Claudiosaurus germaini (Carroll 1981) Late Permian ~260 mya, 60 cm in length, was originally described as a close relative of Thadeosaurus, and indeed it is. However, previous workers have ignored the large number of characters that also unite Claudiosaurus with Adelosaurus, Hovasaurus, Stereosternum and Mesosaurus. Claudiosaurus nests at the base of the Enaliosauria, a clade that includes many marine reptiles including Sinosaurosphargis, Pachypleurosaurus and Colobomycter.
The skulls of the predecesor taxa Adelosaurus, Hovasaurus are poorly known, so distinct from Petrolacosaurus, the reduced skull of Claudiosaurus had a premaxilla enlarged to a third or more of the rostral length. The premaxilla ascending process split the nasals. The naris is elongated horizontally nearly reaching the orbit, but blocked by the lacrimal. The jugal was gracile. The supraoccipital was narrower than the exoccipitals. The ophisthotic contacted the supratemporal. Both the squamosal and supratemporal were relatively large. The internal nares were in a medial position, excluded from premaxillary contact. The vomers contacted the maxilla. Parasphenoid teeth were present. No teeth were canine-like. The posterior mandible dorsal rim was parallel to the ventral rim. A retroarticular process was present.
The number of cervicals increased to at least nine and they decreased in size cranially. The posterior cervicals were as tall as the little skull. The cervical neural spines were taller than each centrum. Intercentra were absent. The presacral of vertebrae dropped to 24.
The humerus was torsioned less than 30 degrees. Metacarpals 3 and 4 were subequal. Manual 4.4 was not longer than m4.3. Metatarsal 1 was longer than half of mt4.
A few cranial elements were originally misidentified and this led to some confusion regarding the phylogenetic nesting of Claudiosaurus. Here the elements are returned to their natural positions. Claudiosaurus had a large quadratojugal that closed off the base of its lateral temporal fenestra and making it a traditional diapsid, similar in most respects to Petrolacosaurus.