Patagosaurus

Patagosaurus (meaning "Patagonia reptile"[1]) is an extinct genus of eusauropodan dinosaur from the Middle Jurassic of Patagonia, Argentina. It was first found in deposits of the Cañadon Asfalto Formation, which date from around 165 to 161 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 such as Piatnitzkysaurus, Condorraptor, Volkheimeria and another unnamed sauropod.

Since Patagosaurus is known from many specimens, including at least one juvenile, its anatomy and growth are fairly well known. 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 Amydgalodon, and more derived sauropods like Diplodocus and Camarasaurus.

Discovery and naming
Many specimens of the taxon were all found associated together in the same bed and locality, a pebbly stratum near a route to Cerro Condor.[2] The specimens were first described by Jose Bonaparte in 1979. He named for them the genus Patagosaurus, as well as its type species P. fariasi. The genus was originally known from an almost complete postcranial skeleton lacking a skull,[3] although in 2003 it was found that a dentary was referable to the species.[4] Its skeleton was found near those of Piatnitzkysaurus and Volkheimeria in the Callovian to Oxfordian aged Patagonian deposits[3] of the Cañadon Asfalto Formation.[4] It can be distinguished from Cetiosaurus, a similar genus, by features of the ischium and vertebrae.[3] Over twelve specimens have been referred to the species,[1] although some of the material is probably from a unique taxon.[4]

The generic name of Patagosaurus comes from the location of its find in Patagonia, and the fact that it is a reptile.[1] The specific name honours Ricardo Farias, on whose land the initial discovery was made.

Description
Patagosaurus is a more generalised sauropod, similar to Cetiosaurus and other related genera. It has been estimated by Thomas R. Holtz Jr. that it was approximately 15.0 m (49.2 ft) long and weighed about equal to two elephants (between 7.3 and 14.5 t; 7.1 and 14.3 long tons (8 and 16 short tons)[5] when fully grown, although juveniles are known to be much smaller in size.[1] An earlier estimate by John Stanton McIntosh and his colleagues in 1997, found that Patagosaurus was approximately 5 m (16 ft) long, and also 9.4354 metric tons (10.4007 short tons; 9.2864 long tons) in weight,[6] similar to the later estimates by Holtz.[1] Patagosaurus is almost completely known, with many articulated specimens found covering almost all of the skeleton, including parts of the skull.

Skull
The skull of Patagosaurus is not very well known, with currently only some lower jaw material referred to it.[4] Bonaparte (1986) described three specimens, a premaxilla under number PVL 4076, left and right maxillae assigned to MACN CH 934, and a juvenile dentary and postcranium under MACN CH 933 to Patagosaurus.[2][4] MACN CH 933 is directly comparable with the type material of Patagosaurus, PVL 4170, which confirms its association with the genus. A specimen first referred to Patagosaurus in 2003, MPEF-PV 1670, is also very similar to MACN CH 934, and differences can be associated with age, so therefore, MPEF-PV 1670 represents adult cranial material. However, the teeth of MACN CH 934 are very different from those of both lower jaws, so it can be identified as another sauropod from the same deposit as Patagosaurus. Based upon how broad, high and short the adult articulated lower jaws of Patagosaurus are, its snout would have been short, high and broad as well, a typical feature of most sauropods.[4]

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 it has less of a concavity and expansion.[7] The teeth also possess marginal denticles on the crown.

Postcranial skeleton
Most of the postcranial skeleton is known in Patagosaurus. The cervical, caudal, and dorsal vertebrae are similar generally 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 however. Starting from the very end of the first vertebrae, 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 unique, its sacral ribs resemble Camarasaurus. The sacral vertebrae have a total length of 540 mm (21 in), with the total sacral length being 920 mm (36 in).[2]

The pelvic girdle is well preserved as well. 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 for cartilage attachment. The pubic elements are large and robust in adults, more 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.[2]

The hindlimbs of Patagosaurus are based on scant material, some femora, a tibia, and a few nondescript pes bones. Two femora come from an adult, with an additional coming from the juvenile. The adult femora have proportional differences 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 surfaces. In the juvenile, the fourth trochanter is completely in the proximal end. The tibia has a well-developed cnemial crest, and is also short and robust. the surface that would have articulated with the astragalus in life has the anterior half being raised, and the posterior half lowered.[2]

The pectoral girdle is also well-known. Both the left and right scapulae and coracoids are known, if incomplete. The scapulae are large, and robust, and thicken as they near the glenoids. The scapular blades are flat, although they are both convex along the anterior edge. Where the scapulae and coracoids articulate, the coracoids are thickest, as they become thinner and thinner the farther from the scapulae. The younger specimen of Patagosaurus possesses a slightly different morphology of the pectoral girdle, with slightly differing proportions and a slightly smaller scapular blade in proportion. The coracoids resemble Barapasaurus in shape, and differ from Camarasaurus, although they cannot be compared with those of Cetiosaurus.[2]

The forelimbs of Patagosaurus are only based on three bones of a juvenile specimen, as no manual elements are preserved. The humeri are slender and elongate, lacking great proximal and distal expansions. The incomplete deltoid crest, only shows that it was wide, and likely had a projection below of behind. Like the humeri, the radius is slender, and lacks large expansions on either end. On the edge closest to the ulna, the radius possesses a ridge along its edge, which corresponds to where radioulnar ligaments would have attached. The ulna is complete, although sediment-filled breaks might have altered its original shape. The forelimb of Patagosaurus is much more gracile and different from the robust later sauropods like Camarasaurus, and Apatosaurus, and instead resembles more the gracile taxon Diplodocus.