Adriosaurus is an extinct genus of squamate which lived in what is now Slovenia and other parts of Europe during the Late Cretaceous.

It was a small, snake-like reptile, with the type species Adriosaurus suessi measuring up to 30 cm (12 in) in length.

Adriosaurus represents the first occurrence of vestigial limbs in fossil lizards, having lost its manus and forearm completely in order to elongate its axial skeleton.

[3][4][5] These unique anatomical features led to discussions of the evolutionary patterns of limb reduction in Squamata.

[5] Adriosaurus includes three species: A. microbrachis (“micro”, meaning small, and “brachis”, meaning arm, referring to the vestigial forelimb composed of only the humerus),[5] A. skrbinensis (named after the location where they found the fossil, Skrbina, northwest of Komen, Slovenia) and A. suessi.

[7] Four years later, in 2004, Lee and Caldwell went on to reevaluate Acteosaurus crassicostatus, a species that was then based on a small squamate fossil found in Comen, Slovenia.

The description was based on specimen SMNH 2158, which was also found near a small village in Comen, Slovenia.

A phosphatic matter was recovered from its gastric contents, which probably represents the remains of small fishes.

[6] As a pythonomorph with reduced limbs, Adriosaurus is important in regards to discussions on the origin of snakes, a topic on which paleontologists and zoologists have long held different opinions.

[11] After the discovery of mid-Cretaceous snake-like lizards, paleontological research linked snakes to mosasauroid lizards which once lived in marine environments,[12] while zoologists linked snakes to squamates with reduced limbs, which implied a terrestrial origin.

The cladogram is based on Palci and Caldwell (2010a & 2010b):[14][2] Anguidae Xenosaurus Shinisaurus Heloderma Lanthanotus Varanus Aigialosauridae Mosasauridae Coniasaurus Dolichosaurus Pontosaurus lesinensis Pontosaurus kornhuberi Aphanizocnemus Adriosaurus skrbinensis Adriosaurus suessi Acteosaurus Ophidia Adriosaurus mainly would have swum by using lateral undulation because of its laterally compressed body, flattened tail, and small limbs.

The thicked parts of the middle dorsal region of the skeleton would have reduced swimming speed and maneuverability.

[6] Adriosaurus had sharp, recurved teeth and a large skull relative to its body size.