Since Patagosaurus is known from many specimens, including at least one juvenile, its anatomy and growth are fairly well understood. Both ages exhibit the typical features of a sauropod, a long neck, small head, a long tail, and being quadrupedal. The juvenile exhibits features different from the adult in regions like the mandible, pectoral girdle, pelvis and hindlimb, although overall their anatomy is quite similar. The many known specimens help fill in gaps in the anatomy of the genus, such as the forelimb and skull. Parts of the skeleton, like the pectoral girdle, tibia and pubis, are more robust, while others, like the forelimb and ischium, are more gracile. The material of Patagosaurus is similar to closely related taxa like Cetiosaurus and Volkheimeria, more primitive genera such as Barapasaurus and Amygdalodon, and more derived sauropods like Diplodocus and Camarasaurus.
Discovery and naming
In the 1970s, many specimens of a previously unidentified were found associated together in the same bed and locality, a pebbly stratum near a route to Cerro Condor. The specimens were first described by Jose Bonaparte in 1979. For the fossil he erected the genus Patagosaurus, as well as its type species P. fariasi. The generic name of Patagosaurus comes from the location of its find in Patagonia, and the fact that it is a reptile. The specific name honours Ricardo Farias, on whose land the initial discovery was made. The genus was originally known from an almost complete postcranial skeleton lacking a skull as the holotype, and many referred specimens however in 2003 it was found that a dentary was referable to the species, so more specimens are probably this taxon. Its skeleton was found near those of Piatnitzkysaurus and Volkheimeria in the Callovian to Oxfordian aged Patagonian deposits of the Cañadon Asfalto Formation. Patagosaurus is almost completely known, with many articulated specimens found covering almost all of the skeleton, including parts of the skull. Over twelve specimens have been referred to the species, although some of the material is probably from a unique taxon. Bonaparte (1986) assigned three specimens other than the holotype PVL 4170, PVL 4076, MACN CH 934, and MACN CH 933 to the genus. While the holotype includes a post cranial skeleton, the others are known from cranial material and a nearly complete juvenile skeleton and skull. MACN CH 933 is directly comparable with the type material of Patagosaurus, which confirms its association with the genus. A specimen first referred to Patagosaurus in 2003, MPEF-PV 1670 (which includes just a lower jaw), is also very similar to MACN CH 934, and differences can be associated with age, so therefore, MPEF-PV 1670 presumably represents adult cranial material. However, the teeth of MACN CH 934 are very different from those of both lower jaws (MACN CH 933 and MPEF-PV 1670), so it can be identified as another sauropod from the same deposit as Patagosaurus. Thus, the taxon only certainly includes PVL 4170, MACN CH 933, and MPEF-PV 1670.