Keilhauia sp.

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 : 720-729

Keilhauia sp.

Material.— PMO 222.667 ( Figs. 3 –7, SOM 4: table 1), a partially articulated anterior portion of a skeleton. The carcass landed ventrally on the sea floor, and the elements are three-dimensional with few signs of distortion and compression compared to many other specimens from the same unit ( Delsett et al. 2016). The skull suffered a collapse during excavation. Approximately 50 incomplete teeth, two partial quadrates, a basioccipital and a basisphenoid, both articulars and a partial stapes were preserved disarticulated, as well as two partial hyoids. The atlas-axis and 23 additional vertebrae are preserved, with 15 neural arches, 35 ribs and several broken gastralia. Seventeen of the vertebrae are articulated with neural arches and with the gastralia, while the atlas-axis and four vertebrae were found in the proximity. Both scapulae are preserved, but only the left is complete. A partial interclavicle and two partial clavicles are preserved, in addition to one complete and one incomplete coracoid and the right humerus, which is preserved with 24 epi- and autapodial elements .

Description.— Premaxilla, nasal, and vomer ( Fig. 4A ): The preserved portion of the upper rostrum in PMO 222.667 ( Keilhauia sp. ) consists of partial premaxillae, nasals and vomers. The elements lack the anterior- and posteriormost ends and are fractured, but three-dimensional with surface details preserved. The premaxilla has the typical longitudinal groove dorsal to the alveolar groove as in many ophthalmosaurids such as Ophthalmosaurus icenicus ( Moon and Kirton 2016) and Platypterygius australis ( Kear 2005) , but the groove is deeper in Undorosaurus? kristiansenae ( Druckenmiller et al. 2012) . The alveolar groove is shallow and lacks tooth impressions. In anterior view the anterior elongated portion of the nasals have a triangular cross section with a flattened ventral margin. They are visible in dorsal view between the two premaxillae and decrease in dorsoventral height posteriorly. The anterior portions of the vomers are shifted laterally towards the left side of the rostrum. They are oval in cross section and increase in dorsoventral height posteriorly.

Basioccipital (Fig. 5A): The basioccipital of PMO 222.667 ( Keilhauia sp. ) is complete and three-dimensional, with only a few fractures. The anteroposterior length of the basioccipital is approximately the same as the dorsoventral height in lateral view, whereas in Janusaurus lundi the element is anteroposteriorly longer than tall ( Roberts et al. 2014). In dorsal view, the element is mediolaterally wider than anteroposteriorly long, as Gengasaurus nicosiai ( Paparella et al. 2016) and Ophthalmosaurus icenicus ( Moon and Kirton 2016) . Most of the anteroventral surface of the basioccipital (Fig. 5A 6) articulated with the basisphenoid and bears a shallow dorsoventrally oriented notochordal groove, as in Arthropterygius chrisorum (Maxwell 2010) and Ophthalmosaurus icenicus ( Moon and Kirton 2016) , whereas a groove is absent in Simbirskiasaurus birjukovi ( Fischer et al. 2014b) and Palvennia hoybergeti ( Druckenmiller et al. 2012; Delsett et al. 2018). Dorsally, the groove terminates in a notochordal pit, similar to Platypterygius australis ( Kear 2005) . The specimen lacks a basioccipital peg as Palvennia hoybergeti ( Druckenmiller et al. 2012) and Platypterygius hercynicus ( Kolb and Sander 2009) , whereas this feature is variably present in Ophthalmosaurus icenicus ( Moon and Kirton 2016) .

The occipital condyle (Fig. 5A 1) is approximately circular in posterior view as in Arthropterygius chrisorum (Maxwell 2010) and Acamptonectes densus ( Fischer et al. 2012) , whereas it is mediolaterally wider than tall in Janusaurus lundi ( Roberts et al. 2014) and Palvennia hoybergeti ( Druckenmiller et al. 2012) . The surface of the condyle is smooth. The notochordal pit is eight-shaped and situated near the middle of the condyle as in the SML Ophthalmosauridae indet. specimen PMO 224.250 ( Delsett et al. 2018). The condyle is poorly demarcated in posterior view and only a small portion of the extracondylar area is visible, as in Brachypterygius extremus , Muiscasaurus catheti , Sveltonectes insolitus , Janusaurus lundi , and “ Grendelius ” alekseevi ( McGowan 1976; Fischer et al. 2011; Roberts et al. 2014; Maxwell et al. 2015; Zverkov et al. 2015a). In Palvennia hoybergeti ( Druckenmiller et al. 2012) and Simbirskiasaurus birjukovi , no extracondylar area is visible in posterior view ( Fischer et al. 2014b), whereas a large portion is visible in Ophthalmosaurus icenicus ( Moon and Kirton 2016) , Undorosaurus? kristiansenae (see description of this element below) and Leninia stellans ( Fischer et al. 2013b) . The extracondylar area immediately surrounding the condyle laterally (Fig. 5A 3) and ventrally consists of unfinished bone, and anterior to this is an area of finished bone on each lateral surface that do not meet ventrally, as in Palvennia hoybergeti ( Delsett et al. 2018) . The floor of the foramen magnum (Fig. 5A 5) is elevated and bears a shallow groove on the dorsal surface. The exoccipital facets are less prominent than in Sisteronia seeleyi ( Fischer et al. 2014a) and Mollesaurus periallus ( Fernández 1999) . The opisthotic facets are raised as in Palvennia hoybergeti and Sveltonectes insolitus ( Fischer et al. 2011; Delsett et al. 2018), in contrast to Simbirskiasaurus birjukovi ( Fischer et al. 2014b) ; but they are not dorsoventrally elongated, in contrast to P. hoybergeti Delsett et al. 2018 ). The ventral surface of the basioccipital is convex. It lacks a ventral notch, as many other ophthalmosaurids (e.g., Druckenmiller et al. 2012; Fischer et al. 2014a; Roberts et al. 2014), in contrast to Ophthalmosaurus icenicus ( Moon and Kirton 2016) .

Basisphenoid (Fig. 5B): The basisphenoid of PMO 222.667 ( Keilhauia sp. ) is three-dimensional, with a ventral and anterior surface missing some parts. It differs from most other ophthalmosaurids in overall shape. We interpret the rugose and pentagonal surface as dorsal (Fig. 5B 4) because it has a median furrow, whereas the ventral surface is flat and smooth (following McGowan 1976; Fischer et al. 2011; Moon and Kirton 2016). The foramina for the carotid are thus situated on the anterior and posterior surfaces. On the dorsal surface, one third of the anteroposterior distance from the anterior margin is a dorsally tall ridge on each side of the furrow representing the anterior margin of the basioccipital facet, which extends to the posterior margin of the element. The dorsal surface (Fig. 5B 4) is strikingly similar to the posterior surface of the basisphenoid in Acamptonectes densus ( Fischer et al. 2012) in having a pentagonal shape with a middle furrow for most of its length. In our preferred orientation, the apex of the pentagon in dorsal view is directed anteriorly and represents the dorsum sellae, based on similarity to Ophthalmosaurus icenicus ( Moon and Kirton 2016) . This contrasts the interpretation of the basisphenoid in Acamptonectes densus , which implies that the basioccipital has a dorsal crest instead of a flattened dorsal surface ( Fischer et al. 2012). As in Ophthalmosaurus icenicus, PMO 222.667 has a large carotid foramen in the middle of the anterior surface (Fig. 5B 5, B 6), with the dorsum sellae overhanging the opening in anterior view ( Moon and Kirton 2016). On each side of the carotid foramen are facets probably for articulation to the pterygoids. In ventral view the articulation for the basipterygoid do not form processes as in other ophthalmosaurids, which means that they are smaller than the reduced processes in Sveltonectes insolitus and Sisteronia seeleyi ( McGowan 1976; Fischer et al. 2011, 2014a; Moon and Kirton 2016; Delsett et al. 2018). The parasphenoid probably originated ventral to the anterior carotid foramen as in e.g., Platypterygius australis and Sisteronia seeleyi ( Kear 2005; Fischer et al. 2014a), but this area is incomplete. The carotid exits posteriorly as in Arthropterygius chrisorum (Maxwell 2010) and Palvennia hoybergeti ( Delsett et al. 2018) , in contrast to other ophthalmosaurids where it exits ventrally ( McGowan 1976; Fernández 1999; Maxwell and Caldwell 2006; Fischer et al. 2011, 2014a; Moon and Kirton 2016). In Platypterygius australis the carotid arteries run from the anterodorsal to the posteroventral surfaces ( Kear 2005).

Stapes (Fig. 5C): The preserved portion of the stapes of PMO 222.667 ( Keilhauia sp. ) is the medial head as well as a minor portion of the shaft. The medial head has a smooth surface interpreted as the posterior surface. Compared to the shaft,themedialheadismoredorsoventrallyexpandedinone direction, and based on its similarity to Palvennia hoybergeti ( Delsett et al. 2018) , it is interpreted to be the dorsal portion. Following from this, the element is the right stapes. The medial head is roughly triangular in medial view with a dorsal,

Fig. 5. Basicranium elements of ophthalmosaurid ichthyosaur Keilhauia sp. ( PMO 222.667) from Spitsbergen, Svalbard, Slottsmøya Member Lagerstätte, → Tithonian. A. Basioccipital in posterior (A 1, A 2), left lateral (A 3, A 4), dorsal (A 5), and anterior (A 6) views. B. Basisphenoid in ventral (B 1), left lateral (B 2), dorsal (B 3, B 4), and anterior (B 5, B 6) views. C. Medial head of stapes in posterior view. D. Left quadrate in posterior view. E. Right quadrate in posterior view. Photographs (A 1, A 3, A 5, A 6, B 1, B 2, B 4, B 6, C–E) and interpretative drawings (A 2, A 4, B 3, B 5). Scale bars 10 mm.

triangular opisthotic facet.A ridge runs dorsoventrally across the medial surface and represents the posterior margin of the facet for the basioccipital, which is more well-defined than in Palvennia hoybergeti ( Druckenmiller et al. 2012; Delsett et al. 2018). Ventral and anterior to the basioccipital facet is the facet for the basisphenoid. The preserved portion of the shaft has a dorsoventrally narrower posterior than anterior margin, resulting in a pyriform cross section as in Platypterygius australis ( Kear 2005) . The shaft is slender in posterior view as in Janusaurus lundi and Palvennia hoybergeti , and narrower than in Ophthalmosaurus icenicus , Acamptonectes densus and Undorosaurus? kristiansenae ( Druckenmiller et al. 2012; Fischer et al. 2012; Roberts et al. 2014; Moon and Kirton 2016; Delsett et al. 2018). The disarticulated piece interpreted as the lateral head resembles that of Janusaurus lundi ( Roberts et al. 2014) in being dorsoventrally and anteroposteriorly narrow compared to the more robust lateral head of Undorosaurus? kristiansenae (see description of this element below).

Quadrate (Fig. 5D, E): The quadrates of PMO 222.667 Keilhauia sp. ) are oriented based on Ophthalmosaurus icenicus ( Moon and Kirton 2016) . The left quadrate is the most complete, but is missing dorsal and lateral portions of the occipital lamella. The element lacks a dorsoventral ridge separating it into defined occipital and pterygoid lamellae, which is found in the Ophthalmosaurinae indet. specimen UAMES 34111 ( Druckenmiller and Maxwell 2013). In posterior view, the medial margin of the pterygoid lamella is straighter than the more convex outline in Palvennia hoybergeti and Acamptonectes densus ( Fischer et al. 2012; Delsett et al. 2018). The medial margin of the pterygoid lamella bears a dorsoventrally oriented groove interpreted as the facet for the supratemporal. The shallow stapedial facet has a thickened ventral margin, as is common in ophthalmosaurids, e.g., Acamptonectes densus Fischer et al. 2012 ) and Sisteronia seeleyi ( Fischer et al. 2014a) . In ventral view, the articular condyle is rhomboid and mediolaterally wider than anteroposteriorly long, as the Ophthalmosaurinae indet. specimen UAMES 3411 Druckenmiller and Maxwell 2013). The articular condyle bears two facets separated by a shallow mediolaterally oriented groove, similar to Sveltonectes insolitus ( Fischer et al. 2011) and Ophthalmosaurus icenicus . The posterior and triangular facet is interpreted to be the articular facet and is the largest, while the mediolaterally elongated anterior facet is for articulation with the surangular ( Druckenmiller and Maxwell 2013; Moon and Kirton 2016).

Lower jaw ( Fig. 6A 1 , A 2 ): Only a small portion of what is interpreted as the right dentary is preserved, with an intact tooth row. Medially to the teeth is a narrow element that probably represents the dorsal anterior process of the splenial.

Articular ( Fig. 6B ): The articular of PMO 222.667 ( Keilhauia sp. ) is complete and interpreted to be a right articular based on its similarity to those of Palvennia hoybergeti and Platypterygius australis ( Kear 2005; Delsett et al. 2018). The element is mediolaterally compressed, similar to most other ophthalmosaurids but unlike Acamptonectes densus (Fischer 2012) . In anterior view, the articular surface is oval and not triangular as in Palvennia hoybergeti ( Delsett et al. 2018) .In medial view ( Fig.6B 1 ); the dorsal margin is slightly convex in contrast to the concave margin of Palvennia hoybergeti , but less convex than in Platypterygius australis ( Kear 2005; Delsett et al. 2018). The medial surface is convex, decreasing in mediolateral thickness into a thin flange on the ventral surface in the posterior half of the element, with a longer ventral reach than in Palvennia hoybergeti ( Delsett et al. 2018) . The articular of Sisteronia seeleyi has a similar medial surface but lacks the ventral flange ( Fischer et al. 2014a). The lateral surface ( Fig. 6B 2 ) is flat and featureless except for a small diagonal ridge, similar to but less well-defined than in Mollesaurus periallus ( Fernández 1999; personal observations AJR on MOZ 2282 V). The posterior margin is mediolaterally thickened in comparison to the middle of the element, in contrast to Palvennia hoybergeti ( Delsett et al. 2018) .

Dentition ( Fig. 6D–F ): Some teeth are still attached to the dentary in PMO 222.667 ( Keilhauia sp. ) ( Fig. 6A 2 , A 3 ) and are tightly packed as in Aegirosaurus leptospondylus ( Bardet and Fernández 2000; personal observations LLD on SNSB-BSPG 1954 I 608). As preserved, they are posteriorly inclined. In the posterior portion are two rows of teeth adjacent to each other. μCT scan showed that the apex of the crowns belonging to one tooth row are ventrally directed and are interpreted as belonging to the dentary ( Fig. 6A 2 , A 3 ). None of the teeth are preserved in entirety, but based on the remains; none of them seem to have surpassed 30 mm in total length (crown + root). The crown is slightly curved ( Fig. 6D ) as in Aegirosaurus leptospondylus ( Bardet and Fernández 2000; personal observations LLD on SNSB-BSPG 1954 I 608) and Platypterygius australis ( Kear 2005) . The crown is finely striated with parallel striations that extend from the base of the crown to the tip ( Fig. 6D–F ), as in Platypterygius australis ( Kear 2005) and Athabascasaurus bitumineus ( Druckenmiller and Maxwell 2010) . The base of the enamel is well defined and forms a straight line, as in Pervushovisaurus bannovkensis ( Fischer et al. 2014b) and Paraophthalmosaurus ( Efimov 1999b) , but the crown is not as narrow compared to the root as in Paraophthalmosaurus ( Efimov 1999b) , nor has it a constriction at the base, as in Acamptonectes densus ( Fischer et al. 2012) . In cross section, the root is rounded, as in Undorosaurus? kristiansenae and Keilhauia nui ( Druckenmiller et al. 2012; Delsett et al. 2017), but in contrast to Palvennia hoybergeti ( Delsett et al. 2018) . A single root is squared in cross section, but this seems to be due to erosion or resorption. In Ophthalmosaurus icenicus , the roots are transversely compressed ( Moon and Kirton 2016), but this is not seen in PMO 222.667. Quadrangular roots are found in several ophthalmosaurids e.g., Acamptonectes densus ( Fischer et al. 2012) and Sisteronia seeleyi ( Fischer et al. 2014a) . Unlike Simbirskiasaurus birjukovi , the teeth lack apicobasal ridges in the root ( Fischer et al. 2014b).

Hyoid ( Fig. 6C ): Two partial hyoids from of PMO 222.667 ( Keilhauia sp. ) are preserved; one is in pieces, while the second consists of two larger portions that likely belong to the same element. The hyoid is more strongly curved than Platypterygius hercynicus ( Kolb and Sander 2009) , P. australis ( Kear 2005) , Janusaurus lundi ( Roberts et al. 2014) and Palvennia hoybergeti ( Delsett et al. 2018) . The element has an oval cross section, and is more flattened at the end interpreted to be anterior than the posterior, as in Platypterygius hercynicus ( Kolb and Sander 2009) and Gengasaurus nicosiai ( Paparella et al. 2016) . In anterior view, the anterior end is mediolaterally narrow and pitted. The posterior portion has a depression on both sides as in Palvennia hoybergeti ( Delsett et al. 2018)

Interclavicle (Fig. 7A): The interclavicle of PMO 222.667 ( Keilhauia sp. ) has the T-shape in ventral view (Fig. 7A 1) typical of ophthalmosaurids, with incomplete lateral and posterior margins. The element is dorsoventrally thin and fused to the medial portions of the two clavicles with a visible suture (Fig. 7A 2), a feature found in some of the largest and presumably more mature Ophthalmosaurus icenicus specimens ( Moon and Kirton 2016). The dorsal surface of the element is flat and makes a 90° angle to the medial portions of the clavicles (Fig. 7A 3). In ventral view, the transverse bar is relatively tall compared to the width, in contrast to the narrower transverse bar in Ophthalmosaurus icenicus and Undorosaurus? kristiansenae ( Druckenmiller et al. 2012; Moon and Kirton 2016). The ventral surface of the transverse bar is rugose, but lacks the triangular structure found in Caypullisaurus bonapartei (personal observations AJR on MLP 83-XI-16-1). The transition posteriorly to the median stem is gradual, as in Janusaurus lundi ( Roberts et al. 2014) and Paraophthalmosaurus ( Efimov 1999b, personal observations LLD on UPM EP-II- 7[1235]), in contrast to Undorosaurus? kristiansenae where the transition between the two parts is more abrupt ( Druckenmiller et al. 2012).

Clavicle (Fig. 7B): The preserved left clavicle is fractured, but almost complete, only missing some pieces in the medial portion. As in the holotype of Keilhauia nui , the clavicles in PMO 222.667 have dorsoventrally narrow medial ends without the interdigitating margin often found in ophthalmosaurids ( Moon and Kirton 2016; Delsett et al. 2017), but in PMO 222.667 the medial portion is thicker than in K. nui . The anteroposterior length of the medial portion of the clavicle is less than in Paraophthalmosaurus (personal observations LLD on UPM EP-II- 7[1235]). The curvature between the anterior and posterior portions is approximately similar to Aegirosaurus leptospondylus ( Bardet and Fernández 2000) and some specimens of Ophthalmosaurus icenicus (personal observations LLD on CAMSM J68689) but more curved than in other O. icenicus specimens (personal observations LLD on LEIUG 90986). The facet for the scapula on the posterior portion is demarcated by a ridge running along the anterior margin on the ventral surface.

Coracoid (Fig. 7C): PMO 222.667 ( Keilhauia sp. ) preserves a complete left and an incomplete right coracoid. The elements are three-dimensional but fractured. The element is anteroposteriorly longer than mediolaterally wide, similar to the holotype of Keilhauia nui and Undorosaurus? kristiansenae ( Druckenmiller et al. 2012; Delsett et al. 2017); but it is not as mediolaterally narrow as Paraophthalmosaurus ( UPM EP-II- 7[1235], personal observations LLD) ( Arkhangelsky 1997; Efimov 1999b). Due to the almost straight lateral and medial margins, the outline of the coracoid is more square than Acamptonectes densus ( Fischer et al. 2012) , and more similar to some Ophthalmosaurus icenicus specimens (e.g., CAMSM J65813 and LEICT 100 1949 2, personal observations LLD). As preserved, the anterior notch is mediolaterally wider and anteroposteriorly shallower than in Keilhauia nui ( Delsett et al. 2017) and Janusaurus lundi ( Roberts et al. 2014) . Compared to Janusaurus lundi ( Roberts et al. 2014) , the coracoid of PMO 222.667 has dorsoventrally taller glenoid and intercoracoid facets, giving the ventral surface of the coracoid a pronounced saddle-shape, whereas the dorsal surface is flat. The glenoid and scapular facets are rugose and not well demarcated, similar to Acamptonectes densus Fischer et al. 2012 ) but in contrast to Sveltonectes insolitus ( Fischer et al. 2011) , and less than in the holotype of Keilhauia nui ( Delsett et al. 2017) . There is less of an angle between the two facets than in Arthropterygius chrisorum Maxwell 2010 ) and Platypterygius hercynicus ( Kolb and Sander 2009) , and the scapular facet is the smaller. Similar to Arthropterygius chrisorum (Maxwell 2010) , the intercoracoid facet runs along the entire anteroposterior length of the coracoid, in contrast to Undorosaurus? kristiansenae where it covers only the anterior half ( Druckenmiller et al. 2012). The facet is pyriform in medial view and tallest anteriorly. The posterior margin of the coracoid is gently convex in dorsal view and dorsoventrally thin, with a groove running along the entire margin in posterior view, which is not found in Acamptonectes densus ( Fischer et al. 2012) .

Scapula (Fig. 7D): The left scapula of PMO 222.667 Keilhauia sp. ) is complete, whereas the right is incomplete, but both of them are three-dimensionally preserved. The scapula has a dorsoventrally expanded anterior portion and a straight posterior shaft. In lateral view, the anterior portion is less evenly dorsally and ventrally expanded than in Platypterygius australis and P. americanus ( Maxwell and Kear 2010; Zammit et al. 2010). PMO 222.667 resembles Keilhauia nui in having a slightly emarginated dorsal margin producing an acromion process that is less dorsally prominent than in Sveltonectes insolitus ( Fischer et al. 2011) and Acamptonectes densus ( Fischer et al. 2012) but larger than in Undorosaurus? kristiansenae ( Druckenmiller et al.

2012). The dorsolateral flange is small as in Keilhauia nui ( Delsett et al. 2017) . Ventral to the acromion process, the anterior margin is mediolaterally narrow and widens ventrally to form the coracoid and glenoid facets, which are poorly demarcated. The glenoid facet is oval in anterior view, coarsely rugose and slightly less than twice the dorsoventral height of the coracoid facet as in Acamptonectes densus ( Fischer et al. 2012) . In Undorosaurus? kristiansenae ( Druckenmiller et al. 2012) the two facets are more similar in height. The shaft is mediolaterally compressed as in Keilhauia nui and Acamptonectes densus ( Fischer et al. 2012; Delsett et al. 2017) in contrast to the rounded cross section in Platypterygius hercynicus ( Kolb and Sander 2009) . The posterior shaft has approximately the same dorsoventral height for all of its proximodistal length, in contrast to most ophthalmosaurids where the distalmost margin is dorsoventrally expanded in lateral view (e.g., Zammit et al. 2010; Fischer et al. 2011; Druckenmiller et al. 2012). As in K. nui , the distal end is angled so that the dorsal margin runs further posteriorly than the ventral margin ( Delsett et al. 2017).

Humerus (Fig. 7E): One humerus of PMO 222.667 ( Keilhauia sp. ) is complete and well preserved, and interpreted as a right humerus based on McGowan and Motani (2003) because of the anteriorly directed, larger and more “platelike” process interpreted as the dorsal process. The proximal surface is relatively flat, with a low ridge along the articular facet and is slightly dorsoventrally taller than the distal, as in Janusaurus lundi ( Roberts et al. 2014) . The dorsal process (Fig. 7E 1) is larger than the deltopectoral crest (Fig. 7E 2) and originates posterior to the midline of the element, in contrast to Janusaurus lundi where it originates in the middle ( Roberts et al. 2014). The dorsal process extends slightly beyond the proximodistal midpoint, which is relatively longer than Aegirosaurus leptospondylus ( LLD personal observations on SNSB-BSPG 1954 I 608), but shorter than Arthropterygius chrisorum (Maxwell 2010) and Undorosaurus? kristiansenae ( Druckenmiller et al. 2012) . As in Keilhauia nui ( Delsett et al. 2017) , the deltopectoral crest is restricted to the proximal and anterior portion of the ventral surface. The deltopectoral crest almost reaches the midpoint of the humerus, as in Janusaurus lundi ( Roberts et al. 2014) and Arthropterygius chrisorum (Maxwell 2010) , whereas it is longer in Ophthalmosaurus icenicus ( Moon and Kirton 2016) and Sisteronia seeleyi ( Fischer et al. 2014a) . The humerus has three distal articular facets for the preaxial accessory element, a radius and an ulna, typical of most ophthalmosaurids ( Maxwell and Caldwell 2006; Maxwell 2010; Roberts et al. 2014; Fernández and Campos 2015; Paparella et al. 2016; Delsett et al. 2017).

Fig. 7. Postcranial elements of ophthalmosaurid ichthyosaur Keilhauia sp. ( PMO 222.667) from Spitsbergen, Svalbard, Slottsmøya Member Lagerstätte, → Tithonian. A. Interclavicle fused to medial portion of clavicles in ventral (A 1, A 2) and dorsal (A 3) views. Dotted lines represent incomplete margins.

B. Left clavicle in ventral (B 1) and dorsal (B 2) views. C. Left coracoid in ventral view. D. Left scapula in dorsal (D 1) and ventral (D 2) views. E. Right humerus, radius, and ulna in dorsal (E 1) and ventral (E 2) views. F. Atlas-axis in anterior (F 1), posterior (F 2), and left lateral (F 3) views. G. Articulated portion of dorsal vertebral column with ribs and neural arches in dorsal view. Photographs (A 1, A 3, B–G) and interpretative drawings (A 2, A 4).

ulna

In contrast, Sveltonectes insolitus ( Fischer et al. 2011) and Nannopterygius enthekiodon ( Hulke 1871) have two facets, whereas Brachypterygius extremus ( Boulenger 1904) and Aegirosaurus leptospondylus ( Bardet and Fernández 2000) have a third facet for the intermedium. The ulna and radius facets are separated by a prominent ridge, whereas the facets for the radius and the preaxial accessory element are separated only by a minute ridge. The facet for the preaxial accessory element is circular in proximal view. The ulnar and radial facets are equally anteroposteriorly long, but the facet for the radius is significantly dorsoventrally taller and anteroposteriorly elongated, whereas the facet for the ulna is rectangular. It differs from Janusaurus lundi ( Roberts et al. 2014) in that the ulnar facet is not as tall relative to the radial facet. The ulnar facet deflects posteriorly, as in Janusaurus lundi ( Roberts et al. 2014) and Keilhauia nui ( Delsett et al. 2017) , unlike Gengasaurus nicosiai where it is not deflected Paparella et al. 2016).

Epi- and autopodial elements (Fig. 7E): The radius and ulna were complete and found articulated to the humerus. The radius is oval in dorsal view and dorsoventrally taller than the ulna, whereas the ulna is proximodistally longer. The radial facet of the ulna is straight in dorsal view and much taller dorsoventrally than the posterior margin, which is rounded and dorsoventrally very thin. It is not possible to determine the identity of the remaining 20 flattened and more or less circular elements that were found together with the humerus and are interpreted to belong to the same limb. Five elements are approaching the ulna and radius in size and probably represent the preaxial accessory element, proximal and possibly some distal carpals. One element is sickle-shaped, and is probably the pisiform. The 14 smaller elements represent distal carpals and phalanges. One additional forefin element was found close to the skull remains, and it is unknown whether it belongs in the right or left forefin.

Vertebral column and ribs (Fig. 7F, G): The elements in the vertebral column of PMO 222.667 ( Keilhauia sp. ) are well-preserved and complete. The atlas and axis are fully fused to each other (Fig. 7F 3). The axis is almost twice as anteroposteriorly long as the atlas and the suture is well defined in the dorsal half of the element, similar to Platypterygius australis ( Zammit et al. 2010) . The diapophysis fuse with the neural arch facet, whereas the parapophyses are confluent with the anterior edge of the element and are situated in the dorsal half of the centra. In anterior view (Fig. 7F 1), the atlantal surface is rhomboid, whereas the axial surface (Fig. 7F 2) has a more rounded outline as in Platypterygius australis ( Zammit et al. 2010) and Arthropterygius chrisorum (Maxwell 2010) . The atlantal surface has the deepest articular surface of the two, as in Platypterygius americanus ( Maxwell and Kear 2010) . The seventeen articulated vertebrae (Fig. 7G) are interpreted as anterior dorsal centra, based on the presence of distinct diapophyses confluent with the neural arch facet ( Fischer et al. 2011; McGowan and Motani 2003). The vertebrae are deeply amphicoelous and pentagonal in anterior view, similar to the anteriormost vertebrae in Platypterygius americanus ( Maxwell and Kear 2010) . The neural canal is flat and bordered by dorsoventrally low, but distinct neural arch facets. The parapophysis and diapophysis are situated on the anterior margin of the vertebrae, which differs from Arthropterygius chrisorum , where the parapophyses in the anterior dorsal region are not connected to the anterior edge (Maxwell 2010). In the anteriormost centra, a ridge connects the diapophysis with the parapophysis. Posteriorly, the parapophysis is situated in an increasingly ventral position on the lateral surface, and in the posteriormost preserved vertebrae in the dorsoventral midpoint, similar to Platypterygius americanus ( Maxwell and Kear 2010) . The two facets become deeper and more well-defined posteriorly, as do the neural arch facet. The vertebrae are at least twice as high the length and slightly wider than high, a common relationship for Ophthalmosauridae (Fischer 2012) . Their mediolateral width increases rapidly posteriorly in the column, whereas the anteroposterior length only increases slightly. The dorsoventral height has a net increase posteriorly. This is different from Ophthalmosaurus icenicus where both width and height increase rapidly in this region ( Buchholtz 2001). Two disarticulated vertebrae, found close to the articulated column, are similar in morphology and likely connect to this series.In addition, four disarticulated centra that most likely originate from a more posterior position in the column are preserved. Two of these centra possess smaller apophyses than those in the articulated column, and a third differs from any other by having the two rib facets situated at the same dorsoventral position, one on the anterior and the other on the posterior margin.

Fifteen neural arches were preserved, not fused to the centra. The neural spines are between 20 and 30 mm long and posteriorly inclined. The dorsal end of the neural spines is flat, with small pits indicating a cartilage extension, without the notch found in Undorosaurus? kristiansenae ( Druckenmiller et al. 2012) . The prezygapophysis is square or trapezoidal in anterior view and in some neural arches has a dorsoventrally oriented ridge, whereas the postzygapophyses are oval.

The ribs are fractured, and none of them are preserved in their entire length. The most complete rib measures 56 cm. The rib heads are bicipital, with a shorter tuberculum than capitulum. The ribs are unique among ophthalmosaurids in being anteroposteriorly flattened and T-shaped in cross section proximally, with a thickened dorsal margin that is larger on the posterior surface than on the anterior. The midshaft is oval in cross section, whereas the distalmost portion is almost circular. Most ophthalmosaurids have figure eight-shaped ribs at least in the proximal portion (e.g., Maxwell and Kear 2010; Druckenmiller et al. 2012; Moon and Kirton 2016), the exceptions being Acamptonectes densus ( Fischer et al. 2012) and an Ophthalmosauridae indet. specimen from the SML ( PMO 222.670) ( Delsett et al. 2017), but both of the latter have instead a rounded cross section.

Remarks.—The proximal articular surface of the humerus is flattened, traditionally used as a criteria for immaturity ( Johnson 1977), but many specimens possess this trait regardless of ontogenetic stage ( Roberts et al. 2014). The surface of the humerus consists of finished bone, the dorsal and deltopectoral crest are well developed, and the distal articular facets and forefin elements are properly ossified, all indicating an adult stage ( Johnson 1977; Kear and Zammit 2014). The fusion of the clavicles to the interclavicle suggest adult (or mature) stage. ( Moon and Kirton 2016). Compared to the size of the late juvenile to adult holotype of Keilhauia ( Keilhauia nui, PMO 222.655), the overlapping elements of the new specimen (proximodistal length of humerus and scapula, and anteroposterior length of coracoid) are 40–60% larger ( Delsett et al. 2017).

Figures

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. 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.

Fig. 6. Cranial elements of ophthalmosaurid ichthyosaur Keilhauia sp. (PMO 222.667) from Spitsbergen, Svalbard, Slottsmøya Member Lagerstätte, Tithonian. A. Lower jaw fragment in dorsal view (anterior to the left). A3, detail of inner structures with arrow showing position of ventral-pointing tooth from the upper jaw, anterior to the right, in dorsal view. B. Right articular in medial (B1) and lateral (B2) views. C. Hyoid in lateral or medial view (anterior to the left). D–F. Teeth. Photographs (A1, B–F), μCT scan images (A2, A3).

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. 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).