Alierasaurus is an extinct genus of caseid synapsid that lived during the early Middle Permian (Roadian) in what is now Sardinia. It is represented by a single species, the type species Alierasaurus ronchii. Known from a very large partial skeleton found within the Cala del Vino Formation, Alierasaurus is one of the largest known caseids. It closely resembles Cotylorhynchus, another giant caseid from the San Angelo Formation in Texas. The dimensions of the preserved foot elements and caudal vertebrae suggest an estimated total length of about for Alierasaurus. In fact, the only anatomical features that differ between Alierasaurus and Cotylorhynchus are found in the bones of the feet; Alierasaurus has a longer and thinner fourth metatarsal and it has ungual bones at the tips of the toes that are pointed and claw-like rather than flattened as in other caseids. Alierasaurus and Cotylorhynchus both have very wide, barrel-shaped rib cages indicating that they were herbivores that fed primarily on high-fiber plant material. Etymology The generic name refers to Aliera, the name in local dialect of the town of Alghero, and ‘saurus’ meaning lizard. The specific name is in honor of Ausonio Ronchi the discoverer of the specimen. Description The paleontologists Marco Romano and Umberto Nicosia have identified several autapomorphies in the feet anatomy of Alierasaurus: metatarsal IV with distinct axial region, length about twice that of the corresponding proximal phalanx, not short and massive as in other large caseids; metatarsal IV proximal head not orthogonal to the bone axis, forming an angle of 120° with the shaft: with this conformation, the proximal and distal heads are much closer along the medial side of the metatarsal; claw-shaped ungual phalanges proportionally shorter than in Cotylorhynchus, with a double ventral flexor tubercle very close to the proximal rim of the phalanx; ungual phalangeal axis bent downward and medially; distal transverse section subtriangular, not spatulate as in Cotylorhynchus. Discovery The holotype of Alierasaurus was discovered in the uppermost levels of the Permian Cala del Vino Formation, on top of the Torre del Porticciolo promontory, which separates the Porticciolo Gulf from the northern coast (near the town of Alghero, Nurra, northwest Sardinia). Some bones were found loose on the ground surface, and others still embedded in mudstone-siltstone layer. These sediments were deposited in a former alluvial plain under a relatively mild semi-arid climate. The known material consist of eight articulated caudal vertebrae, two isolated caudal vertebrae,