Arthropterygius

Arthropterygius is a widespread genus of ophthalmosaurid ichthyosaur which existed in Canada, Norway, Russia, and Argentina from the late Jurassic period and possibly to the earliest Cretaceous.

Arthropterygius appears to have been a relatively large ichthyosaur, with all species measuring between 3–5 m (9.8–16.4 ft) long. The partially preserved specimen PMO 222.655 has been estimated at 3.8–4.3 metres (12–14 ft) based on comparisons to the contemporary ophthalmosaurid Undorosaurus. This specimen was probably mature or close to maturity at time of death, judging by the convex head of the humerus and the smooth texture of the humeral shaft.

The skull of Arthropterygius has become well known due to the discovery of a well preserved skull from Svalbard. It is unusual among ichthyosaurs in having a very short, yet also robust rostrum for its skull length. As a result, the orbit appears very large (about 0.34× the skull length). The skull bears a very large pineal foramen, which is larger proportionately than that of any other ophthalmosaurid. This may potentially be an adaptation to the high latitude environment of Arthropterygius, but it is not yet well understood how the size of the pineal eye relates to latitude in ichthyosaurs. The presence of the foramen for the internal carotid artery (a major artery that supplies blood to the brain) on the posterior surface of the parabasisphenoid is a distinguishing trait of this genus. The teeth are relatively large, with robust, conical crowns, and are largely straight or slightly recurved.

It is difficult to estimate the exact number of vertebral segments that were present in front of the sacrum; the preserved centra suggest that there are at least 43. This is more than the 42 in Athabascasaurus, the 41 in Nannopterygius, and the 37 in Platypterygius americanus, but less than the 46 in Platypterygius australis and the 50 or more in Undorosaurus, Aegirosaurus, and Platypterygius platydactylus. The posterior dorsal vertebrae are wider and taller than those near the front, and their front and rear faces are rounded. The first few caudal vertebrae are the proportionally tallest and widest of all the vertebrae, but the vertebrae quickly become shorter vertically prior to the bend in the tail that supported the tail fluke.

Each neural spine is about the same length as the underlying centrum, which is similar to Undorosaurus but not Ophthalmosaurus (in which they are proportionally longer in the posterior cervical vertebrae). Similar to Gengasaurus, the first few neural spines are taller than the corresponding centra, but then decrease in height gradually. The tops of the neural spines, which are the thinnest parts of the bones, are straight, instead of being notched like Undorosaurus or Platypterygius australis. The ribs are shaped like a figure eight in cross section near their top ends, but this is less obvious closer to the bottom; such a morphology is typical among ophthalmosaurids, except for Acamptonectes and Mollesaurus. The dorsal ribs are around 75–81 cm (30–32 in) long, while the first few caudal ribs are only 2–5 cm (0.79–1.97 in) long.

In Arthropterygius, the scapula has a relatively straight blade that was broadened at its front end into a fan-like acromion process. This process is less prominent than that of Acamptonectes and Sveltonectes, instead being more similar to that of Undorosaurus, Platypterygius hercynicus, and Sisteronia. The scapula formed more of the glenoid than the coracoid; like Sveltonectes but unlike Undorosaurus, the glenoid does not extend onto the bottom face of the scapula. The de facto blade of the scapula was widest near the middle, and angled slightly downwards; like Acamptonectes but unlike all other ophthalmosaurids, the blade has a relatively uniform thickness along its entire length.

The clavicle, which is not fused to other elements in the pectoral girdle, bears a thickened process on its frontal bottom edge which points towards the midline of the torso. This process is relatively short, square-shaped from the front, and bordered by a rim on its back edge. In most other ophthalmosaurids, this same process is more finger-like, although some specimens of Ophthalmosaurus have similar processes. The back part of the clavicle gradually narrows into a curved point, as in Ophthalmosaurus and Baptanodon.

The coracoid is a kidney-shaped bone that is about as long as it is wide, compared to Undorosaurus, Nannopterygius, and Sveltonectes, where it is not as wide proportionally. Like Ophthalmosaurus, the coracoid bears a prominent notch on its front edge; it also bears a ridge on the frontal part of its midline, like Ophthalmosaurus and Acamptonectes, but unlike Caypullisaurus and Platypterygius australis. Its articular facets with the scapula and glenoid are clearly separate, as in Sveltonectes but not Acamptonectes. The former is only about 45% of the latter in length; this figure is 50% in Undorosaurus.