Undorosaurus? kristiansenae Druckenmiller, Hurum, Knutsen, and Nakrem, 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 : 734-737

Undorosaurus? kristiansenae Druckenmiller, Hurum, Knutsen, and Nakrem, 2012

Fig. 10 .

Holotype: PMO 214.578 , complete skeleton primarily described by Druckenmiller et al. (2012); basioccipital, basisphenoid, and stapes are described herein for the first time.

Material.— Type material only ( Fig. 10 , SOM 4: table 3).

Emended diagnosis.—Large ophthalmosaurid ichthyosaur (estimated 5.5 m total body length) with the following autapomorphies and unique character combinations: robust rostrum with snout ratio of 0.61 (relatively longer and more gracile in Aegirosaurus , Nannopterygius ); orbital ratio of 0.19 (relatively larger in Ophthalmosaurus , Nannopterygius ); supranarial process of premaxilla strongly reduced and not contacting the external naris (well-developed supranarial process contacting the external naris in Brachypterygius and Caypullisaurus ); subnarial process does not contact the jugal (contacts jugal in Brachypterygius ); lacrimal does not contact the external naris (contacts external naris in Ophthalmosaurus , Caypullisaurus , Aegirosaurus , Sveltonectes ); posterior margin of lacrimal forms distinct, nearly 90° angle (autapomorphic); maxilla with 23 teeth (10–13 in Ophthalmosaurus ); maxilla with extensive lateral exposure along the tooth row, extending as far posteriorly as the midpoint of the orbit (shorter exposure laterally in Brachypterygius , Aegirosaurus , Ophthalmosaurus ); jugal nearly straight (bowed in Aegirosaurus , Ophthalmosaurus ); postorbital bar anteroposteriorly broad(narrow in Ophthalmosaurus , Aegirosaurus , Nannopterygius ); element (supratemporal?) located posterior to the quadratojugal with a narrow, ventrally projecting process (autapomorphic); large exposure ofextracondylarareainposteriorview(littleornoexposurein Palvennia , Janusaurus, Simbirskiasauru s, Arthropterygius , Sisteronia , and Platypterygius australis ); large basipterygoid processes on basisphenoid (small or non-existent in Palvennia hoybergeti , Arthropterygius chrisorum , Sisteronia seeleyi ); teeth robust and large with numerous, fine, enamelled ridges (relatively smaller and more gracile in Aegirosaurus and Sveltonectes ); 52 presacral vertebrae (39– 42 in Ophthalmosaurus , 37? in Platypterygius americanus ); conspicuous V-shaped notch along the dorsal margin of presacral neural spines as seen in lateral view (autapomorphic among ophthalmosaurids); ribs 8-shaped in cross section (round in Acamptonectes ); relatively small forelimb bearing 5–6 digits (relatively larger with 6+ digits in Caypullisaurus , Platypterygius ); humerus with two distal facets only (three facets in Ophthalmosaurus , Aegirosaurus , Caypullisaurus , Undorosaurus , Brachypterygius , Arthropterygius , Acamptonectes , Palvennia ); rounded phalanges (rectangular in Platypterygius , Sveltonectes ); ischiopubis expanded and unfused distally (unlike Ophthalmosauru s, Keilhauia , Platypterygius australis , Sveltonectes , Aegirosaurus , Caypullisaurus ); femur anteroposteriorly broad with two facets distally (three distal facets in Platypterygius americanus , P. australi s).

Description.— Basioccipital ( Fig. 10A ): The dorsal and parts of the left lateral surfaces of the basioccipital from PMO 214.578 ( Undorosaurus? kristiansenae ) are damaged. The floor of the foramen magnum is narrowest anteriorly and increases in mediolateral width posteriorly ( Fig. 10A 4 ). In posterior view ( Fig. 10A 1 ), the occipital condyle is slightly mediolaterally wider than dorsoventrally tall and the extracondylar area is largely visible. Surrounding the condyle on all sides is a ring of unfinished bone. Anterior to this is a wider extracondylar area forming a second ring on the lateral and ventral surfaces, also visible in posterior view ( Fig. 10A 2 ). This is different from PMO 222.667, Simbirskiasaurus birjukovi ( Fischer et al. 2014b) , Arthropterygius chrisorum Maxwell 2010 ), Sisteronia seeleyi ( Fischer et al. 2014a) , Palvennia hoybergeti ( Delsett et al. 2018) and Platypterygius australis ( Kear 2005) , which have little or no extracondylar area visible in posterior view. The morphology resembles Ophthalmosaurus icenicus , Athabascasaurus bitumineus and Acamptonectes densus ( Druckenmiller and Maxwell 2010; Fischer et al. 2012; Moon and Kirton 2016), but this specimen has even more extensive extracondylar area exposed in posterior view and the condyle is more clearly set off from the extracondylar area. The basioccipital is not preserved in the holotype of Undorosaurus gorodischensis , but in another specimen attributed to the species, the basioccipital displays a similar overall shape ( Zverkov and Efimov 2019). The extracondylar area visible in posterior view in Undorosaurus gorodischensis ( Zverkov and Efimov 2019: fig. 6B, D) is relatively wide for an ophthalmosaurid, but not as extreme as that found in PMO 214.578. The notochordal pit in PMO 214.578 consists of only one pit, in contrast to PMO 222.667 ( Keilhauia sp. ) and Undorosaurus gorodischensis Zverkov and Efimov 2019 : fig. 6A) and the Undorosaurus sp. specimen UPM-EP-II-23(744). The notochordal pit is situated more dorsally than in Arthropterygius chrisorum (Maxwell 2010) and Janusaurus lundi ( Roberts et al. 2014) but more ventrally than in Brachypterygius extremus McGowan 1976 ). In lateral view, the stapedial and opisthotic facets are poorly separated from each other. Anteriorly on the ventral surface is a depression, probably representing a shallow ventral notch. The anteriormost portion of the element is significantly larger and more drawn out ventrally in PMO 214.578 than in Undorosaurus gorodischensis UPM EP-II-21(1075) ( Zverkov and Efimov 2019). Palvennia hoybergeti and Acamptonectes densus lack a ventral notch, whereas this structure is found in Ophthalmosaurus icenicus Fischer et al. 2012 ; Moon and Kirton 2016; Delsett et al. 2018). Because of preservation it is unknown whether the specimen possessed a basioccipital peg.

Basisphenoid ( Fig. 10B ): The basisphenoid from PMO 214.578 ( Undorosaurus? kristiansenae ) is damaged on the left side and has suffered deformation along the sagittal plane. The element is anteroposteriorly short compared to mediolateral width and dorsoventrally tall relative to anteroposterior length, compared to almost all other ophthalmosaurids, except Undorosaurus gorodischensis and Sveltonectes insolitus which are also relatively dorsoventrally tall ( Fischer et al. 2011; Zverkov and Efimov 2019). This might be affected to some degree by the deformation. The dorsal plateau covers a relatively small area and is coarsely rugose with a wide median furrow. The anterior, posterior and lateral surfaces slope steeply from the dorsal surface. The anterior carotid foramen is situated in the mediolateral and dorsoventral midpoint of the anterior surface and is smaller than the ventral foramen. Almost the entire posterior surface is taken up by the basioccipital facet. The ventral surface ( Fig. 10B 1 ) is flat with the base of the parasphenoid anterior to the carotid foramen, as in Ophthalmosaurus icenicus , Mollesaurus periallus , and Platypterygius australis ( Fernández 1999; Kear 2005; Maxwell 2010; Moon and Kirton 2016). The parasphenoid has an oval cross section in anterior view and the same mediolateral width as the foramen. The carotid foramen is bordered by ridges laterally and anteriorly as in Sisteronia seeleyi ( Fischer et al. 2014a) . A ventral carotid foramen is found in Undorosaurus gorodischensis , Ophthalmosaurus icenicus , and Brachypterygius extremus ( McGowan 1976, Moon and Kirton 2016, Zverkov and Efimov 2019) in contrast to the posterior foramen in Arthropterygius chrisorum (Maxwell 2010) , Palvennia hoybergeti ( Delsett et al. 2018) and PMO 222.667. In ventral view the basipterygoid process is among the largest in any ophthalmosaurid ( Fischer et al. 2014 a, Kirton 1983, Maxwell 2010), even larger than in Platypterygius australis ( Kear 2005) , Mollesaurus periallus ( Fernández 1999; AJR personal observation on MOZ 2282 V), Brachypterygius extremus ( McGowan 1976) and Undorosaurus gorodischensis ( Zverkov and Efimov 2019) .

Stapes ( Fig. 10C ): The two stapes from PMO 214.578 ( Undorosaurus? kristiansenae ) were oriented based on the articulation of the left to the basioccipital and their similarity to Palvennia hoybergeti ( Delsett et al. 2018) with a flatter posterior surface. The left stapes ( Fig. 10C ) is better preserved than the right, which is compressed and distorted. The overall morphology, especially relative size of the medial and lateral heads and shaft outline is most similar to Platypterygius australis ( Kear 2005) in constrast to the more slender stapes in Janusaurus lundi ( Roberts et al. 2014) . The medial head is dorsoventrally taller than anteroposteriorly long in medial view. The facet for the opisthotic is small and poorly demarcated. The facet for the basioccipital is the largest as in the Ophthalmosaurinae indet. specimen UAMES 3411 ( Druckenmiller and Maxwell 2013) and faces posteromedially with an anterior margin that is well defined by a dorsoventrally oriented ridge. Ventral to the basioccipital facet is the less well-defined basisphenoid facet. The shaft of the stapes has approximately the same relative thickness as in Platypterygius australis ( Kear 2005) and Ophthalmosaurus icenicus ( Moon and Kirton 2016) , which is anteroposteriorly and dorsoventrally thicker than in Janusaurus lundi ( Roberts et al. 2014) and Palvennia hoybergeti ( Druckenmiller et al. 2012) , but more gracile than in Sisteronia seeleyi ( Fischer et al. 2014a) and Leninia stellans ( Fischer et al. 2013b) . The shaft is pyriform in cross section, in contrast to the rounded cross section in the shaft of Janusaurus lundi ( Roberts et al. 2014) as in Platypterygius australis ( Kear 2005) . The lateral head is expanded both dorsally and ventrally as in Platypterygius australis ( Kear 2005) , in contrast to Acamptonectes densus ( Fischer et al. 2012) , Janusaurus lundi ( Roberts et al. 2014) and Palvennia hoybergeti ( Druckenmiller et al. 2012) , which have small lateral heads barely expanded relative to the shaft. The lateral head bears a triangular facet for the quadrate on its lateral surface. With regard to Undorosaurus gorodischensis , overlapping braincase material of the two holotype specimens PMO 214.578 and UPM EP-II-20(572) is only based on the stapes. It is not as slender and constricted in UPM EP-II-20(572) (Zverkov and Efrimov 2019: fig. 5E, F) as in PMO 214.578. In medial view UPM EP-II-20(572) is teardrop shaped ( Zverkov and Efimov 2019: fig. 5I), while PMO 214.578 is oval. The fragmentary stapes in UPM EP-II-23(744) cannot be interpreted ( Zverkov and Efimov 2019: fig. 6H, I).

Remarks.—Most of the reinterpretations by Zverkov and Efimov (2019) seem to be correct, but some misinterpretations are summarized here, as it is of value to the hypothesized synonymy of Undorosaurus gorodischensis and Cryopterygius kristiansenae ( Zverkov and Efimov 2019) .

The only preserved rostrum and orbital region elements in the holotype of Undorosaurus gorodischensis (UPM EP-II-20[572]) are a well preserved nasal, a parietal, and broken jugal and quadratojugal, that are barely overlapping with the preserved elements in PMO 214.578 ( Undorosaurus? kristiansenae ). The parietal is hard to interpret in PMO 214.578 as the skull is laterally compressed. The posterior portion of the nasals in Undorosaurus gorodischensis is not preserved, and the preserved portion is similar both to Ophthalmosaurus icenicus ( Moon and Kirton 2016) and PMO 214.578. The quadratojugal fragment of UPM EP-II-20(572) has no features that are comparable to the well preserved element in PMO 214.578. The straight suborbital bar in the jugal fragment of UPM EP-II-20(572) resembles the complete element of PMO 214.578, but there is no contact between the jugal and the subnarial process of the premaxilla in PMO 214.578, contrary to the reconstruction by Zverkov and Efimov (2019: fig. 3C and SOM 3), even when the deformation is considered. A ventral exposure of the maxilla between the two elements is clearly visible in PMO 214.578 as in Ophthalmosaurus icenicus ( Moon and Kirton 2016) . In PMO 214.578, the sutures of the postorbital are easily observed and the element is surprisingly small compared to Ophthalmosaurus incenicus ( Moon and Kirton 2016: fig. 4), and more similar to Ichthyosaurus communis and Leptonectes moorei ( McGowan and Motani 2003: pl. 2, text-fig. 69). The large anterodorsal projection of the postorbital limiting the posterior margin of the orbit in most ichthyosaurs is not present. The element contributing to the posterodorsal part of the orbit is clearly continuing medially to the postorbital and is covered by it ventralmost, and might represent a projection of the postfrontal. The quadrate is covered by the quadratojugal and postorbital in PMO 214.578 and only partly visible and thus cannot be compared to the complete element in UPM EP-II-20(572).

The scapula of UPM EP-II-20(572) ( Zverkov and Efimov 2019: fig. 9A, D, E) lacks the acromion process seen in PMO 214.578 ( Druckenmiller et al. 2012: fig. 8). Only two clavicle fragments are preserved in UPM EP-II-20(572), and cannot be compared to the complete element in PMO 214.578. The humeri are superficially similar, but the trochanter dorsalis is more pointed and well pronounced in proximal view in PMO 214.578 than in UPM EP-II-20(572) ( Zverkov and Efimov 2019: fig. 11B). The radius in UPM EP-II-20(572) is eroded and with a pentagonal shape, whereas it is polygonal and anteroposteriorly wider than proximodirstally long in PMO 214.578 ( Druckenmiller et al. 2012: fig. 9A–C). The radius and ulna of Undorosaurus gorodischensis are longer than wide in UPM EP-II-20(572) and YKM 44028-7 ( Zverkov and Efimov 2019: fig. 11A, F). In this respect the holotype of Undorosaurus nessovi (UPM EP-II-24[785]) ( Zverkov and Efimov 2019: fig. 17A) is more similar to PMO 214.578 than Undorosaurus gorodischensis .

Stratigraphic and geographic range.—Spitsbergen, Svalbard, Slottsmøya Member Lagerstätte, Tithonian.

Figures

Fig. 10. Basicranium of ophthalmosaurid ichthyosaur Undorosaurus? kristiansenae Druckenmiller, Hurum, Knutsen, and Nakrem, 2012 (PMO 214.578, holotype) from Spitsbergen, Svalbard, Slottsmøya Member Lagerstätte, Tithonian. A. Basioccipital posterior (A1), right lateral (A2), ventral (A3) and dorsal (A4) views. B. Basisphenoid in ventral (B1), dorsal (B2), and anterior (B3) views. C. Left stapes in posterior (C1), medial (C2), anterior (C3), and dorsal (C4) views.

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. 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).