML K 1776 ( Fig. 4B). However, Hulke (1871) considered the teeth of the holotype identical to those of ‘ Enthekiodon ’ (see Fig. 2 and Remarks above). Thus, the teeth might be small (not exceeding 13 mm in their apicobasal length, including root), with slender and poorly ornamented crowns and markedly expanded bulbous roots: width of the root nearly twice exceeding the maximum diameter of the crown .

Vertebral column ( Fig 6): In the holotype, 66 vertebra are present, presumably 45 of which are presacral, as was originally identified by Hulke [45 th centrum is the first in which the diapophysis and parapophysis are merged; cf. 42 identified by Kirton (1983) and by Moon & Kirton (2018)]. Posterior to them, at least six more vertebra in the transitional region bear a fused 8-shaped rib facet.

Isolated presacral vertebral centra are present in MJML K 1776 and OUMNH J. 10574 ( Fig. 6). Their morphology is similar to those of Ophthalmosaurus . Anteriormost centra are tapered ventrally, and more posteriorly located centra have circular articular faces. The atlas–axis complex preserved in MJML K 1776 bears a marked lateral suture between the atlas and axis ( Figs. 3D, 6A).

Ribs: The ribs are long (the longest rib of the holotype is c. 60 cm when measured directly from proximal to distal end. Thus, it is as long as the skull and comprises c. 20% of the total animal length). We found no support for the suggestion of Moon & Kirton (2018) that the ribs bear only a single groove proximally. Instead, in the holotype, the observed proximal cross-sections are characteristically 8-shaped with longitudinal grooves running on both the anterior and posterior faces terminating close to the midlength; distally, the rib becomes circular in cross-section. Identical condition could be observed in MJML K 1776 .

Pectoral girdle ( Fig. 7): The pectoral girdle elements of the holotype were recently characterized in detail by Moon & Kirton (2018), but not all of the interpretations proposed in that work can be supported by our observations. Primarily, this concerns the medial contact of the coracoids, which is present only in the anterior half of the medial border ( Fig. 7A, B), but not along the entire medial length, as was supposed by Moon & Kirton (2018). The posterior portions of the coracoids are slightly divergent and their posterior edges are rounded ( Fig.7A,B). The anterolaterally faced scapular facet is relatively large (only slightly shorter than the glenoid contribution) and clearly separated from the glenoid contribution forming an angle of c. 120°. The glenoid contribution is concave and faces posterolaterally, unlike laterally facing and parallel to the medial facet in most other ophthalmosaurids. The coracoids available for MJML K 1776 and MJML K 1174 (here referred to as Nannopterygius sp. ) ( Fig. 7P, Q) show no marked differences from those of the holotype, except for minor variation in size and proportions, which is partially due to deformation and also could partially reflect an intraspecific variation. Compared to a wide range of coracoid shape variation reported for Ophthalmosaurus icenicus ( Moon & Kirton, 2016) , the morphology of the coracoid in Nannopterygius is remarkably stable. The intercoracoidal facet could be observed from MJML K 1776: it has a simple lenticular outline with the ventral edge more convex than the dorsal edge.

The left scapula of the holotype is completely preserved and exposed in the lateral view and the right scapula could be observed only in its proximal part, which is articulated to the corresponding coracoid ( Fig. 7A, B). The left scapula of the holotype demonstrates a peculiar morphology among ophthalmosaurids, so that several isolated scapulae from the Kimmeridge Clay Formation that, having comparable size and identical morphology, could be referred to this taxon (see Supporting Information, Table S1) and are used below to supplement the description, along with MJML K 2010 and MJML K 1776. The scapula of Nannopterygius is peculiar in its extensive and concave coracoid facet, which is wide and rounded anteriorly, and a small glenoid contribution, as well as in an extensive notch of finished ossification between the coracoid facet and the acromial process. The coracoid facet of the scapula is dorsoventrally thickened and terminates anteriorly with the rounded edge being clearly separated from the acromial process by an extensive notch. This condition is autapomorphic among ophthalmosaurians, as typically the coracoidal facet tapers anteriorly. Although scapular notches were reported as a rare condition for several mature individuals of Ophthalmosaurus ( Moon & Kirton, 2016) , in Nannopterygius this appears to be a typical state. The acromial process is well pronounced and extends anteriorly, curving ventrally for articulation with the clavicle. Another characteristic feature of the scapula in Nannopterygius is its mediolaterally compressed and distally expanded shaft. This latter condition is similar to that of Ophthalmosaurus icenicus , although in most of the referred specimens of O. icenicus the distal end of the scapular shaft is markedly less expanded anteroposteriorly, whereas in N. enthekiodon the marked distal expansion is a typical condition ( Fig. 7C, L, T, U).

The clavicle is described based on holotype, MJML K 1776 and MJML K 2010 ( Figs. 3E, F, 4B, S). The clavicle is robust and similar to that of Arthropterygius ( Zverkov & Prilepskaya, 2019) . It is high dorsoventrally, having a dorsoventral height to mediolateral length ratio of 0.26 (estimated from MJML K 1776). However, unlike in Arthropterygius , it is not thickened anteroposteriorly but thin.

Forelimb ( Figs. 3C, D, 8): The incomplete left forelimb of the holotype is exposed in dorsal view [cf. interpreted as being exposed in ventral view by Moon & Kirton (2018)]. This interpretation is supported by the shape and orientation of the process that originates close to the posterior edge and is obliquely directed to the radial facet, thus having a typical position and morphology of the trochanter dorsalis (see e.g. McGowan & Motani, 2003), whereas the left humerus of this specimen, exposed in ventral view, has a welldeveloped deltopectoral crest typically shifted to the anterior edge of the humeral proximal end ( Fig. 7A, B). The humerus is stocky with proximal and distal ends of nearly equal anteroposterior width and robust diaphysis. The humerus is slightly compressed along its posterior edge ( Fig. 8C, I). There are two distal articular facets: a posterodistally deflected ulnar facet and anterodistally facing radial facet. The radial facet is slightly anteroposteriorly longer than the ulnar facet. The presence of a small facet anterior to radial facet is equivocal.

The forelimbs are also preserved in MJML K 2010 and MJML K 1776 . Additionally, several isolated humeri and one incomplete forelimb are deposited in the OUMNH collection (Supporting Information, Table S 1). All these materials are consistent with the holotype in their morphology and size, and thus herein referred to the same taxon (see Supporting Information, Table S 1). All the referred humeri have a poorly developed dorsal trochanter and deltopectoral crest, but in OUMNH J. 10346 and MJML K 2010 the deltopectoral crest is better pronounced than the dorsal trochanter ( Fig. 8E, Q). The distal facets have equal dorsoventral width and similar outline, although the radial facet slightly tapers anteriorly, while the ulnar facet is more rounded posteriorly ( Fig. 8J, P). Judging from MJML K 1776 and OUMNH J 10346 , there was no contact between the humerus and anterior accessory epipodial element ( Fig. 8F, L). However, anterior to the contact with the radius there is a small, free surface, poorly demarcated from the radial facet. This may be interpreted either as a part of the radial facet or as the rudimentary facet of the anterior accessory epipodial element .

At least five elements are preserved in articulation with the right humerus of the holotype. These are the radius, ulna, ulnare, intermedium and presumably the distal carpal four ( Fig. 8A–C). Seven epipodial and mesopodial elements are in articulation in the limb of MJML K 1776 ( Fig. 8F) , and a number of isolated forelimb elements, including the radius, ulna and intermedium, are preserved in association with forelimbs in MJML K 2010 and OUMNH J. 10346.

The ulna is characterized by a tapered and concave posterior edge ( Fig. 8B, C, D, F, E, L), which is a synapomorphy of ophthalmosaurines ( Fischer et al., 2012). The element is roughly pentagonal in dorsal outline and bears three distal facets for the intermedium, ulnare and pisiform. The proximal articular surface is slightly convex. The radius is anteroposteriorly longer and proximodistally shorter than the ulna. It is roughly trapezoidal in dorsal outline with the widest surface for articulation with the humerus ( Fig. 8A, D, F, L). Distally it articulates with the anterior accessory element, radiale and intermedium. The medial articulation with the ulna was probably poorly developed in the holotype; it is relatively short in MJML K 2010 and MJML K 1776 , and is nearly lost in OUMNH J. 10346 ( Fig. 8B, D, F, L). The anterior edge of the radius is not involved in ossification. In some specimens, a short, free surface (facet) separates the facet of the anterior accessory epipodial element from the humerus proximally ( Fig. 8F, L). The anterior accessory epipodial element preserved in MJML K 1776 and MJML K 2010 is semicircular in outline with straight anterior edge lacking ossification ( Fig. 8E, F) .

The intermedium wedges between the ulna and radius so that it is nearly in contact with the humerus ( Fig. 8A, B, F, L). The element is roughly pentagonal in dorsal view and bears six facets for the following elements (clockwise for the right limb in dorsal view): ulna, ulnare, distal carpal three, distal carpal two, radiale and radius.

Hindlimb: Based on photographs, Zverkov & Efimov (2019) suggested that the femur of NHMUK PV 46497a could be in articulation with three epipodial elements. Our personal examination of the specimen confirmed the interpretations of previous workers ( Kirton, 1983; Lyddeker, 1889; Moon & Kirton 2018) that only the two elements are articulated with the femur. However, anterior to the tibia there is a free surface of the distal femur that may or may not serve as a facet for an anterior accessory element (Supporting Information, Fig. S2). For details on the morphology of NHMUK PV 46497a we direct the reader to Moon & Kirton (2018). It is worth mentioning that the fibula of NHMUK PV 46497a lacks posterior perichondral ossification, which is present in the fibula of the holotype (NHMUK PV 46497). Similar variation occurs in Ophthalmosaurus (NGZ pers. obs. on NHMUK specimens, April 2019).