Differential diagnosis.

An ophthalmosaurid with the following autapomorphies: ilium anteroposteriorly expanded at the dorsal end; ischiopubis shorter or subequal in length to the femur (differs from all other Middle-Late Jurassic and Cretaceous ophthalmosaurids, but found in some specimens of Stenopterygius sp).

The species also possesses the following unique character combination: Posterior dorsal and anterior caudal centra less than 3.5 times as high as long (4 times or more in Ophthalmosaurus icenicus and Arthropterygius chrisorum)[52, 53]; glenoid contribution of the scapula larger than coracoid facet (smaller in Caypullisaurus bonapartei and Sveltonectes insolitus)[24, 54]; small acromion process of scapula (prominent in Brachypterygius extremus, Acamptonectes densus, Platypterygius americanus, Caypullisaurus bonapartei and Sveltonectes insolitus)[19, 24, 52, 54, 55]; anteromedial process of coracoid present (not present in Caypullisaurus bonapartei and Platypterygius australis)[26, 54]; absence of a strongly developed deltopectoral crest of the humerus (present in Caypullisaurus bonapartei, Platypterygius americanus and Sveltonectes insolitus)[24, 54, 55]; humerus with facet for preaxial accessory element anterior to radius (absent in Brachypterygius extremus, Maiaspondylus lindoei, Aegirosaurus leptospondylus and Sveltonectes insolitus)[24, 52, 56, 57]; posteriorly deflected ulnar facet (absent in Brachypterygius extremus, Maiaspondylus lindoei, Caypullisaurus bonapartei, Platypterygius australis, Aegirosaurus leptospondylus, Sveltonectes insolitus and Cryopterygius kristiansenae)[24–26, 52, 54, 56, 57]; lack of a contact between the humerus and intermedium (present in Brachypterygius extremus, Maiaspondylus lindoei and Aegirosaurus leptospondylus)[52, 56, 57]; complete fusion of the ischium and pubis lacking an obturator foramen (only proximal fusion in Cryopterygius kristiansenae and Undorosaurus gorodischensis, present with foramen in Ophthalmosaurus icenicus)[25, 52, 58]; medial margin of ischiopubis straight in dorsal and ventral view (shared with Janusaurus lundi only)[29]; femur with two distal facets (three in Platypterygius americanus, P. australis, P. hercynicus and Paraophthalmosaurus saratoviensis)[26, 55, 59, 60].

Preservation.

PMO 222.655 is articulated and partially complete (Fig 3). The individual appears to have landed on the seafloor on its left lateral side. Skeletal preservation improves posteriorly. The only preserved portion of the skull are the tips of the rostral elements (length = 25 cm), including the anterior portions of the dentaries and premaxillae, and a smaller portion of the nasal (Fig 4). A few isolated partial teeth were found in association with fragments of the ribs and gastralia. The teeth most likely originate from the skull of PMO 222.655, and are probably not gut content, judging from their size. Almost the entire vertebral column is preserved in articulation. For the purposes of description, numbers of individual vertebrae (x1-x72) and neural arches (x1-x42) were given during preparation and indicate position relative to the anterior end of the preserved skeleton but do not correspond to their actual position in the column (Figs 5–7). Some vertebrae are probably missing in the cervical region. Anterior to the fault in the anterior dorsal region, the individual vertebrae are difficult to discern, and directly posterior to it, severely crushed. Vertebra x19 is not preserved, but its existence is indicated by a higher number of neural arches than vertebrae in this area. Most of the vertebrae preserve their neural arches. Anterior to the fault in the anterior dorsal region, seven vertebrae have parts of their neural arches. Posterior to this fault 43 neural arches are preserved, the most complete are those belonging to vertebrae x3-x6. Between the body and the preserved part of the tail is a compressed area with fragments of five vertebrae. The entire postflexural column is missing. Many ribs are preserved in articulation. Anterior to the fault in the anterior dorsal region, the ribs are in position but fractured. Posterior to this fault they are relatively well preserved, but almost none are complete. Fourty-one ribs are preserved in position on the left lateral side of the specimen and 21 on the right side. The left side of the pectoral girdle (Fig 8) is missing, as well as the left forefin and one of the ischiopubes. The pelvic girdle and hind fin (Figs 3 and 9) appear to have drifted or been moved anteriorly after death because the elements was found at the position of vertebra x20, whereas the sacral vertebra is interpreted to be vertebra x39 (see below).

Ontogeny.

The proximal head of the humerus is convex (Fig 8H), which is traditionally considered a criterion for skeletal maturity [61]. The surface texture of the humerus shaft is partly eroded, but is relatively smooth and not “sand paper-like” as would be expected in a juvenile specimen [61]. The degree of ossification (when it is possible to observe) resembles mature finished bone [61]. The closure of the gaps between proximal fin elements could not be assessed because the forefin was not in articulation [61]. Criteria relating to the orbit and sclerotic aperture, the basicranium and the neural spines [62] are not applicable because these parts are not preserved or poorly preserved. Based on available evidence and the size of the animal, we infer that PMO 222.655 reached a late juvenile to adult ontogenetic stage.

Total body length.

Estimated total body length is based on a combination of the length of the vertebral column and rostrum, an estimation of missing portions and relative position using the quarry map. The unpreserved part of the skull is assumed to represent two thirds of total skull length (50 cm) based on comparable skull dimensions of Cryopterygius kristiansenae (PMO 214.578) [25]. The preserved part of the vertebral column has a length of 260 cm. The missing cervical vertebrae are estimated to represent 10–20 cm, and the compressed caudal area might have extended the measureable body length up to 10 cm. These lengths depend on how many vertebrae are missing and how much the compression expanded the area anterior to the tail. We assume that the missing part is the fluke and that it was approximately 60–80 cm in length, based on the relative size of the fluke of Ophthalmosaurus icenicus [7]. This provides an estimated total body length of 3.8–4.3 metres.

Skull.

The premaxillae and nasals are not preserved well enough to present any details; however, the rostrum of Keilhauia nui (PMO 222.655) (Fig 4A and 4B) becomes dorsoventrally taller compared to the equivalent point in the rostrum of Aegirosaurus leptospondylus Wagner 1853 [56], and is more similar to that of Cryopterygius kristiansenae [25] and Caypullisaurus bonapartei Fernández 1997 [17, 54]. Nutrient foramina are visible on the lateral sides of the dentaries in a shallow longitudinal groove reaching almost to the anterior tip of these elements. Fragments from approximately ten teeth are preserved. None of them are complete, and none preserve the enamel, so that length and possible ornamentation is not possible to describe. The crowns that are preserved are conical, with a narrow crown apex and a slight lingual curvature.

Vertebral column.

Thirty-eight more or less well preserved vertebrae are preserved in the presacral region and 33 in the caudal (Figs 3 and 4C–4L). Additionally, poorly preserved remains of 5–7 vertebrae are located in the anteriormost area of the body, which are neither given numbers in the “x” series nor measured. Centrum number x39 (Figs 4E, 5 and 6) marks the approximate position of the sacral vertebra based on the observations that it possesses only one apophysis and thus might represent the transition from bicipital to unicipital ribs, and that rib length rapidly decreases from vertebra x28 (10 cm) to x39 (4 cm). However, the transition from bicipital to unicipital ribs could lie anterior to this because not all of the centra were removed from the matrix for closer examination. An exact presacral vertebral count is not possible to determine; however, based on the preserved centra, the presacral number must have been at least 43. This is more than in Nannopterygius enthekiodon Hulke 1871 (41 vertebrae [52]), Platypterygius americanus Nace 1939 (37 vertebrae [55]) and Athabascasaurus bitumineus Druckenmiller and Maxwell 2010 (42 vertebrae[22]). There is uncertainty regarding the number of presacral vertebrae in Ophthalmosaurus icenicus Seeley 1874, with estimates ranging from 38–43 [7, 52]. Platypterygius australis McCoy 1867 has 46 presacral vertebrae, [26], and Cryopterygius kristiansenae, Aegirosaurus leptospondylus and Platypterygius platydactylus Broili 1907 have more than 50 presacral vertebrae [25, 56, 63].

The height and width of the centra in the dorsal region increase posteriorly. In vertebra x18 (Fig 4C and 4D), the diapophysis and the parapophysis are situated midway between the dorsal and ventral edges on the lateral side, indicating that this is an anterior dorsal vertebra, and the apophyses on vertebra x29 (Fig 4F and 4G) are situated on the ventral half, typical of a posterior dorsal vertebra. The majority of the vertebrae from the dorsal region are circular in anterior or posterior view.

Generally, the caudal centra in the anterior part of the preflexural stock are the highest and widest in the entire column [7]. In Keilhauia nui (PMO 222.655), that is found at vertebra x40 (Figs 5 and 6). Similar to the dorsal vertebrae, the anterior caudal vertebrae are nearly circular in anterior or posterior view [7]. The largest height: length ratio is found in vertebra x59 (Fig 6) and decreases posterior to this. In the posterior part of the preflexural stock the height of the vertebrae decreases rapidly but the width decreases less rapidly. This results in dorsoventrally shortened vertebrae, which can be seen in centra x59-x71 (Fig 4I–4L). Vertebra x72 (Fig 4K and 4L) is higher than wide, an indication of the fluke base [7]. The centrum edges change from being sharp to broadly rounded shortly before the fluke [7], a feature that is observed from vertebra x69 and posteriorly. The apical centra were not preserved. Some small, rounded bones (diameter 10–15 mm) that were found in the region of vertebrae x50-x60 may be chevrons.

The neural spines are posteriorly inclined and of the same anteroposterior length throughout the axial skeleton, although in some cases the length expands slightly in the dorsalmost part (Fig 7). The anteroposterior length is similar to that of their corresponding vertebral centra. This is similar to Cryopterygius kristiansenae [25] but differs from Ophthalmosaurus icenicus, where the anteroposterior length increases posterior to vertebra 25 [52]. The mediolateral width of the neural spine is narrowest at the dorsal end (approximately 5 mm), and increases ventrally. The dorsal margins of the spines are straight and lack the dorsal notches as seen in Cryopterygius kristiansenae [25] or Platypterygius australis [64]. Whether the dorsal margins are rugose or have a longitudinal groove as in Ophthalmosaurus icenicus [52] for an extension of soft tissue is not possible to determine. The dorsal end of the neural spines in Aegirosaurus leptospondylus (SNSS-BSPG 1954 I 608) is smoother than in Keilhauia nui (PMO 222.655) and slightly convex. The neural spines are similar in height but the height drops by 30% at vertebra x20, and continues to decrease after that (Fig 7). In Ophthalmosaurus icenicus the neural spines decrease rapidly in height in the posterior trunk region. However, the maximum neural spine height is encountered approximately at vertebra 25 in Ophthalmosaurus icenicus [52], but this happens in the anterior dorsal section in Keilhauia nui (PMO 222.655). In the anteriormost preserved vertebrae in Keilhauia nui, the neural spine height is larger than the corresponding centrum height, and the relationship is reversed posterior to this, resembling the situation in Gengasaurus nicosiai Paparella, Maxwell, Cipriani, Roncacè and Caldwell 2016 [65]. For vertebra x39, the centrum height is twice as high as the neural spine height. The transition from the prezygapophysis to the pedicel is smooth, rather than abrupt. The pedicels, where visible, have a triangular shape in lateral view and are not thickened, rather they are mediolaterally narrow.

A single dorsal rib is preserved almost in entirety, probably belonging to vertebra x7, and is 81 cm long. More posterior ribs measure approximately 75 cm as preserved. Posterior to the dorsal-caudal transition, the ribs are between 2 and 5 cm in length until vertebra x60. The ribs are figure eight-shaped in cross section in the proximal portion, but this becomes less pronounced distally. This condition is typical of ophthalmosaurids, except for Acamptonectes densus Fischer, Maisch, Naish, Kosma, Liston, Joger, Krüger, Pérez, Tainsh and Appleby 2012, in which the ribs have a rounded cross-section with only a small groove on one side of some dorsal ribs [19] and Mollesaurus periallus Fernández 1999 in which the ribs bear a dorsal crest [66]. A few disarticulated gastralia fragments were preserved close to the caudal region. The fragments have a circular cross-section and are much smaller in diameter than the ribs.

Pectoral girdle.

The scapula (Fig 8A and 8B) has a dorsoventrally expanded proximal end and a straight distal blade. The anterior third of the element widens gradually into what we interpret as a relatively small acromion process with a triangular shape in lateral view. It has a partially broken ridge dorsally on the lateral side. The acromion process is small also in Cryopterygius kristiansenae [25], Platypterygius hercynicus Kuhn 1946 [59] and Sisteronia seeleyi Fischer, Bardet, Guiomar and Godefroit 2014 [18]. The acromion process is taller and more prominent in Acamptonectes densus [19], Sveltonectes insolitus Fischer, Masure, Arkhangelsky and Godefroit 2011[24] and Platypterygius australis [26] than in Keilhauia nui. The shape and size of the acromion process varies in different specimens of Ophthalmosaurus sp. [52] (from small in e.g. OUMNH J48007 to prominent and pointed LEIUG 90986; pers. obs. LLD. Specimens assigned to Ophthalmosaurus because of their similarity to published specimens in e.g. [52], and that both were found in the Oxford Clay.). In anterior view, the proximal end consists of the acromion process dorsally, and ventral to it, a mediolaterally narrower section, as well as a wider coracoid facet and a more prominent glenoid contribution ventrally. Similar to Sveltonectes insolitus [24] and an indeterminate Albertan ophthalmosaurid specimen (TMP 92.41.01 8]) the glenoid facet does not continue on the ventral side of the scapula in Keilhauia nui, as it does in Cryopterygius kristiansenae [25].

The scapular blade has its maximum mediolateral thickness approximately midlength between the proximal and distal end. In lateral view, the distal end of the blade is angled diagonally, so that the dorsal edge reaches approximately 5 mm further distally than the ventral portion. The blade has the same dorsoventral height throughout, similar to Acamptonectes densus [19]; in contrast, all other ophthalmosaurids have a dorsoventrally expanded distal end. Additionally, Platypterygius hercynicus and Sisteronia seeleyi have shafts that are more thickened mediolaterally [18, 59].

The right clavicle (Fig 8C and 8D) is nearly complete and not fused to any other pectoral element. The medial margin bears a medially-projecting process along its ventral edge that is squared in anterior view. It widens dorsoventrally at approximately 15 mm from the medial edge. Anteroposteriorly the projection is thin and surrounded by a rim posteriorly. This differs from most other ophthalmosaurids, which are dorsoventrally thicker medially, often with a digitiform outline along the midline in dorsal view [6], although a morphology resembling Keilhauia nui has been observed in some specimens of Ophthalmosaurus sp. (e.g., GLAHM:V1861, pers. obs. AJR). Distal to the dorsoventral thickening, the clavicle gradually narrows again towards the distal tip. The posterior side of the clavicle is concave for articulation with the interclavicle, and this part is longer mediodistally than the distal projection, compared to Ophthalmosaurus icenicus [52] and Mollesaurus periallus [66], where the two parts are approximately equal in length. In Janusaurus lundi the concave section is longer than that seen in Keilhauia nui [29]. The distal projection is curved posteriorly, and narrows into a 10 mm wide tip, resembling the condition seen in Ophthalmosaurus icenicus [52], Baptanodon natans Marsh 1879 [67] and Janusaurus lundi [29]. This differs from TMP 92.41.01 (Ophthalmosauridae indet.), which is at its anteroposteriorly widest at the distal end [8].

The right coracoid (Fig 8E) is poorly preserved. In ventral view, it is reniform in outline, and as preserved approximately equal in anteroposterior length and mediolateral width. In Cryopterygius kristiansenae [25], the coracoid is somewhat anteroposteriorly longer than wide and is subcircular, whereas the coracoid of Nannopterygius enthekiodon [52] and Sveltonectes insolitus [24] are considerably narrower in mediolateral width than long. The anterior notch is prominent, similar to that in Arthropterygius chrisorum Russell 1993 [53] and Ophthalmosaurus icenicus [52]. The coracoid also possesses an anteromedial process, as in Ophthalmosaurus icenicus [52] and Acamptonectes densus [19]. This feature is absent in Caypullisaurus bonapartei [54] and in Platypterygius australis [26]. The glenoid and scapular facets are well demarcated, similar to Sveltonectes insolitus [24], but in contrast to Acamptonectes densus, where the two facets are not separated [19]. In Arthropterygius chrisorum, the angle between the two facets is less acute than in Keilhauia nui [53]. The scapular facet is only 45% of the length of the glenoid facet. In Cryopterygius kristiansenae the scapular facet is 50% of the length of the glenoid [25], and they are close to equal length in Janusaurus lundi [29]. The thickness of the intercoracoid facet is not possible to assess due to the preservation.

Forefin.

The orientation of the humerus (Fig 8F and 8H) is based on McGowan and Motani [6] and Druckenmiller et al. [25]. Because of their position relative to the vertebral column, the pectoral girdle and forefin elements are interpreted to be from the right side. Of the two major processes on the humerus, we interpret the longer and more ridge-like (“plate-like”) to be the dorsal process, which makes this process more posteriorly situated than the deltopectoral crest. Following this interpretation, the postaxial edge is dorsoventrally narrower and sharper than the anterior edge in posterior view. The smallest facet is for the preaxial accessory element in most ophthalmosaurids, which also supports the interpretation of the processes. In proximal view (Fig 8H), the dorsal process is placed midway anteroposteriorly, and the more prominent deltopectoral crest is located in a relatively more anterior position. Although a small portion of the dorsal process (Fig 8F) might be lacking dorsally, the process is remarkably small in proximodistal length compared to that seen in other ophthalmosaurids, including Ophthalmosaurus icenicus [52] (e.g. OUMNH 48754, OUMNH 68542, LEIUG 90986 pers. obs. AJR, LLD) and Cryopterygius kristiansenae [25]. The dorsal process fails to meet the midpoint of the shaft, as in some specimens of Undorosaurus gorodischensis [58] and Aegirosaurus leptospondylus (SNSB-BSPG 1954 I 608, pers. obs. LLD). In Ophthalmosaurus icenicus, Brachypterygius extremus von Huene 1922 [52], Paraophthalmosaurus saveljeviensis Arkhangelsky 1997 [68] and Cryopterygius kristiansenae [25] the dorsal process extends to the humeral midpoint, and it extends further yet in Arthropterygius chrisorum [53], Platypterygius americanus, P. australis and P. hercynicus [55, 59, 63, 69]. The deltopectoral crest in Keilhauia nui is generally more prominent than the dorsal process (Fig 8G). As preserved, it is placed in the proximal and anterior region of the ventral surface. Similar to Arthropterygius chrisorum [53], Platypterygius hercynicus [59] and Janusaurus lundi [29], the deltopectoral crest of Keilhauia nui extends less than half of the total shaft length. In contrast, the deltopectoral crest extends distal to the mid-point in Ophthalmosaurus icenicus [52] and nearly reaches the distal end in Sisteronia seeleyi [18], Acamptonectes densus [19] and Platypterygius americanus [55].

The distal end of the humerus is anteroposteriorly wider than the proximal end. The shaft has only a weakly developed mid-shaft constriction, with the minimum width approximately 20% less than the proximal maximum width. The humerus bears three distal facets (Fig 8F and 8G). The anterior facet is partly eroded, but is interpreted to be for the preaxial accessory element, followed by those for the radius and ulna, as in the majority of ophthalmosaurids [25, 29, 30, 52, 65]. Of the three distal facets, that for the preaxial accessory element is the smallest. It is distorted, but a small rim can be seen on its anterior half. The radial facet is slightly longer anteroposteriorly than the ulnar facet and the dorsoventrally tallest of the three facets. The facet for the radius is oval in distal view, but is distorted along the anteriormost edge. The ulnar facet is posteriorly deflected, in contrast to Gengasaurus nicosiai [65], with an angle of approximately 120° to the radius facet. Sveltonectes insolitus [24], Nannopterygius enthekiodon [70], Platypterygius hauthali von Huene 1927 [69] and Platypterygius platydactylus [63] bear only two facets. Maiaspondylus lindoei Maxwell and Caldwell 2006 [71], Aegirosaurus leptospondylus [56], Brachypterygius extremus [72] and Platypterygius americanus [55] all have three distal facets, but the middle one belongs to the intermedium in the three first; in contrast Platypterygius americanus has one postaxial facet for an accessory element. Cryopterygius kristiansenae possesses two facets on the left humerus but three on the right [25]. A fourth facet for a postaxial element is seen in Platypterygius hercynicus [59] and in some specimens of Platypterygius australis [26]. Eight small forefin elements are associated with the humerus, of which the largest two might be the radius and ulna, based on their size and a possible articulation with the humerus.

Pelvic girdle.

The ilium (Fig 9A–9D) is mediolaterally narrower than other ophthalmosaurids (with the possible exception of Aegirosaurus leptospondylus) and as both ilia are preserved and show the same feature, it is not likely that it is a taphonomic feature. The ilia are oriented similar to those in Ophthalmosaurus icenicus [52] with the concave side facing posteriorly. The concavity is more pronounced in Athabascasaurus bitumineus [22] and Janusaurus lundi [29]. This is probably also the case in Aegirosaurus leptospondylus, however the preservation is rather poor (SNSB-BSPG 1954 I 608, pers. obs. LLD). It is not possible to distinguish between the right and the left ilium. The element is equally wide anteroposteriorly and widens dorsally, making the anteroposterior length of the dorsal end 1.5 times the length of the acetabular end, which is autapomorphic among ophthalmosaurids. The end interpreted to be the acetabulum is mediolaterally thicker than the rest of the element and lacks distinct distal facets. The acetabular edge is diagonal with respect to the two parallel sides of the shaft, with the anterior side extending further ventrally.

The ischiopubis (Fig 9E) is a proximodistally short element. It is partly broken and may be slightly flattened, but the proximodistal length appears preserved in its entirety, based on the unbroken medial margin. Notably, and unique among ophthalmosaurids, the ischiopubis is slightly shorter than the femur. The ischium and pubis are completely fused without any trace of a foramen. The portion of the element that is dorsoventrally thicker than the other is interpreted to be the pubis. The complete fusion of the elements is shared with Janusaurus lundi [29], Sveltonectes insolitus [24], Athabascasaurus bitumineus [22], Aegirosaurus leptospondylus [56], Caypullisaurus bonapartei [17] and possibly Platypterygius australis [26]. This differs from Ophthalmosaurus icenicus [52], Cryopterygius kristiansenae [25], Undorosaurus gorodischensis [58] and Paraophthalmosaurus (“Yasykovia”) [73], all of which have a foramen or open notch (see also Discussion). The ischiopubis has an elongate trapezoidal shape, and is oriented similar to Ophthalmosaurus icenicus [52], Stenopterygius quadriscissus [6] and Cryopterygius kristiansenae [25], such that the anteroposteriorly widest portion is interpreted as the medial end. The medial end is dorsoventrally thicker than the acetabular end, but whether this is a real or a taphonomic artifact is difficult to determine. Similar to Janusaurus lundi, the medial end of the ischiopubis is straight in dorsal view [29] whereas in other ophthalmosaurids it is typically more rounded (with the possible exception of Athabascasaurus)[24–26, 52, 56, 58, 73] (Caypullisaurus bonapartei: Fernández pers. comm.). The medial edge is 1.4 times anteroposteriorly wider than the acetabular end. In contrast, in Aegirosaurus leptospondylus the medial end is more than twice as long anteroposteriorly than the acetabular end [56] and in Janusaurus lundi it is 1.8 times as long [29]. Ophthalmosaurus icenicus [52] and Athabascasaurus bitumineus [22] are also wider medially, but Sveltonectes insolitus is unique in having an ischiopubis equally expanded at both ends [24].

Hind fin.

In PMO 222.655 both femora (Fig 9F–9H) are preserved; however, they are eroded and compressed (confirmed by чCT-scanning that indicates crushed trabeculae). The more complete femur is used primarily here for description, but because the processes are poorly preserved it is not possible to distinguish the left from the right. The anteroposterior orientation of the femora is based on the distinction between the tibial and fibular facets. Typically in ophthalmosaurids, the tibial facet is oriented perpendicular to the long axis of the femur, and the fibular facet is distinctly angled posterolaterally [14]. The proximal and distal ends are equally wide anteroposteriorly in dorsal view, with a modest mid-shaft constriction. The proximal end is slightly taller dorsoventrally along its anterior edge near the dorsal process. The dorsal and ventral processes are not well preserved, but do seem to have been dorsoventrally and proximodistally short compared to higher and longer processes in most ophthalmosaurids such as PMO 222.670, described below, and many specimens of Ophthalmosaurus icenicus ([52] and LEIUG 90986, pers. obs. LLD), as well as Cretaceous isolated femora from the Cambridge Greensand (e.g. CAMSM B58337-61 and TN1753 pers. obs. LLD and "Platypterygius" CAMSM B58058, pictured in [18]). However, in Arthropterygius chrisorum, the dorsal and ventral processes are poorly developed as in Keilhauia nui [53]. The femur bears two distal facets, typical of most ophthalmosaurids [24, 25, 29, 52, 53, 56, 58, 74], although Platypterygius hercynicus, P. americanus, P. australis [26, 55, 59] and Paraophthalmosaurus saratoviensis [60] have three. In Keilhauia nui the fibular facet faces posterolaterally, approximately 120 degrees relative to the tibial facet. Both facets have the same anteroposterior length. One of the femora was found with three associated, small rounded bones, presumably more distal elements of the hind fin, but of uncertain identity.

Ichthyosauria de Blainville 1835

Neoichthyosauria Sander 2000

Thunnosauria Motani 1999

Ophthalmosauridae Baur 1887

Ophthalmosauridae indet.

Referred material: PMO 222.670; posterior half of a large ichthyosaur, with a complete pelvic girdle and both femora. PMO 222.662; associated ilium, ischiopubis, femur and fibula, and articulated caudal vertebrae. PMO 227.932; a single ischiopubis, lacking only a part in the acetabular end.

PMO 222.670.

Figs 10 and 11, Table 2

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Fig 10.

Vertebral column of PMO 222.670 showing A, photograph of stratigraphically down side as preserved and prepared in the four jackets and B, illustration of the complete, restored, articulated series seen from the stratigraphic up side. Dashed line represents the edge of the cliff. Scale bar equals 50 cm. Modified from Delsett et al. 2016.

https://doi.org/10.1371/journal.pone.0169971.g010

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Fig 11. Pelvic girdle and femora of PMO 222.670.

Ilium in A and B lateral (?) and medial (?) views. Posterior is to the right in A. The other ilium in C and D lateral (?) and medial (?) views. Posterior is to the right in C. Right ischiopubis in E ventral (anterior to the right) and F dorsal views. Left ischiopubis in G dorsal (anterior to the left) and H ventral views. Right femur in I dorsal (anterior to the right) and J ventral views. Left femur in K dorsal (anterior to the left), L ventral, M proximal and N distal views. Scale bar equals 20 mm. Abbreviations: ac acetabulum end, d dorsal end, dp dorsal process, ff fibular facet, isch ischium, m medial end, n notch in acetabular end, p pubis, tf tibial facet, vp ventral process, vr ventral ridge.

https://doi.org/10.1371/journal.pone.0169971.g011

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Table 2. Selected measurements for PMO 222.670, Ophthalmosauridae indet.

Lengths are given in millimetres.

https://doi.org/10.1371/journal.pone.0169971.t002

Locality: Island of Spitsbergen, north side of Janusfjellet, approximately 13 km north of Longyearbyen, Svalbard, Norway. UTM WGS84 33X0519609 8695600.

Horizon and stage: Slottsmøya Member, Agardhfjellet Formation, Janusfjellet Subgroup, early middle Tithonian. 14.5 m above the echinoderm marker bed.

Preservation.

PMO 222.670 represents the posterior half of an individual that landed on the seafloor on its left lateral side (Fig 10). The posterior dorsal, preflexural and some postflexural vertebrae are preserved in articulation with the dorsal ribs and associated with a complete pelvic girdle and both femora. The pelvic girdle elements and femora (Fig 11) are three-dimensional and were found close to the vertebral column in a region where the ribs experience a rapid decrease in length and then disappear (Fig 10B), suggesting that they were preserved nearly in life position. Fragments of gastralia and some severely compressed neural arches are also preserved. The preservation of the rest of the specimen is poor, due in part to compaction, the presence of siderite in the surrounding matrix and the abundant occurrence of the bivalve Buchia [75] on and around the skeleton. The skeleton was also covered by a thick layer of calcium sulphate [76], all of which made it difficult to assess bone texture and see facets. Finally, the posteriormost part of the skeleton, which was preserved in permafrost, is better preserved than portions of the skeleton in the active layer.

Total length estimate and ontogenetic status.

The preserved length of PMO 222.670 is 279 cm. Based on the vertebral count of Ophthalmosaurus icenicus [52] and Cryopterygius kristiansenae [25], we estimate that 25–35 presacral vertebrae are missing, in addition to the skull. The distal end of the tail is curled up and probably not complete. Based on the length of preserved individual vertebrae (25–43 mm in anteroposterior length), the eroded presacral series had a length of 88–123 cm. Based on the body proportions of Cryopterygius kristiansenae, the skull was 20–25% of total body length. Assuming approximately 40 cm of the tail is missing the total body length is estimated at 510–590 cm. In Cryopterygius kristiansenae, which has a total length of 5.5 metres, the longest rib is 92 cm. In PMO 222.670 the longest rib exceeds 110 cm. Thus, it is likely the specimen was closer to 6, rather than 5 metres in total length. Based on its large size, the specimen is interpreted as an adult. Other ontogenetic criteria [61, 62] are not easily applicable for this specimen.

Vertebral column.

PMO 222.670 preserves a total of 65 vertebrae (Fig 10). Sixteen centra are assumed to belong to the dorsal series and were found articulated to long dorsal ribs, 23 belong to the preflexural series and 26 to the postflexural. The floor of the neural canal is consistently about 35 mm wide at its widest point. The centra in the transition between the dorsal and preflexural series have a diameter of 80–90 mm and their anteroposterior length varies between 32–42 mm as preserved. The vertebrae are compressed to some degree, which might affect the measurements. The preflexural centra associated with the pelvic girdle have a diameter of approximately 80 mm and an anteroposterior length of 25–35 mm. Posterior to this, anteroposterior length decreases gradually.

Eighty-six dorsal ribs were preserved in articulation with the vertebrae. Many ribs are well preserved for almost their entire length. The longest ribs exceed 110 cm in length and the precaudal ribs are double-headed. Some long ribs immediately anterior to the pelvic girdle have a shallow groove on the anterior and posterior surfaces, resulting in a figure eight cross-section, whereas the remaining dorsal ribs do not show this feature. This is unusual among ophthalmosaurids, which normally have figure eight-shaped dorsal ribs. Acamptonectes densus and Platypterygius americanus also lack the figure eight shape, but Acamptonectes densus has a small groove on one side on some dorsal ribs and Platypterygius americanus and Gengasaurus nicosiai have dorsal ribs with spatulate distal ends [19, 55, 65]. In the few figure eight-shaped ribs in PMO 222.670, the grooves are deeper on the anterior side, and the dorsal portion more robust than the ventral portion. The distal end of all the ribs lacks the groove, as is the case for most ophthalmosaurids. Instead, the distal 25–35% of each rib is longitudinally striated. This feature has not been previously mentioned for any other ophthalmosaurid. A few disarticulated and broken gastralia, with a circular cross-section, were preserved in the caudal region.

Pelvic girdle.

In lateral view, the ilium (Fig 11A–11D) is mediolaterally narrow and curved, presumably with the concave side facing posteriorly. The ilium is more curved than Keilhauia nui (PMO 222.655), but less so than Athabascasaurus bitumineus [22] and Janusaurus lundi [29]. The ilium of Janusaurus lundi also bears a prominent anterior process that is absent in PMO 222.670. It is not possible to interpret which element is left or right. The dorsoventral orientation of the ilium is interpreted on the basis of a small notch at one end, which is likely for articulation with the ischiopubis and is thus the acetabular end (Fig 11). The dorsal end is anteroposteriorly expanded, but less so than in Keilhauia nui. PMO 222.670 resembles TMP 92.41.01 (Ophthalmosauridae indet.) in that the element is anteroposteriorly widest dorsally [8]. The dorsal end is more slender mediolaterally than the rest of the element, and the ilia are in general more flattened mediolaterally than in Ophthalmosaurus icenicus (LEIUG 90986, pers. obs. LLD). This might be partly taphonomic, but as the two ilia are similar and other elements from the pelvic girdle are three-dimensional, it is interpreted to be an actual feature.

The ischium and pubis (Fig 11E–11H) are completely fused. The element is anteroposteriorly short, dorsoventrally thin and mediolaterally elongate. The two ischiopubes are identified as right (Fig 11E and 11F) and a left (Fig 11G and 11H). This is based on the observation that one surface is flatter than the other, which is interpreted to be the dorsal and visceral side. The dorsoventrally thicker and more rounded side is interpreted as the pubis, and is thus anterior. The left ischiopubis is longer than the right, which is probably a taphonomic artefact, as it is also less three-dimensional than the right. For the calculations, we used the mean length of each pair of elements, as was done for all other specimens which preserves both sides of a given element. In dorsal view, the anterior margin is straight, and the posterior margin is broadly concave. A prominent ridge on the ventral surface extends from the mid-point of the acetabular end towards the anterior edge at the approximate mid-length of the element. The acetabular portion is triangular in cross-section and thickened, bearing a rugose acetabular surface. The facet for the ilium is interpreted to be at the dorsoventrally tallest portion of the acetabular end. The medial end of the ischiopubis is anteroposteriorly expanded and dorsoventrally flattened. In medial view, it is slightly curved and bears a groove along its entire length, approximately 5 mm deep. The medial end is straight in dorsal view, as in Keilhauia nui, and 1.5 times wider anteroposteriorly than the acetabular end, similar to Keilhauia nui.

Hind fin.

The femora (Fig 11I–11N) are oriented based on Maxwell et al. [14]. The left femur (Fig 11K–11N) is three-dimensional and almost complete, but slightly compressed on the dorsal side. The right femur (Fig 11I and 11J) is distorted and eroded, missing parts of the posterior margin and the ventral process, but not very flattened due to compression. The dorsal and the ventral process are approximately the same height relative to the shaft, but the dorsal process is narrower than the ventral. The processes are most similar to Arthropterygius chrisorum in being distinct, but not large, and approximately of the same height [53], in contrast Sveltonectes insolitus has relatively larger processes [24]. In Athabascasaurus bitumineus [22] and “Otschevia zhuralevi” Efimov 1998 [74] the dorsal process is more prominent, but the ventral process is larger in Cryopterygius kristiansenae [25]. In Keilhauia nui and Platypterygius hercynicus the dorsal process extends a third of its proximodistal length [59], in Janusaurus lundi [29] and Arthropterygius chrisorum [53], it extends midway, and in Platypterygius americanus it covers almost the entire length [55]. The ventral process is straight and extends approximately two-thirds of total femoral length. This morphology differs from Caypullisaurus bonapartei, where the ventral process extends midway [17] and less than that in Platypterygius hercynicus [59] and Keilhauia nui. The anterior margin of the femur is dorsoventrally taller than the posterior margin. The anterior and posterior margins are concave in dorsal view, the posterior more so. A large, concave area posterior to the dorsal process covers two-thirds of the dorsal surface. Where this area ends, the bone is at its narrowest anteroposteriorly. The proximal end is slightly wider anteroposteriorly than the distal. The proximal surface (Fig 11M) is triangular in medial view, with each of the three sides being roughly the same length. The outline of the distal end (Fig 11N) is oval seen in lateral view, bearing two poorly demarcated facets. The tibial facet is more than twice the length of the fibular facet. In dorsal view, the fibular facet faces posterolaterally at almost 45 degrees relative to the tibial facet, similar to Cryopterygius kristiansenae [25], but more than Sveltonectes insolitus [24] and less than Keilhauia nui.

PMO 222.662.

Locality: Island of Spitsbergen, east side of Knorringfjellet, approximately 13 km north of Longyearbyen, Svalbard, Norway. UTM: WGS84 33X 528940 8692530.

Horizon and stage: Slottsmøya Member, Agardhfjellet Formation, Janusfjellet Subgroup, early middle Tithonian. 0.2 metres below the echinoderm marker bed.

Fig 12A–12F

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Fig 12. Pelvic girdles and hind fin material.

Ilium of PMO 222.662 in A medial or lateral view. Posterior is to the right. Femur and fibula of PMO 222.662 in B dorsal, C ventral and D proximal views. Ischiopubis of PMO 222.662 in E dorsal and F ventral views. Ischiopubis of PMO 227.932 in G and H dorsal (?) and ventral (?) views. Dashed line shows where pieces are missing. Scale bar for A-G equals 20 mm. The Bauer (1898) specimen pelvic girdle in I, not to scale. Compare I to the right ilium of PMO 222.654 Janusaurus lundi holotype in J lateral view and the left ischiopubis of PMO 222.654 in K dorsal view. Scale bar for J-K equals 10 mm. Abbreviations: ac acetabular end, d dorsal end, dp dorsal process, f fibula, m medial end, vp ventral process, vr ventral ridge.

https://doi.org/10.1371/journal.pone.0169971.g012

PMO 227.932.

Locality: Island of Spitsbergen, north side of Janusfjellet, approximately 13 km north of Longyearbyen, Svalbard, Norway. UTM: WGS84 33X 0519622 8695649

Horizon and stage: Slottsmøya Member, Agardhfjellet Formation, Janusfjellet Subgroup, early middle Tithonian. 15.7 metres above the echinoderm marker bed.

Fig 12G and 12H

Ilium.

From PMO 222.662, a single ilium (Fig 12A) is preserved. In lateral view, the element is curved, and the concavity is interpreted to be directed posteriorly. The curvature of the element is strongest at the approximate midpoint. Dorsal and posterior to this, the element narrows to the dorsal tip. The shape of the ilium is most similar to that of PMO 222.670, but the acetabular end is more anteroposteriorly expanded, and the posterior curvature more pronounced in PMO 222.662. The acetabular end bears a single large, posterior oval facet and a smaller rounded facet.

Ischiopubis.

Both PMO 222.662 and PMO 227.932 preserve a single ischiopubis, which are described and compared together here (Fig 12E–12H). The ischium and pubis are completely fused and lack a foramen. PMO 222.662 (Fig 12E and 12F) is interpreted to be the left based on the criteria used for PMO 222.670 (above), but this is uncertain for PMO 227.932. PMO 222.662 is similar in overall morphology to PMO 222.670, described above. PMO 227.932 shares some features with PMO 222.670; it is anteroposteriorly and dorsoventrally slender and proximodistally elongate, and has one sharp margin and the other thickened and rounded. Based on the orientation of PMO 222.670, the thickened and rounded margin is interpreted to be located posteriorly, even though the thickened part is narrower anteroposteriorly than in PMO 222.670. Compared to the width of the medial end, the minimum anteroposterior width of PMO 227.932 is less narrow than that of PMO 222.670. The anterior margin of PMO 222.662 is sharp and slightly curved in dorsal view, and the posterior margin is dorsoventrally taller and more curved than the anterior. The posterior margin of PMO 227.932 is straight and the anterior margin curves slightly, but less than in PMO 222.670. The acetabular surface of PMO 222.662 is triangular in lateral view and is rugose. Its facets are not well demarcated and the ischiopubis is somewhat eroded in the acetabular end. A triangular area in the posteroventral area is interpreted to be the facet for the femur and the facet for the ilium more dorsally placed. The medial margin of PMO 222.662 is tallest dorsoventrally at the midpoint, and is coarsely rugose. The medial end of PMO 227.932 is expanded, but straighter than in PMO 222.670 and bears a notch along its entire distance, 5–10 mm deep.

Femur.

A single femur is preserved in PMO 222.662 (Fig 12B–12D). The femur is constricted at midshaft and the distal end is anteroposteriorly wider than the proximal end. This differs from PMO 222.670, where the proximal end is anteroposteriorly wider, but resembles some specimens of Ophthalmosaurus icenicus (e.g. NHMUK 3534 and NHMUK 4531, pers. obs. AJR). It is oriented based on the processes and the facets according to Maxwell et al. [14] and the articulated hind fin of Cryopterygius kristiansenae [25]. The element has two processes, of which the smaller and less developed is assumed to be the dorsal process (Fig 12B). This process is barely expressed, but appears to extend past the midpoint of the shaft. The larger ventral process (Fig 12C) is more prominent and extends far past the midpoint of the shaft. Both processes are less developed than in many ophthalmosaurids, including PMO 222.670, but dorsoventrally taller than in Keilhauia nui. In proximal view (Fig 12D), the femoral head is an almost equilateral triangle, where the ventral process makes up one of the three corners. The distal end bears three facets, in contrast to Keilhauia nui, PMO 222.670 and most ophthalmosaurids, which have two facets, but similar to Platypterygius hercynicus, P. americanus, P. australis [26, 55, 59] and Paraophthalmosaurus saratoviensis [60]. One of the facets has a more acute angle relative to the proximodistal axis of the femur and is interpreted to be for the fibula. Thus, the femur is from the right side. The fibular and middle facets are longer anteroposteriorly than the anterior-most facet. The fibula (Fig 12B and 12C), which was found in articulation, is a square-shaped element.

Ilium.

There are several Triassic genera with anteroposteriorly broad ilia, such as Guanlingsaurus, whereas other ilia were relatively straight and simple, such as Toretocnemus (Fig 13). Californosaurus bears a process on the anterior side of the ventral half of the ilium. In Jurassic parvipelvians, the ilium became narrower and simpler, with a varying degree of posterior curvature. Suevoleviathan is a possible exception in that it might have possessed a process (see discussion on orientation in McGowan and Motani 2003 [6]).

One important result from this study is that there is no relative size reduction of the ilium, compared to either femoral length or to ischiopubis length through the Jurassic and Cretaceous. McGowan and Motani [6: 40] noted that the ilium of Stenopterygius “is often relatively much smaller than it is in Ichthyosaurus and Leptonectes”, presumably in relation to the other pelvic elements. Our calculations indicate that that the ilium of Stenopterygius is not particularly short relative to the femur or ischiopubis when compared to other Early–Middle Jurassic taxa (Fig 14B and 14C). There is generally little variation in the ilial shape of Stenopterygius; the curvature is similar in all investigated specimens, and the acetabular end is somewhat mediolaterally thicker than the dorsal end.

The morphology of the ilia from the Slottsmøya Member ichthyosaurs (Figs 9A–9D, 11A–11D, 12A, 12J and 13) differ from each other and from most other known genera. Cryopterygius kristiansenae has an anteroposteriorly broader ilium than most other parvipelvians, whereas PMO 222.670 and PMO 222.662 have simpler, more typical ilia (Figs 11A–11D and 12A). The most unusual ilia are probably those of Janusaurus lundi, which have a prominent anterodorsal process (Fig 12J). They resemble a specimen figured (and later lost in WWII) by Bauer [82] as “Ichthyosaurus posthumus” from Solnhofen (Lower Tithonian), which is possibly closely related to Nannopterygius [56] (Fig 12I). The element had previously been interpreted as an ischium [56, 82], but judging from its similarity to Janusaurus lundi, it is likely an ilium. A specimen of Ophthalmosaurus icenicus (NHMUK 3013, pers. obs. AJR) and a Slottsmøya Member specimen (PMO 222.662) (Fig 12A) possess an anterior margin with a distinct dorsal bend, but lack a distinct process as seen in Janusaurus lundi.

Ischium and pubis.

In Triassic ichthyopterygians, the ischium and pubis consist of two large, rounded bones (Fig 13) [83], similar to some terrestrial squamates. In most genera the obturator foramen is present. In basal ichthyopterygians, cymbospondylids as well as mixosaurids, the pubis is larger than the ischium (Fig 13). In shastasaurids, the two elements are similar in size, whereas among basal parvipelvians with a tripartite pelvis the ischium is the larger element [6, 11]. During the Early Jurassic, the ischium and pubis generally evolved into mediolaterally elongated elements. Stenopterygius and Hauffiopteryx were the stratigraphically earliest ichthyosaurs with a fused ischium and pubis, a feature also found to be subject to individual variation in Leptonectes and Temnodontosaurus [84]. In Hauffiopteryx, the two elements are fused only at the acetabular end, whereas Stenopterygius possesses only a small foramen. In Stenopterygius, the ischiopubis shows little intraspecific variation except in size. In Ichthyosaurus several species possess a pubis that is longer than the ischium, and the genus shows morphological variation in the pelvic girdle that might be important taxonomically [12].

The ischium and pubis are completely fused in the Slottsmøya Member ichthyosaurs (Figs 9E, 11E–11H, 12E–12H, 12K and 13) except for Cryopterygius kristiansenae, which shows a medial opening between the two elements. Undorosaurus gorodischensis also shows incomplete fusion [58] and an ischiopubis very similar to Cryopterygius kristiansenae. It has been suggested that the fusion might have been completed in life by cartilage [58]. Some specimens of Ophthalmosaurus icenicus exhibit a medial opening (probably between the two elements) that varies in size intraspecifically, and some specimens possess a foramen (e.g. Andrews 1910: 58; NHMUK 4754, AJR pers. obs.) [52, 85]. The loss of the ischiopubic foramen in Ophthalmosauridae occurred by the Late Jurassic, as shown in the Late Jurassic forms Aegirosaurus leptospondylus, Janusaurus lundi, Caypullisaurus bonapartei (Fig 13), Paraophthalmosaurus, PMO 222.670, PMO 222.662 and PMO 227.932.

Maisch and Matzke [11] suggested that complete proximal and distal fusion was a synapomorphy of the clade including Stenopterygius and Ophthalmosauridae. This is however not the result of the current phylogenetic analysis, where fusion of the ischiopubis does not appear as a synapomorphy for any clade. The varying degree of fusion of the ischium and pubis within Ophthalmosauridae could result from homoplasy or from intraspecific variation in some species. Complete fusion of the ischiopubis, including the lack of a foramen, can be observed in all known platypterygiine species for which an ischiopubis is known (Fig 13). However, complete fusion is also seen in two out of four known ophthalmosaurine species with a preserved ischiopubis, including Athabascasaurus bitumineus (alternatively recovered as a platypterygiine ophthalmosaurid in some studies [19]) and Janusaurus lundi [22, 29] and Keilhauia nui, whose phylogenetic position is considered uncertain. Thus, it is possible that complete fusion of the ischiopubis may be a valid synapomorphy for Platypterygiinae but not Ophthalmosaurinae [19].

The ischiopubis is long relative to the femur in Late Jurassic ophthalmosaurids, compared to Stenopterygius, Hauffiopteryx and the few known Cretaceous specimens (Fig 14D). Ischiopubes as large (twice as long or almost twice as long) compared to femur or ilium as those of Janusaurus lundi (PMO 222.654), PMO 222.670 and PMO 222.662 have not been described from other localities (Fig 14C and 14D). The large variation in relative size of the ischiopubis in Slottsmøya Member specimens may prove to be an informative new character in phylogenetic analyses, where the states are divided between specimens with an ischiopubis: ilium length ratio ≥ 2:1 and those with a lower ratio.