Athabascasaurus is a medium-sized ichthyosaur, measuring long. The
premaxillae (front upper toothed bones) of
Athabascasaurus extend beneath the horizontally-oriented
external nares (openings for the nostrils), overlying the
maxillae (rear upper toothed bones). The maxillae are long, and extend upwards and fork into two ascending processes flanking the external naris, the rear one being the taller of the two and potentially reaching the
prefrontal, one of the paired bones that form the upper edge of the
orbit (eye socket). Much of the top of the snout consists of the enlarged
nasals. The front portion of the orbit is formed by the slender
lacrimals, which lack extensions reaching below the external nares. Within the orbits are the
scleral rings, sets of small bones that supported the eyeballs. Each
postfrontal (a paired skull roof bone) of
Athabascasaurus bears a triangular projection that extends on top of the upper part of the adjacent
postorbital (paired bones located behind the orbits), a
unique feature of the genus. The
frontals (paired skull roof bones) are not involved in the borders of the
supratemporal fenestrae (large openings behind the orbits), but do enclose the majority of the
pineal foramen (a small opening on the skull roof's midline), which expands from a slit into a
rhomboidal shape at its front. The
parietals (rear pair of skull roof bones), which bound the inner edges of the supratemporal fenestrae, do not form a
sagittal crest.
Athabascasaurus possesses long, rectangular
squamosals (a pair of skull roof bones). The
supratemporals (paired rear skull roof bones) bear long processes that extend forwards, forming the exterior sides of the supratemporal fenestrae, though how far forward they reach is uncertain. The
quadrates are bowed inwards, with their upper ends flared and their lower ends forming the jaw joint. The rear surfaces of the
exoccipitals (a pair of braincase bones) are rugged, considered to provide a site where the
occipital muscles anchored. The
basioccipital's rear face is dominated by the large
occipital condyle for articulation with the vertebral column. The conydle is not very strongly rounded and is surrounded by grooves. The inner ends of the shafted
stapedes (paired braincase bones) are prominently expanded. Above the stapedes and to the sides of the basioccipital are the
opisthotics, through which
cranial nerve X passed through. The rear part of the lower jaw externally is formed by the
surangular and
angular, with the latter overlapping the former and constituting the majority of the posterior
mandible's outer surface. The
articulars, which formed the jaw joint with the quadrate, are approximately quadrangular, with roughened ends that may have supported cartilage. The teeth of
Athabascasaurus are implanted in a groove. While there appear to be rather substantial gaps between each tooth in the holotype, this may be an artifact of teeth falling out after death. Sparse, fine enamel ridges extend vertically along the sides of the thin, cone-shaped tooth crowns, which are otherwise smooth. There are 42 vertebrae in front of the hips
Athtabascasaurus, an unusually low number for ophthalmosaurs, with this series measuring long in the holotype. The first two vertebrae (the
atlas and
axis) are entirely coalesced. Unique to
Athabascasaurus, there is a forwards-pointing spur at the apex of the short atlantal
neural spine. The
clavicles (collarbones) straighten and flatten out towards the animal's midline. The two lower hip bones, the
ischium and
pubis, are fused into a single unit known as an ischiopubis. The ischiopubis narrows towards the
hip socket. Unlike
Ophthalmosaurus, there is no opening in the ischiopubis. There is an upwards-facing ridge on the
femur. ==Classification==