This process is remarkably short compared to other ophthalmosaurids, being less than half of the length of the humeral shaft, a condition also seen in Undorosaurus and Aegirosaurus; in Ophthalmosaurus, Brachypterygius, Nannopterygius, and Undorosaurus, the process reaches the midpoint of the humerus, while it is even longer in Platypterygius americanus, Platypterygius australis, and Platypterygius hercynicus. The other process is the deltopectoral crest, which is about half of the shaft length in Ophthalmosaurus, and is almost as long as the entire shaft in Sisteronia, Acamptonectes, and Platypterygius americanus. At the midpoint of the humeral shaft, the bone is mildly constricted, being approximately 20% narrower than the maximum width. The bottom end of the humerus is larger than the top end; it bears three articular facets, with one for the preaxial accessory element (the smallest of the three), one for the radius (the tallest of the three), and one for the ulna (which forms an angle of 120° with the radial facet). Sveltonectes, Nannopterygius, Platypterygius hauthali, and Platypterygius platydactylus all only have two; Maiaspondylus, Aegirosaurus, Brachypterygius, and Platypterygius americanus also have three, but the second facet articulates with a different bone; Undorosaurus has two on the left humerus and three on the right; and Platypterygius australis and Platypterygius hercynicus have up to four facets. Hindlimb and pelvic girdle The ilium of Arthropterygius is rather short compared to that of other ophthalmosaurids; Aegirosaurus is the only other ophthalmosaurid that possibly approaches Arthropterygius in this respect. The side of the ilium directed towards the back of the body is concave, like Ophthalmosaurus and Athabascasaurus, with the curve being more pronounced in the latter two. The acetabulum, on the lower end of the ilium, is thickened relative to the rest of the bone, and does not bear any distinct articular facets. Autapomorphically, the top end of the ilium is 1.5 times the width of the acetabular end. The ischium and the pubis are fused together into a single, continuous, solid trapezoid-shaped element known as the ischiopubis, with the wider edge (1.4 times the width of the other end, which is shorter proportionally than that of Aegirosaurus, Ophthalmosaurus, and Athabascasaurus) being at the midline of the body. This complete fusion is also seen in Sveltonectes, Athabascasaurus, Aegirosaurus, Caypullisaurus, and possibly Platypterygius australis; meanwhile, Ophthalmosaurus, Undorosaurus, and Nannopterygius retain a small hole in the ischiopubis. The probable portion of the ischiopubis that represents the pubis is thicker