Ophthalmosauridae (Fischer, 2012)

Delsett, Lene L., Roberts, Aubrey J., Druckenmiller, Patrick S. & Hurum, Jørn H., 2019, Osteology and phylogeny of Late Jurassic ichthyosaurs from the Slottsmøya Member Lagerstätte (Spitsbergen, Svalbard), Acta Palaeontologica Polonica 64 (4), pp. 717-743 : 729-734

Ophthalmosauridae indet.

Figs. 8 , 9 .

Material.—PMO 224.252 ( Figs. 8 , 9 , SOM 4: table 2), a partly articulated ichthyosaur that consists of a dorsoventrally compressed and fractured skull with a few postcranial elements. The skull roof, mandibles and a partial palate, consisting of an incomplete parabasisphenoid and pterygoids, are preserved in approximate life position. A large number of teeth are preserved in the anterior portion of the rostrum. The orbital area is not preserved, except for two displaced lacrimals. Both jugals are preserved, but disarticulated and turned 180° so that the anterior ends are directed posteriorly. One hyoid was found disarticulated on the right side of the skull. The quadrates are preserved together in the posterior and left area of the skull, whereas the remainder of the basicranium is missing. The atlas-axis complex is preserved as well as the remains of four smaller vertebrae scattered on top of the skull together with a few ribs and two partial forefin elements. All elements from the specimen share the same preservation: they are severely fractured and some are incomplete, and are dorsoventrally compressed; from Slottsmøya Member Lagerstätte, Tithonian.

Description.— Premaxilla ( Fig. 8 ): The rostrum of the Ophthalmosauridae indet. PMO 224.252 is more slender than in Brachypterygius extremus ( McGowan 1976; SMC J68516, personal observations LLD) and Undorosaurus? kristiansenae ( Druckenmiller et al. 2012) . The anterior tip of the right premaxilla is situated 70–90 mm posterior to the tip of the dentaries, but because the elements are displaced to some extent it is unknown whether this is due to taphonomic reasons or because the specimen possessed an underbite in life. The premaxilla increases in dorsoventral height posteriorly, as in Ophthalmosaurus icenicus and Palvennia hoybergeti , in contrast to Brachypterygius extremus ( McGowan 1976; Druckenmiller et al. 2012; Moon and Kirton 2016), and the element is concave in medial view with a thickened dorsal margin. The anteriormost 15 cm of the lateral surface bears two rows of foramina, one dorsal to the other, that coalesce into one anteroposteriorly directed groove posteriorly. The groove has a dorsal overhang formed by a sharp ridge and becomes shallower posteriorly. Posteriorly, the dorsal margin of the premaxilla is projected into a minute supranarial process, and the posteroventral margin forms the subnarial process. Athabascasaurus bitumineus also has an extremely reduced or absent supranarial process as well as a subnarial posterior process ( Druckenmiller and Maxwell 2010). This morphology contrasts the deeply forked posterior end with equal-sized sub- and supranarial processes in Gengasaurus nicosiai , Caypullisaurus bonapartei , and Platypterygius australis ( Kear 2005; Fernández 2007; Paparella et al. 2016). A few cm posterior to the anterior tip in ventral view, starts a series of 12 shallow but clearly demarcated tooth impressions, as in Ophthalmosaurus icenicus , Platypterygius hercynicus and Acamptonectes densus ( Kolb and Sander 2009; Fischer et al. 2012; Moon and Kirton 2016).In Platypterygius australis there are 40 shallow impressions ( Kear 2005). The tooth impressions do not continue posteriorly as far as the anterior tip of the maxilla, a feature also found in Pervushovisaurus campylodon , Acamptonectes densus and Aegirosaurus leptospondylus ( Bardet and Fernández 2000; Fischer et al. 2012; Fischer 2016), and the alveolar groove is very shallow in this area.

Maxilla ( Fig. 8A 1 , A 3 ): The overall shape of the maxilla in the Ophthalmosauridae indet. specimen PMO 224.252 resembles Ophthalmosaurus icenicus ( Moon and Kirton 2016) . The lateral surface is convex, with a triangular and mediolaterally thin dorsal flange and a thickened ventral portion. The maximum dorsoventral height of the element is encountered approximately midway anteroposteriorly, and approaches that of the premaxilla, in contrast to Sveltonectes insolitus where the maxilla is very reduced compared to other elements in this region ( Fischer et al. 2011). The dorsal margin and the lateral surface in the anterior portion of the element is smoother than the more posterior portion, which has a crenulated margin and a lateral surface with longitudinal ridges, probably for contact with the lacrimal and/or the jugal ( Druckenmiller et al. 2012; Fischer et al. 2014b; Moon and Kirton 2016). The dorsal margin is not made up of a series of processes as in Platypterygius australis ( Kear 2005) . The posterior portion of the element is drawn out into a long, narrow process, as in many ophthalmosaurids, e.g. Palvennia hoybergeti and Brachypterygius extremus ( Druckenmiller et al. 2012; McGowan 1976). The alveolar groove does not show tooth impressions.

Jugal ( Fig. 8 ): The suborbital bar of the jugal in Ophthalmosauridae indet. PMO 224.252 tapers anteriorly and is mediolaterally thicker than the posteriorly ascending process. The posteroventral corner is dorsoventrally and anteroposteriorly wider and more similar to Janusaurus lundi ( Roberts et al. 2014) and Leninia stellans ( Fischer et al. 2013b) than to the narrow corner of Palvennia hoybergeti ( Delsett et al. 2018) . The amount of curvature from the suborbital bar to the posteriorly ascending process is similar to Palvennia hoybergeti and Ophthalmosaurus icenicus , contrasting the straighter jugal in Undorosaurus? kristiansenae ( Druckenmiller et al. 2012, Moon and Kirton 2016). The posteriorly ascending process is relatively anteroposteriorly wide, similar to some Ophthalmosaurus icenicus specimens (e.g., CAMSM J29861; personal observations LLD) and Palvennia hoybergeti ( Delsett et al. 2018) . The posterior outline of the posterior process is crenulated.

Nasal ( Fig. 8 ): The anterior process of the nasal is thin and increases steadily in mediolateral width posteriorly. Except for the thin anteriormost portion, the nasal has the typical 90° bend forming a lateral extension found in most ophthalmosaurids ( Kear 2005; Fischer et al. 2012; Moon and Kirton 2016; Delsett et al. 2018). As preserved there is a 30 mm wide gap between the anterior portions of the nasals, which abruptly decreases in width posteriorly. The gap is probably taphonomic, as the nasals meet in a medial butt joint in Ophthalmosaurus icenicus ( Moon and Kirton 2016) , but it might partly represent a vacuity or an internasal foramen, a structure that is found in some ophthalmosaurids, e.g., Sveltonectes insolitus , however usually more posteriorly ( Fischer et al. 2011). In the posterior portion the element flares out into a lateral wing as in other ophthalmosaurids ( Kear 2005; Fischer et al. 2012). On the left side the lateral wing covers the prefrontal posterior to the narialis process. The medial margin of the nasal in this area is crenulated as in Platypterygius australis ( Kear 2005) . The posterior margins of the nasals are incompletely preserved, but clearly overlap the frontals on the left side of the skull where the preservation is best, and there is a lack of contact between the nasals and the parietals, similar to other ophthalmosaurids e.g., Platypterygius australis ( Kear 2005) .

Frontal ( Fig. 8A 1 , A 3 ): The frontals are preserved in articulation in PMO 224.252 ( Ophthalmosauridae indet.). They are overlapped by the nasals anteriorly and overlap the parietals posteriorly, but with incompletely preserved margins. There is a lack of interdigitating structures between the skull roof elements, in contrast to Acamptonectes densus ( Fischer et al. 2012) . The exact relationship with the parietal foramen is unknown, but the frontals meet its anterior bor- der and possibly surround it laterally. In contrast to platypterygiine ophthalmosaurids, the frontals seem not to have extended to the anterior margin of the supratemporal fenestra ( Kear 2005; Fischer et al. 2014b). The two frontals meet in a straight suture as in Ophthalmosaurus icenicus ( Moon and Kirton 2016) in contrast to the crenulated and interlocking margin in Platypterygius australis and Athabascasaurus bitumineus ( Kear 2005; Druckenmiller and Maxwell 2010). The element is flat, in contrast to the deeply concave frontal in Platypterygius australis ( Kear 2005) , and it possesses a processus temporalis, which is thin and stretches laterally under the jugal on the left side.

Parietal ( Fig. 8A 1 , A 3 ): The parietals are not complete posteriorly, and the right parietal preserves incomplete sutures to other elements. The dorsal surface of the parietal is slightly convex as in Palvennia hoybergeti ( Druckenmiller et al. 2012) , and not concave as in Gengasaurus nicosiai ( Paparella et al. 2016) . As preserved, the parietals border most of the parietal foramen laterally and posteriorly, but might have been overlain by the frontals in the anterior portion. In contrast, the parietals of Platypterygius hercynicus are excluded from contact with the parietal foramen (Fischer 2012). The parietals do not contact the prefrontal. The parietal foramen is anteroposteriorly elongated, and of the same relative size as in other ophthalmosaurids ( Maxwell et al. 2015; Moon and Kirton 2016) and not enlarged as is autapomorphic for Palvennia hoybergeti ( Druckenmiller et al. 2012; Delsett et al. 2018). The supratemporal fenestra is not reduced and its anterior margin reaches halfway into anteroposterior length of the parietal foramen.

Lacrimal ( Fig. 8 ): The left lacrimal is preserved in medial view and the right in lateral view in Ophthalmosauridae indet. PMO 224.252. The element has an anterior, a posterodorsal and a ventral process and is similar in overall shape to Palvennia hoybergeti ( Druckenmiller et al. 2012) . The anterior process is dorsoventrally taller than the posterior and drawn out into a small, triangular process that is similar to in shape but dorsoventrally taller than in Palvennia hoybergeti ( Druckenmiller et al. 2012) and Janusaurus lundi ( Roberts et al. 2014) . The dorsal process is of the same width as the anterior process. The anterior process is mediolaterally thin anteriorly and thicker along the posterior margin, where it has a ridge on the medial surface. The posterior process is a single narrow process, not a number of finger-like projections as in Platypterygius australis ( Kear 2005) . Similar to Leninia stellans ( Fischer et al. 2013b) the posterior process is anteroposteriorly long compared to the middle portion, compared to Undorosaurus? kristiansenae where the posterior process is relatively shorter ( Druckenmiller et al. 2012). The middle portion of the element is relatively large as in Caypullisaurus bonapartei ( Fernández 2007) and Platypterygius americanus ( McGowan 1972) , but smaller than in Simbirskiasaurus birjukovi ( Fischer et al. 2014b) . The lateral surface bears a diagonal ridge for the anteroposterior margin of the orbit, as in Athabascasaurus bitumineus and Sveltonectes insolitus ( Druckenmiller and Maxwell 2010; Fischer et al. 2012). On the medial surface are two slight depressions and a small ridge, probably for articulation with the maxilla ( Moon and Kirton 2016). The posterior margin is curved although not to the near to 90° autapomorphic bend of Undorosaurus? kristiansenae ( Druckenmiller et al. 2012) . The ventral surface of the middle portion is grooved.

Pterygoid ( Fig. 8A 2 , A 4 ): Two partial pterygoids are preserved in the Ophthalmosauridae indet. specimen PMO 224.252. The posterior portion is missing in both elements, and the medial sheet and anterior processes do not preserve details. As in other ophthalmosaurids, the medial sheet is dorsoventrally flattened and mediolaterally wider than the more anterior portion that is instead dorsoventrally thicker.

Parabasisphenoid ( Fig. 8A 2 , A 4 ): The only preserved part of the parabasisphenoid in PMO 224.252 is the majority of the anterior parasphenoid process (cultriform process). The preserved process is equally mediolaterally wide for occipital lamella its entire length, and widens mediolaterally for articulation with the basisphenoid posteriorly.

Quadrate ( Figs. 8A 2 , A 4 , 9A ): Both quadrates are preserved in Ophthalmosauridae indet. PMO 224.252, and the description is based on the better preserved left element which is nearly complete, with only an incomplete medial margin Fig. 9A ). The dorsal portion of the occipital lamella is triangular in posterior view and has a lateral reach approximately similar to that of the articular condyle. It is of approximately the same relative size as Ophthalmosaurus icenicus and Palvennia hoybergeti ( Moon and Kirton 2016; Delsett et al. 2018), while Platypterygius australis and P. hercynicus lack the lateral extension of the occipital lamella ( Kear 2005; Kolb and Sander 2009). The quadrate foramen ventral to the occipital lamella is relatively smaller than in Ophthalmosaurus icenicus ( Moon and Kirton 2016) . The pterygoid lamella covers a smaller area than in Ophthalmosaurus icenicus and Palvennia hoybergeti and is dorsoventrally straight in posterior view compared to the rounded outline in these taxa ( Moon and Kirton 2016; Delsett et al. 2018). Quadrate outline is however variable in Ophthalmosaurus icenicus Moon and Kirton 2016 ; e.g., rounded in GLAHM V1852 and straighter in MANCH L10304 personal observations AJR and LEIUG 90986 personal observations LLD). The pterygoid lamella is dorsoventrally taller relative to mediolateral width than in Sisteronia seeleyi ( Fischer et al. 2014a) . A ridge separates the occipital and pterygoid lamellae as in the Ophthalmosaurinae indet. specimen UAMES 3411 Druckenmiller and Maxwell 2013) but in contrast to PMO 222.667 which has no such ridge. The ridge is formed by a convex surface in the dorsal portion of the quadrate and a distinct dorsoventrally oriented ridge ventral to the stapedial facet as in Palvennia hoybergeti ( Delsett et al. 2018) .

The stapedial facet on the posterior surface is large and as in Palvennia hoybergeti it is situated in a more dorsal position than in Ophthalmosaurus icenicus and Platypterygius australis ( Kear 2005; Moon and Kirton 2016; Delsett et al. 2018). The facet is dorsoventrally elongated with a thickened lateral and ventral margin as in Sisteronia seeleyi and Sveltonectes insolitus ( Fischer et al. 2011; Fischer et al. 2014a). The articular condyle is relatively small compared to the rest of the element, compared to Acamptonectes densus and “ Grendelius ” alekseevi ( Fischer et al. 2012; Zverkov et al. 2015a). In ventral view, the articular condyle bears the surangular and articular facets on each side of a depression in ventral view. The anterior surface of the element is concave and featureless.

Dentary ( Fig. 8 ): The anterior tip of the dentary is straight as in Acamptonectes densus ( Fischer et al. 2012) . The dorsal and ventral margins curve around the dorsal and ventral margins of the surangular. The dentary is dorsoventrally narrow compared to anteroposterior length and resembles Aegirosaurus leptospondylus ( Bardet and Fernández 2000; LLD personal observations on SNSS-BSPG 1954 I 608) more than the more robust dentaries in Palvennia hoybergeti , Undorosaurus? kristiansenae ( Druckenmiller et al. 2012) and Brachypterygius extremus ( McGowan 1976) . In the anterior portion of the lateral surface are two rows of anteroposteriorly elongated foramina, as in Acamptonectes densus ( Fischer et al. 2012) and the Palvennia hoybergeti specimen PMO 222.669 ( Delsett et al. 2018). Posteriorly the foramina coalesce into a deep longitudinal groove that becomes shallower posteriorly as in Sveltonectes insolitus and Platypterygius australis ( Kear 2005; Fischer et al. 2011). The alveolar groove is shallower than in Palvennia hoybergeti ( Delsett et al. 2018) , and the anterior portion is partitioned into shallow tooth impressions as in Platypterygius australis ( Kear 2005) . On the medial surface the ventral margin is thickened, which forms a groove to the ventral margin that represents the anterior portion of the Meckelian canal ( Moon and Kirton 2016; Kear 2005). The element has a diagonal posterior margin.

Splenial ( Fig. 8 ): The total length of the splenial is less than the surangular, and it is slightly dorsoventrally shorter anteriorly than posteriorly as in Acamptonectes densus ( Fischer et al. 2012) . As in Palvennia hoybergeti and Ophthalmosaurus icenicus ( Moon and Kirton 2016; Delsett et al. 2018), the anterior, bifurcated portion, (“anterior fork”) is strongly elongated in contrast to Platypterygius australis ( Kear 2005) . In contrast to all of these, the anterior fork in PMO 224.252 has two additional, smaller processes between the dorsal and ventral processes. The opening between the two small processes might correspond to the tiny foramen found in Baptanodon natans ( Gilmore 1906) . As in Palvennia hoybergeti ( Delsett et al. 2018) one of the long processes is concave. Posterior to the fork, the element has a thickened dorsal ridge on the lateral surface, decreasing in size posteriorly, as in Ophthalmosaurus icenicus ( Moon and Kirton 2016) . The element lacks the ventral semicircular ridge which is found in Pervushovisaurus bannovkensis ( Fischer et al. 2014b) .

Angular ( Fig. 8A 2 , A 4 ): The right angular is preserved in articulation with the surangular and the prearticular, and lacks the posteriormost portion. The element is medially and laterally slightly convex, in contrast to Sveltonectes insolitus , which is medially and laterally concave ( Fischer et al. 2011). It has an extensive lateral exposure, common to all ophthalmosaurids ( Bardet and Fernández 2000; Fernández and Campos 2015; Fischer et al. 2012). In contrast to Palvennia hoybergeti , the element has a long and narrow anterior process ( Delsett et al. 2018) as found in other ophthalmosaurids (e.g., Druckenmiller et al. 2012; Moon and Kirton 2016). It does, however, extend shorter anteriorly than the surangular, in contrast to Aegirosaurus leptospondylus and Platypterygius australis where the two elements are of the same length ( Bardet and Fernández 2000; Kear 2005). The element increases in dorsoventral height posteriorly as in Ophthalmosaurus icenicus ( Moon and Kirton 2016) and is dorsoventrally tallest anterior to the dorsoventral maximum height of the prearticular, where it contributes less than half to the total height of the ramus in medial view.

Prearticular ( Fig. 8A 2 , A 4 ): The right prearticular is preserved in articulation between the surangular and angular in medial view, and lacks the posterior bar. The angular reaches further anteriorly than the prearticular in medial view, as in Janusaurus lundi ( Roberts et al. 2014) . The dorsal margin is gently curved in lateral view as in Ophthalmosaurus icenicus , Janusaurus lundi and Palvennia hoybergeti ( Roberts et al. 2014; Moon and Kirton 2016; Delsett et al. 2018) in contrast to the pointed margin in Platypterygius australis ( Kear 2005) . The dorsoventral maximum height is approximately one centimeter shorter than the height of the surangular.

Surangular ( Fig. 8 ): The description is mainly based on the right surangular, which is preserved articulated to the prearticular, angular and dentary. The anterior tip is dorsoventrally short and increases in height posteriorly as in other ophthalmosaurids (e.g., Druckenmiller et al. 2012; Moon and Kirton 2016). The dorsal and ventral margins are thickened and rounded, and due to this, the anterior portion of the medial surface is more concave than in Palvennia hoybergeti ( Delsett et al. 2018) but resembles Platypterygius hercynicus and P. australis in this aspect ( Kear 2005; Kolb and Sander 2009). In lateral view, the surangular possesses a fossa surangularis, as in e.g. Pervushovisaurus bannovkensis , in contrast to Sveltonectes insolitus , which lacks this feature ( Fischer et al. 2011, 2014b). The element is dorsoventrally tallest in the posterior portion, corresponding to the maximum dorsoventral height of the prearticular. The posteriormost portion is not preserved.

Dentition ( Fig. 8A 1 , A 3 ): Approximately 140 teeth are preserved in PMO 224.252 ( Ophthalmosauridae indet.), not in life position, but what is likely an almost correct order, i.e. the smallest teeth are preserved at the anteriormost portion of the rostrum. The teeth are poorly preserved, and few details from the surface can be described. Compared to other ophthalmosaurids, they are of an intermediate size and robustness: smaller than in Brachypterygius extremus and larger than in Aegirosaurus leptospondylus , and more similar to e.g., Palvennia hoybergeti ( McGowan 1976; Bardet and Fernández 2000; Delsett et al. 2018; LLD personal observations on SMC J68516 and SNSS-BSPG 1954 I 608). There is a significant increase in height and diameter of the teeth moving posteriorly in the jaw, from a total height of 16 mm in the anteriormost teeth to 41 mm in the largest, more posterior teeth, which is a wider size range than in Palvennia hoybergeti ( Delsett et al. 2018) . The crown and root are subtly ridged, with ridges that are more uneven in the root. The roots have an approximate rectangular cross section, with the shorter sides in the rectangle perpendicular to the curvation direction of the teeth. The corners of the rectangle are not well-defined.

Hyoid ( Fig. 9B ): The hyoid of Ophthalmosauridae indet. PMO 224.252 was found disarticulated on the right side of the skull, and is tentatively interpreted as the right. The element is almost straight as in Janusaurus lundi and Sveltonectes insolitus ( Fischer et al. 2011; Roberts et al. 2014) and differs from the more curved hyoids in Platypterygius hercynicus ( Kolb and Sander 2009) and PMO 222.667. The anterior end is mediolaterally thicker than the posterior end, as in Janusaurus lundi ( Roberts et al. 2014) and has an oval cross section. The anterior end is, in contrast, flattened in Gengasaurus nicosiai ( Paparella et al. 2016) and Sveltonectes insolitus ( Fischer et al. 2011) . The lateral surface (facing the mandible) of the hyoid in PMO 224.252 is flattened and bears a shallow groove, while the medial surface is convex.

Vertebral column and ribs: The five preserved vertebrae of Ophthalmosauridae indet. PMO 224.252 vary largely in size. The atlas-axis is the largest, and has one pentagonal surface whereas the other is too poorly preserved for description. On the lateral surface is a single rib facet in the dorsal half on each side, but there was very likely a more ventrally placed facet as the short, presumably “cervical” rib is bicipital. One vertebra is interpreted as dorsal because its two lateral rib facets are confluent with the anterior margin ( McGowan and Motani 2003). The smallest vertebral remains are interpreted to belong to caudal centra.

The neural arch of either the atlas or the axis is anteroposteriorly narrow, but too poorly preserved to warrant a description.

The rib fragments vary in size. The shortest, which is the only complete rib, is 11 cm, and is interpreted to be from the anteriormost portion (“cervical”) of the vertebral column. The longest rib was at least four times as long ( Fig. 9C ). The proximal heads of the ribs, where preserved, are bicapitate and they have a thickened dorsal margin, resulting in a T-shaped cross section in contrast to the typical figure eight cross section in ophthalmosaurids, but similar to PMO 222.667 in this aspect. The distalmost portion of the rib is subcircular in cross section and longitudinally striated which is uncommon among ophthalmosaurids, but found in the Ophthalmosauridae indet. specimen PMO 222.670 ( Delsett et al. 2017) and Palvennia hoybergeti ( Delsett et al. 2018) .

Remains from some gastralia are present, but none of them are complete. As in Ophthalmosaurus icenicus , Janusaurus lundi , Palvennia hoybergeti and Keilhauia nui they are circular to subcircular in cross section ( Roberts et al. 2014; Moon and Kirton 2016; Delsett et al. 2017, 2018).

Forefin elements: One complete but deformed, and one partial forefin element are preserved of the Ophthalmosauridae indet. specimen PMO 224.252. The complete element is oval in dorsal and ventral view and strongly thickened, and based on comparison to Palvennia hoybergeti Delsett et al. 2018 ), it might be a metacarpal. The less complete element is relatively small and circular in dorsal and ventral view, and is most likely a phalanx.

Remarks.—The preserved skull lacks the posteriormost portion of the lower mandibles and the basicranium, but by comparison to the holotype of Undorosaurus? kristiansenae and Ophthalmosaurus icenicus ( Druckenmiller et al. 2012; Moon and Kirton 2016), it is estimated that the preserved remains represent approximately 85% of total skull length. This gives an estimated total skull length in life of 1280 mm, which is longer than the holotypes of Palvennia hoybergeti (SVB 1451) and Undorosaurus? kristiansenae (PMO 214.578), the latter with a total body length of 5.5 meters Druckenmiller et al. 2012). The surface of the ribs and the best preserved skull elements display finished bone. Even though the typical ontogenetic criteria cannot be assessed Johnson 1977, Kear 2005), the large size and surface texture where accessible are valid indicators of an adult stage.

Figures

Fig. 8. Skull of Ophthalmosauridae indet. (PMO 224.252) from Spitsbergen, Svalbard, Slottsmøya Member Lagerstätte, Tithonian; in dorsal (A1, A3) and ventral (A2, A4) views. Photographs (A1, A2) and interpretative drawings (A3, A4). L, left; R, right; paf, parietal foramen; suf, supratemporal fenestra.

Fig. 9. Elements of Ophthalmosauridae indet. (PMO 224.252) from Spitsbergen, Svalbard, Slottsmøya Member Lagerstätte, Tithonian. A. Left quadrate in posterior view. B. Hyoid in anterior or posterior view. C. Rib in anterior or posterior view.

Fig. 1. A. Map of Svalbard archipelago and the main island Spitsbergen with excavation area marked with an asterisk. B. Geological map of the excavation sites for the SML ophthalmosaurid specimens described and discussed in this paper (red dots); see Fig. 2. Adapted from Hurum et al. (2012).

Fig. 3. Explanatory drawing of the skeleton of ophthalmosaurid ichthyosaur Keilhauia sp. (PMO 222.667) from Spitsbergen, Svalbard, Slottsmøya Member Lagerstätte, Tithonian. Modified and corrected from Delsett et al. (2016).

Fig. 2. Composite section of the Slottsmøya Member Lagerstätte with the ophthalmosaurids described and discussed in the text. Specimens described in this paper marked with an asterisk. Modified from Delsett et al. (2017). A bed with a high abundance of echinoderm fossils is set as marker bed (0 m) in the section (Hurum et al. 2012; Rousseau and Nakrem 2012).

Fig. 4. Rostrum fragment of ophthalmosaurid ichthyosaur Keilhauia sp. PMO 222.667) from Spitsbergen, Svalbard, Slottsmøya Member Lagerstätte, Tithonian. Premaxillae, nasals, and vomer in dorsal (A1) and lateral A2) views. Anterior to the right.