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''Amargasaurus'' (; "La Amarga lizard") is a genus of sauropod dinosaur from the Early Cretaceous epoch (129.4–122.46 mya) of what is now Argentina. The only known skeleton was discovered in 1984 and described in 1991, forming the holotype specimen of the single species ''Amargasaurus cazaui''. The skeleton is nearly complete, including a fragmentary skull, making ''Amargasaurus'' one of the best-known sauropods from the Early Cretaceous. ''Amargasaurus'' was small for a sauropod, reaching in length. It sported two parallel rows of tall spines down its neck and back, taller than in any other known sauropod. It is unclear how these spines appeared in life—they could have supported skin sails or stuck out of the body as solitary structures supporting a keratinous sheath. They might have been used for display, combat, or defense. ''Amargasaurus'' was discovered in sedimentary rocks of the La Amarga Formation, which dates back to the Barremian and late Aptian of the Early Cretaceous epoch. 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 above the ground and a maximum height of . Within the Sauropoda, ''Amargasaurus'' is closely related to the Late Jurassic genera ''Dicraeosaurus'', ''Brachytrachelopan'' and ''Suuwassea''. Together, these genera form the family Dicraeosauridae, which differ from other sauropods in showing very short necks and small body sizes. ==Description== ''Amargasaurus'' was small for a sauropod, measuring 〔〔〔 in length and weighing approximately .〔 It followed the typical sauropod bauplan, with a long tail and neck, a small head, and a barrel-shaped trunk supported by four column-like legs. The neck of ''Amargasaurus'' was shorter than in most other sauropods, a common trait within the Dicraeosauridae.〔 Measuring in length,〔 the neck corresponds for 136% of the length of the dorsal vertebral column.〔 This is comparable to ''Dicraeosaurus'' (123%) but greater than in the extremely short-necked form ''Brachytrachelopan'' (75%).〔 The neck consisted of 13 cervical vertebrae, which were opisthocoelous (convex at the front and hollow at the back), forming ball-and-socket joints with neighboring vertebrae.〔 The trunk was made out of 9 dorsal and probably 5 fused sacral vertebrae.〔 The foremost dorsals were opisthocoelous, while the remaining dorsals were amphyplatyan (flat on both ends).〔 Robust transverse processes (lateral projections connecting to the ribs) indicate a strongly developed rib cage.〔 The dorsal vertebrae of ''Amargasaurus'' and other dicraeosaurids lack pleurocoels, the deep lateral excavations that were characteristic for other sauropods.〔 The most striking feature of the skeleton were the extremely tall, upwardly projecting neural spines on the neck and anterior dorsal vertebrae. The neural spines were bifurcated along their entire length, forming a double row. They were circular in cross section and tapered towards their tips. The tallest spines could be found on the middle part of the neck, where they reached on the 8th cervical.〔 On the neck, they were bowed backwards, projecting above the adjacent vertebra.〔 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.〔 The pelvic region was relatively wide, judging from the long, laterally projecting transverse processes of the sacral vertebrae.〔 The forelimbs are somewhat shorter than the hindlimbs, as in related sauropods. Most of the hand and foot bones are not preserved, but probably possessed five digits each as in all sauropods.〔 Only the rear part of the skull is preserved. It likely showed a horselike, broad snout equipped with pencil-like teeth, as seen in related sauropods for which more complete skulls are known.〔 As in other dicraeosaurids, the external naris (nostril opening) was situated in the posterior half of the skull, diagonally above the eye socket, which was proportionally large.〔〔 As in most other dinosaurs, the skull featured three additional openings (fenestrae). The infratemporal fenestra, located below the eye socket, was long and narrow.〔 Behind the eye socket was the supratemporal fenestra, which in dicraeosaurids was uniquely small and can be seen when the skull is viewed from the side. This contrasts with other diapsid reptiles, where these openings were directed upwards, thus being visible only in top view.〔 In front of the eye socket, the antorbital fenestra would have been located, although this region is not preserved. An unusual feature were small openings seen on the backside of the skull, the so-called parietal openings or fontanelles. In other tetrapods, these openings are usually seen only in juveniles and would close as the individual grows.〔〔 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Amargasaurus」の詳細全文を読む スポンサード リンク
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