Pistosaurus (exact etymology uncertain[1]) is an extinct genus of aquatic sauropterygian reptile closely related to plesiosaurs.

However, there are several synapomorphies that make Pistosaurus distinguished: the long, slender, snout; the possession of splint-like nasals that are excluded from the external naris; and the posterior extension of the premaxilla to the frontals.

[2] Based on synapomorphies such as the small nasals size and the presence of interpterygoid vacuity, Pistosaurus is more closely related to Plesiosauria than to Nothosaurus.

[2] Pistosaurus was about 3 metres (10 ft) long, and had a body form resembling that of nothosaurs, aquatic reptiles that flourished during the Triassic.

The head also resembled that of a plesiosaur, but with the primitive palate of a nothosaur, and numerous, sharp teeth ideal for catching and eating fish.

[4] The glenoid region is similar to Nothosaurus in development: both the slight notching of its margin and a distinct facet contact with the humeral head.

[4] This feature is a synapomorphy that appears in plesiosaurs, which is a thickened ridge passes transversely across the anterior portion of the coracoid to connect the glenoid region.

[5] The specimen showing that the axis of Pistosaurus' humerus is straight, with the distal end slightly expanded posteriorly.

[4] According to the specimen provided by paleontologist Sues, the proximal articular end is much more robust than the distal one, and is more or less triangular in transverse section.

[7] A later discovery of a new Pistosauridea from middle triassic of Nevada by paleontologist Sander indicates that Augustasaurus is closely related to Pistosaurus, while there are several difference including axial skeleton.

Plesiosaurus dolichodeirus Elasmosauridae and Cryptoclidia Microcleidus homalospondylus Hydrorion brachypterygius Occitanosaurus tournemiensis There are several different ways for aquatic tetrapods to counteract their positive buoyancy caused by their lungs: pachyostosis, osteosclerosis, pachyosteosclerosis, and calcified cartilage of bone.

The ultimate goal of these processes are to increase density for different parts of the body to offset the buoyancy, in order to live in an aquatic/semi-aquatic environment.

According to Krahl, the small region of medullary of humeri is results from a suppressed perimedullary resorption activity, which is associated with osteosclerosis.

The presence of enlarged coracoid and pubis bone in pelvic girdle indicated that there were possibility for Pistosaurus to develop flipper-like extremities.

[11] Although Pistosauroidea has long been considered as structural antecedents of Plesiosauria, a new specimen of Augustasaurus discovered by paleontologist Sandra from Nevada had raised to against this theory.