Amargasaurus is a genus of sauropod dinosaur from the Early Cretaceous period (129.4–122.46 mya) of what is now Argentina. The only known skeleton was discovered in 1984 and is virtually complete, including a fragmentary skull, making Amargasaurus one of the best-known sauropods of its epoch. Amargasaurus was first described in 1991 and contains a single species, Amargasaurus cazaui. The animal was small for a sauropod, reaching 9 to 10 meters (30 to 33 feet) in length. Most distinctively, it sported two parallel rows of tall spines down its neck and back, taller than in any other known sauropod.
Amargasaurus was discovered in sedimentary rocks of the La Amarga Formation, which dates back to the Barremian and late Aptian stages of the Early Cretaceous. A herbivore, it shared its environment with at least three other sauropod genera, which might have exploited different food sources in order to reduce competition. Amargasaurus probably fed at mid-height, as shown by the orientation of its inner ear and the articulation of its neck vertebrae, which suggest a habitual position of the snout some 80 centimeters (31 inches) above the ground and a maximum height of 2.7 meters (8.9 feet). Within the Sauropoda, Amargasaurus is most closely related to the Late Jurassic genera Dicraeosaurus, Brachytrachelopan and Suuwassea. Together, these genera form the family Dicraeosauridae, which differs from other sauropods in showing shorter necks and smaller body sizes.
The only known skeleton (specimen number MACN-N 15) was discovered in February 1984 by Guillermo Rougier during an expedition led by the famous Argentine paleontologist José Bonaparte. This was the eighth expedition of the project “Jurassic and Cretaceous Terrestrial Vertebrates of South America”, which was supported by the National Geographic Society and initiated in 1975 to improve on the sparse knowledge of the Jurassic and Cretaceous tetrapod life of South America. The same excursion uncovered the nearly complete skeleton of the horned theropod Carnotaurus. The discovery site is located in the La Amarga Arroyo in the Picún Leufú Department of Neuquén Province in northern Patagonia, some 70 kilometers (43 miles) south of Zapala. The skeleton stems from sedimentary rocks of the La Amarga Formation, which dates to the Barremian through early Aptian stages of the Early Cretaceous, or around 130 to 120 million years ago.
Amargasaurus is classified as a member of the Dicraeosauridae, a family ranked clade within the Diplodocoidea. Currently, this clade consists of five species belonging to four genera. These are, besides Amargasaurus cazaui, the species Dicraeosaurus hansemanni and Dicraeosaurus sattleri from the Late Jurassic Tendaguru beds of Tanzania, as well as Brachytrachelopan mesaifrom the Late Jurassic of Argentina. Whitlock (2011) argued that Suuwassea emilieae from the Morrison Formation of the United States has to be placed inside the Dicraeosauridae as well, which was supported by subsequent studies. Amargasaurusis the only named dicraeosaurid from the Cretaceous; however, an unnamed specimen from Brazil indicates that this group persisted until the end of the Early Cretaceous. Most analyses find Dicraeosaurus and Brachytrachelopan to be more closely related to each other than to Amargasaurus. Suuwassea was recovered as the most basal member of the family. A 2015 analysis by Tschopp and colleagues came to the preliminary result that two poorly known genera from the Morrison Formation, Dyslocosaurus polyonychius and Dystrophaeus viaemalae, might be additional members of the Dicraeosauridae.
Amargasaurus stems from sedimentary rocks of the La Amarga Formation, which is part of the Neuquén Basin and dates to the Barremian and late Aptian of the Early Cretaceous. Most vertebrate fossils, including Amargasaurus, have been found in the lowermost (oldest) part of the formation, the Puesto Antigual Member. This member is approximately 29 meters (95 ft) in thickness and mainly composed of sandstones deposited by braided rivers. The Amargasaurus skeleton itself was recovered from a layer composed of sandy conglomerates. The sauropod fauna of the La Amarga Formation was diverse and included the basal rebbachisaurid Zapalasaurus, the titanosaur Amargatitanis, and unnamed remains of basal titanosauriforms.
Other dinosaurs of the La Amarga Formation include the stegosaur Amargastegos; predatory dinosaurs include the small ceratosaur Ligabueino, and the presence of a large tetanuran is indicated by teeth. Other than dinosaurs, the formation is notable for the cladotherian mammal Vincelestes, the only mammal known from the Early Cretaceous of South America. Crocodylomorphs are represented by the trematochampsid Amargasuchus – the holotype of this genus was found in association with the Amargasaurus bones.