Ophthalmosaurinae Baur 1887 sensu Fischer et al. (2012)

Subfamily Ophthalmosaurinae Baur 1887 sensu Fischer et al. (2012)

Ophthalmosaurinae indet.

Material

TY53: An incomplete specimen preserving cranial and postcranial bones ( Fig. 7 View Figure 7 ; Supporting Information, Model S2).

Description

Proportions: TY53 is exposed in less lateral view. The exposed portion is 2.88 m long and constitutes a partly embedded articulated specimen including the skull, a partial axial skeleton, portions of a forefin, the pectoral girdle, and a portion of the pelvic girdle. Prior to glacial erosion, the specimen probably was virtually complete. The eroded caudal vertebrae of the tail and the complete skull could have measured an additional 2 m. Thus, the animal might have had an estimated total length of 5 m.

Skull: The skull is preserved as three discrete sections, which are cut at different angles and showing perimortem breakage, probably incurred during head-first arrival at the seafloor. These sections are: (i) the anterior rostrum; (ii) nasals and the palate in the region of the external nares; and (iii) the right lower jaw ramus.

The first segment (i) is from the anterior rostrum. This is preserved in cross-section at a point estimated to be approximately halfway between the orbit and the anterior tip of the rostrum. The best-preserved components are the less premaxilla and nasal. The latter is exceedingly small and forms a right angle in cross-section, forming dorsal and ventral rami. It is likely that the section is close to its anteriormost extent, and is largely, but not entirely, covered by the premaxilla externally. The premaxilla shows a well-developed premaxillary fossa, defined dorsally by a sharp ridge and the alveolar groove. Medial to the alveolar wall, there is an element with an oval cross-section inferred to be a vomer. The right premaxilla and nasal have been abraded at a lower angle and are thus more difficult to interpret. Ventral to the less upper jaw ramus, there is a semicircular structure, which is likely to correspond to the less mandibular ramus. A notch in its lateral margin is interpreted as the dentary fossa, but aside from this, no anatomical information can be extracted.

The rostrum posterior to the cross-section described above has been abraded at an angle near to the horizontal plane. The palate is exposed in dorsal view and appears as a series of elongate, roughly parallel elements. This makes the interpretation of the palate difficult. Posterior to this cluster of bones, however, a broken fragment of the right nasal is preserved, which runs at a slightly different angle to the palate. The nasal fragment must come from the portion anterior to the excavatio internasalis, because it shows a vertically oriented internasal suture and a convex external surface ( Fig. 8A, B View Figure 8 ).

The second skull section (ii) is a posterior section through the rostrum in the region of the external nares. Both nasals are preserved in cross-section. The right one has shissed slightly anteriorly relative to the less, such that they are not in articulation nor do they show the same anatomical structures. The right nasal is from a cut anterior to the excavatio internasalis and shows a vertically oriented internasal suture and convex external surface. The less nasal is sectioned across the external narial opening and shows the development of a prominent lateral wing roofing the narial opening. It forms a convex bulge dorsally, indicating a well-developed excavatio internasalis. Both nasals show a compact outer cortex and an extremely spongiose inner surface. Ventral to the nasals is a pair of dorsoventrally deep elements, which are laterally flat and medially concave, forming a tube. These elements articulate with the nasals via a weakly ossified connection and are interpreted as the vomers. On the right, a smaller quadrangular element is preserved, potentially representing the palatine but sectioned almost parallel to the long axis. On the less is a plate-like element, here interpreted as the less palatine in cross-section, and lateral to this is a hatchet-shaped element, potentially part of the maxilla or nasal ( Fig. 8C, D View Figure 8 ).

The third component (iii) is here interpreted as a cross-section through the right mandibular ramus. A round element preserved laterodorsally might represent the right jugal in cross-section, although this is uncertain. The mandible consists of the surangular dorsally and laterally, forming the roof of the Meckelian canal and approximately one-third of the lateral surface of the lower jaw. Ventral and internal to the surangular is the angular, which forms the entire medial wall of the Meckelian canal. The angular forms a robust ventromedial articulation with the splenial. The medial surface of the splenial appears to be pierced by several foramina, which appear as bony projections in section. The dentary has not been identified, although a thin fragment of bone between the lateral and dorsal portions of the surangular might represent its posterior extent. Additional elements exposed ventrolateral to the splenial might represent palatal elements ( Fig. 8E, F View Figure 8 ).

Postcranial axial skeleton: The exposed elements of the postcranium consist of a portion of the anterior dorsal vertebral column with articulated ribs and gastralia, a preflexural portion of the caudal vertebral column, the articulated coracoids, a partial forefin, and pelvic girdle elements ( Figs 7 View Figure 7 , 9 View Figure 9 ). The vertebral column is not completely exposed, but the visible part of the vertebral column is articulated. No apophyses or neural spines are preserved, but a series of 10 articulated centra are associated with dorsal ribs ( Fig. 9C, D View Figure 9 ). The vertebrae are polished owing to erosion, but traces of the dorsal portion of ribs are identified, which covered the vertebrae before being polished by the glacier. Vertebrae increase in length along the preserved series, from 39 mm long in the first centrum where the length can be measured accurately to 46 mm in the last measurable dorsal centrum (for measurements, see Supporting Information, Table S3). The height of the last preserved vertebral centrum is 89 mm, giving a height-to-length ratio of 1.9 for the anterior dorsal region. Posteriorly, 22 articulated preflexural caudal vertebrae are preserved. Anteriorly, the centrum length starts at 40 mm and gradually decreases along the preserved series to 32 mm posteriorly. In contrast, the centrum height of the caudal series begins at 94 mm anteriorly, increases slightly to 105 mm in the tallest centrum, then decreases steadily to 80 mm posteriorly. The maximum height-to-length ratio in the anterior caudal region is 3.2, suggesting a relatively non-regionalized vertebral column.

The dorsal ribs exposed in TY53 are dorsoventrally deep with respect to their anteroposterior length and have an hourglass-shaped cross-section ( Fig. 9E View Figure 9 ). Approximately 27 disarticulated gastralia are also preserved and, based on articulation patterns, were arranged as a dorsal and a ventral element on either side of the body, lacking a medial component. No caudal ribs could be identified.

Appendicular skeleton

Pectoral girdle: The coracoids are preserved in articulation, but glacial abrasion makes the shape difficult to discern with accuracy. They appear to be approximately equidimensional or slightly broader than long. The less coracoid preserves the remnants of an anterior notch. The less coracoid overlaps the proximal margin of the less humerus ( Fig. 9A, B View Figure 9 ).

Forefin: The humerus, zeugopodial elements, and part of the posterodistal fin of one of the two forefins are partly exposed in TY53. Based on embedding position and exposure, this is likely to corresponds to the less forelimb ( Fig. 9A, B View Figure 9 ). The humerus is preserved, possibly in dorsoposterior view. Owing to the twisted embedding angle, little detail can be added.

The radius is only partly exposed, but it is clear that it articulates directly with the anterodorsal margin of the ulna. The radius is ≥ 53 mm in anteroposterior width and 38 mm in proximodistal length but is not completely exposed. The ulna is completely exposed, possibly in dorsal view, and has a proximodistal length of 72 mm a nd a maximum anteroposterior width of 69mm.The posterior margin is concave, indicating that a posterior zeugopodial element was absent. The ulna articulates anterodistally with an anterior mesopodial element (presumably the intermedium, which is not preserved) and distally with the ulnare. The ulnare is proximodistally shorter and anteroposteriorly narrower than the ulna (56 mm wide and 50 mm long). Its posterior margin is rounded, whereas the anterior margin is straight. Distally, a straight margin contacts metacarpal V, which is even shorter and narrower than the ulnare (41 mm wide and 34 mm long). It is posteriorly rounded and is anteriorly concave, although the latter is attributable to erosion. A distally exposed phalanx measures 30 mm in proximodistal length. Its anterior margin is not completely preserved. The distal-most element (a distal phalanx) is the smallest. The exposed section is about twice as proximodistally long as anteroposteriorly wide.

Pelvic girdle: Portions of four bones are exposed in the pelvic girdle area. The largest bone corresponds to the ischiopubis in external view. The ischiopubis is 96 mm in length and 48 mm wide at the medial end; the proximal end is incompletely preserved. The ischiopubis shows a slight constriction at the midpoint and a small foramen at its anterodistal margin. No ischiopubic foramen is observed. A second element preserved next to the ischiopubis is roughly triangular in outline. This bone is tentatively identified as the proximal end of the femur ( Fig. 9F, G View Figure 9 ). The last two bone fragments aligned close to those previously described are indeterminate.

Comparison

TY53 is referred to Ophthalmosaurinae because of the large ulna with a concave and ‘edgy’ posterior surface (sensu Fischer et al. 2012). Ichthyosaurs showing this morphology include the Middle–Late Jurassic taxa Ophthalmosaurus icenicus (Seeley, 1874; Moon and Kirton, 2016), Baptanodon natans (Marsh, 1880) , Nannopterogius spp. ( Zverkov and Jacobs, 2020), Ŋalassodraco etchesi Jacobs and Martill (2020) and the Hauterivian species Acamptonectes densus Fischer et al. (2012) . TY53 also shows rounded forefin elements, as in many of these taxa. Acamptonectes densus is the only named ophthalmosaurine from the Valanginian–Barremian interval showing a consistent morphology of the posterior margin of the ulna. However, this genus clearly differs from TY 53 in having ribs that are rounded in cross-section, with a single deep groove ( Fischer et al. 2012), unlike the clearly hourglass-shaped cross-sections observed in TY53. Therefore, we refer TY53 to Ophthalmosaurinae indet., pending excavation and further preparation.

Taphonomy

According to the preservation of the skeleton, including perimortem fracturing of the skull elements, the angle of the skull relative to the body, and the ‘hunch-backed’ proximal dorsal region ( von Huene 1922), a head-first arrival ( Wahl 2009) on the probably somewhat soupy (sensu Martill 1993) seafloor is plausible. The strength of the fall broke the snout into at least two pieces, which now are exposed in the posterior cross-section and the anterior dorsal-to-cross-sectional view ( Fig. 10 View Figure 10 ), suggesting that the soss substrate was thin and underlain by a consolidated seafloor.