Brown (1981) diagnosed Muraenosaurus as follows: “Plesiosauroids in which the teeth are ornamented with many longitudinal ridges; the dentary bears 19 to 22 teeth on each ramus; the premaxillae bear 5 teeth each, of which the 1st and 5th are small, and the 2nd to 4th are large; the most anterior maxilliary tooth (6th upper tooth) is small and the 8th and 9th are large; the parietals form a sagital crest; the quadrate overlaps the quadrate ramus of the pterygoid posteromedially; the paroccipital process of the exoccipital-ophisthotic is relatively long and slender; the occipital condyle is ringed by a groove, and is formed by the basioccipital only; there are about 66 pre-sacral vertebrae, of which usually 44 are cervical; the cervical vertebrae have relatively platycoelous centra, the length of which exceeds the height in the anterior vertebrae, and exceeds or is approximately equal to the height in posterior vertebrae; the clavicles are reduced or absent; the interclavicle is developed and may be plate-like or reduced and lanceolate; the coracoids meet the scapulae in the midline in ‘adults’; the width across the posterior cornua of the coracoids is equal to or only just in excess of the interglenoid width in ‘adults’; the elements of the pectoral girdle tend to fuse in old individuals; there are normally only two epipodials in the manus; a foramen is present between epipodials even in juveniles”
