Temporal range: Early Cretaceous, 129.4–122.46Ma
|Amargasaurus mounted skeleton cast in the Melbourne Museum foyer.|
Salgado & Bonaparte, 1991
Amargasaurus (//; "La Amarga lizard") is a genus of dicraeosaurid sauropod dinosaur from the Early Cretaceous Period (129.4–122.46 mya) of what is now South America. It was small for a sauropod, reaching 10 meters (33 feet) length. It would have been a quadrupedal herbivore with a long, low skull on the end of a long neck, much like its relative Dicraeosaurus. However, this dinosaur sported two parallel rows of tall spines down its neck and back, taller than in any other known sauropod. These spines have been reconstructed supporting skin sails, but the "skin sail" hypothesis was rejected by Gregory S. Paul in 2000. It was found at the La Amarga Arroyo (which means "the bitter creek" in Spanish) in Neuquén Province, Argentina. They are related to Diplodocus, a giant, long-necked sauropod from the Late Jurassic of North America . However they went down a different evolutionary path to their ancestors and evolved a much shorter neck and a drastically reduced overall body size .
Amargasaurus was small for a sauropod, measuring 9:4:172 to 10:304 meters in length and weighing approximately 2.6 tons.:4 It moved on four legs and probably was unable to rear on its hind legs.:344 The neck of Amargasaurus, measuring 2.4 meters in length, was proportionally short for sauropod standards.:5, 6 It consisted of 13 cervical vertebrae which were opisthocoelous (convex at the front and hollow at the back), forming ball-and-socket joints with neighbouring vertebrae.:174 The trunk was made out of 9 dorsal and probably 5 fused sacral vertebrae.:174 While the foremost dorsals were opisthocoelous, the remaining dorsals were amphyplatyan (flat on both ends).:174 Robust transverse processes (lateral projections connecting to the ribs) indicate a strong developed rib cage.:339 The dorsal vertebrae of Amargasaurus and other dicraeosaurids lack pleurocoels, the deep lateral excavations that were characteristic for other sauropods.:339
The most obvious feature of the Amargasaurus' skeleton were the extremely tall, upwardly projecting neural spines on the neck and foremost back vertebrae.:174 The neural spines were bifurcated along their entire length, forming a double row.:174 They were circular in cross section and tapered towards their tips.:174 The tallest spines could be found on the middle part of the neck, where they reached 60 cm on the 8th cervical.:174 On the neck, they were bowed backwards, projecting above the adjacent vertebra.:304 Greatly elongated spines continue along the last two dorsal vertebrae, the hip and foremost tail. However, in these regions the spines were not bifurcated but flared into a paddle-shaped upper end.:1139
The skull is only incompletely preserved––however, it likely had a horselike, broad snout equipped with pencil-like teeth, based on related sauropods for which more complete skulls are known. As in other dicraeosaurids, the external nares (nostril openings) were situated in the posterior half of the skull, diagonally above the eye sockets, which were proportionally large.:338:341 Additional (fenestrae) could be found surrounding the eye sockets: The infratemporal fenestrae, located below the eye sockets, were long and narrow.:341 Behind the eye sockets were the supratemporal feneatrae, which, in dicraeosaurids, were uniquely small and directed laterally (in contrast to other diapsids, where they were directed upwards).:172 In front of the eyes would have been the antorbital fenestra, although this region is not preserved. Small openings, called parietal openings or fontanelles, can be seen on the backside of the skull. In other tetrapods, these openings are usually seen only in juveniles and they close as the individual grows.:212
Discovery and species
The name Amargasaurus was coined in 1991 by Argentine paleontologists Leonardo Salgado and José Bonaparte, because its fossil remains were found alongside the La Amarga Arroyo in the Neuquén province of Argentina. La Amarga is also the name of a nearby town, as well as the geologic formation the remains were recovered from. The word amarga itself is Spanish for "bitter," while sauros is Greek for "lizard." The one named species (A. cazaui) is named in honor of the man who discovered the site, Dr. Luis Cazau, a geologist with the YPF oil company, which at the time was state-owned.
This site is located in the lower (older) sections of the La Amarga Formation, which dates to the Barremian through early Aptian stages of the Early Cretaceous Period, or around 130 to 120 million years ago.
Amargasaurus is known from a relatively complete skeleton from a single individual. This skeleton includes the back of the skull, and all vertebrae of the neck, back, and hips, as well as a bit of the tail. The right side of the shoulder girdle is also known, as are the left forelimb and hind limb, and the left ilium, a bone of the pelvis.
The spines may have supported a pair of tall skin sails. Similar sails are seen in the unrelated dinosaurs Spinosaurus and Ouranosaurus, as well as the pelycosaurs Dimetrodon and Edaphosaurus. There are a variety of hypotheses for the function of these such sails, including defense, communication (for mating purposes or for simple species recognition), or temperature regulation.
Gregory Paul argued that parallel neck sails would have reduced neck flexion. Instead, he proposed that, with their circular rather than flat cross-sections, these spines were more likely covered with a horny sheath. He even suggests that they could have been clattered together for a sound display.
Similar spines are found on the presacral vertebrae of Dicraeosaurus from Africa, although not nearly as tall.
Amargasaurus and Dicraeosaurus are united with the more recently discovered Brachytrachelopan in the family Dicraeosauridae. Dicraeosaurids and diplodocid sauropods are included in a group called Flagellicaudata.
- Paul, Gregory S. (2000) The Scientific American Book of Dinosaurs, p 94. St. Martin's Press. ISBN 0-312-26226-4.
- Mazzetta, G. V.; P. Christiansen; R. A. Farina (2004). "Giants and bizarres: body size of some southern South American Cretaceous dinosaurs". Historical Biology 16 (2-4): 71–83. doi:10.1080/08912960410001715132.
- Novas, Fernando E. (2009). The age of dinosaurs in South America. Bloomington: Indiana University Press. ISBN 978-0-253-35289-7.
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