Patagosaurus

''Patagosaurus'' (meaning "Patagonia lizard") is an extinct genus of eusauropod dinosaur from the Middle-Late Toarcian of Patagonia, Argentina. It was first found in deposits of the Cañadón Asfalto Formation, which date to around 179 to 177 million years ago. Although originally twelve specimens were assigned to the taxon, at least one of them may belong to a different genus. ''Patagosaurus'' probably lived alongside genera as '' Piatnitzkysaurus'', '' Condorraptor'' and '' Volkheimeria''.

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 dinosaur 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 José 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 layers originally suggested as Callovian- to Oxfordian-aged Patagonian deposits of the Cañadón Asfalto Formation. These layers have been recently re-dated, finding out thanks to advanced zircon datation that the bones of the three genera and all the vertebrates of Las Charcitas member where deposited in between 179 and 178 million years, that is Middle-Late Toarcian. ''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 postcranial 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 933, 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.

Description

''Patagosaurus'' is a sauropod that possessed a general and unspecialized bauplan of being quadrupedal, having an elongate neck, a small head and a very long tail. Therefore, it is similar to ''Cetiosaurus'' and other related genera, who possessed the same morphology. It has been estimated that it was about long and weighed about . An earlier estimate by John S. McIntosh and his colleagues in 1997, found that ''Patagosaurus'' was approximately long, and also in weight, similar to the later estimates by Holtz. A 2006 study by Donald M. Henderson calculated the weight of ''Patagosaurus'' to be , a smaller estimate than McIntosh's.

The skull of ''Patagosaurus'' is not very well known, with a 2003 revision by Oliver Rauhut determining only a few jaws are certainly referrable to it, as opposed to nearly the entire skull. MPEF-PV 1670 shows what the morphology of the adult or subadult skull was like, while MACN CH 933 represents a juvenile individual. Based upon how broad, high and short the adult articulated mandibles of ''Patagosaurus'' are, its snout would have been short, high and broad as well, a typical feature of most sauropods.

The teeth of ''Patagosaurus'' are reminiscent of more derived sauropods. They are similar in morphology to ''Euhelopus'', being concave on one side as well as having crowns with fairly great expansions. They are also similar to ''Camarasaurus'', although the latter genus has less of a concavity and expansion. The teeth also possess marginal denticles on the crown. Based on histological studies, an individual of ''Patagosaurus'' would have replaced a tooth every 58 days, similar to 62 days for ''Camarasaurus'', and 34 days for ''Diplodocus''.

Postcranial skeleton

Most of the postcranial skeleton is known in ''Patagosaurus''. The cervical, caudal and dorsal vertebrae are generally similar to ''Camarasaurus'', although the sacrum possesses many distinct features. The sacrum is well preserved, showing that ''Patagosaurus'' possessed five sacral vertebrae. All the vertebrae but the fifth are fused together. All the neural spines are tall, and the centra are occasionally transversely narrow. The neural canal of the vertebrae is unique among sauropods, however. Starting from the very end of the first vertebra, and extending to almost the end of the third there is an enlargement of the canal, forming a well-defined cavity. Even though the sacrum itself is distinguishing, its sacral ribs resemble ''Camarasaurus''. The sacral vertebrae have a total length of , with the total sacral length being .

The pelvic girdle is well preserved and well studied. In the holotype, the pelvic girdle is almost complete, only lacking the proximal ends of each ischium. The ilia of the holotype are well known, and show many distinct features. The pubic peduncle, where the ilium articulates with the pubis, is long and straight and has an expansion on the end, as in many sauropods. The upper edge of the iliac blade is curved and thick, with rugosities (rough spots) for cartilage attachment. The pubic elements are large and robust in adults, more so than in juveniles. They are flat when viewed from in front, and convex when seen from behind. ''Lapparentosaurus'' resembles ''Patagosaurus'' when comparing their pubes. The ischia are much more gracile than the pubes, and only have a small distal expansion. While the ilia resemble ''Barapasaurus'', and the pubes resemble ''Lapparentosaurus'', the ischia are most similar to ''Diplodocus'' and ''Apatosaurus''.

The hindlimbs of ''Patagosaurus'' are based on scant material, some femora, a tibia and a few nondescript pedal bones. Two femora come from an adult, with a single additional bone known from the juvenile. The adult femora are proportionately different from the juvenile, being mostly straighter and more ovoid in cross-section. The femoral head is well preserved, although lacking the greater trochanter. The distal end is rather symmetrical when viewed from behind, with two similarly sized condylar

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