page_title: Amargasaurus
section_title: Description
hierarchical_section_title: Amargasaurus / Description
caption_attribution_description: Photographs and reconstructions of soft−tissues in the neck of Amargasaurus cazaui (MACN−N−15)), La Amarga, Neuquén, Argentina, La Amarga Formation, Hauterivian, Early Cretaceous. A. 7th and 8th cervical vertebra in left lateral (A1) and in cranial aspects (A2) and with close−up cranial view show− ing crests at the cranial face of neural spines (A3). B. Isolated cervical rib in dorsal (B1) and ventral (B2) aspects. C. Vertebral corpus of 5th cervical vertebra in ventral aspect. D. 10th cervical vertebral in left lateral aspect. E. Transverse cross−sections through cervical vertebra in the diapophysis region, with inter− nal extension of pneumatic cavities basing on Dicraeosaurus hansemanni (see also Fig. 8). Scale bars 60 mm, E is not to scale.
context_page_description: Amargasaurus is a genus of sauropod dinosaur from the Early Cretaceous epoch 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 known species, Amargasaurus cazaui. It was a large animal, but small for a sauropod, reaching 9 to 10 meters in length. Most distinctively, it sported two parallel rows of tall spines down its neck and back, taller than in any other known sauropod. In life, these spines most likely could have stuck out of the body as solitary structures that supported a keratinous sheath. An alternate hypothesis, now less favored, postulates that they could have formed a scaffold supporting a skin sail. 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 stages of the Early Cretaceous.
context_section_description: Amargasaurus was small for a sauropod, measuring 9 to 10 meters (30 to 33 feet) in length and weighing approximately 2.6 metric tons (2.9 short tons). It followed the typical sauropod body plan, 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 2.4 meters (7.9 feet) in length, the neck corresponded to 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 thirteen 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 nine dorsal and probably five 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 lacked pleurocoels, the deep lateral excavations that were characteristic for other sauropods.
  The most striking features of the skeleton were the extremely tall, upwardly projecting neural spines on the neck and anterior dorsal vertebrae. From the third cervical onwards, these 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 60 centimeters (24 inches) on the 8th cervical. On the neck, they were bowed backwards, projecting above the adjacent vertebra. A similar elongated neural spine has been described from the neck region of the closely related Bajadasaurus in 2019. Unlike in Amargasaurus, this spine was bowed frontwards and broadened towards the tips. The last two dorsal vertebrae, the hip, and the foremost tail in Amargasaurus also had elongated spines; these 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 were somewhat shorter than the hind limbs, as in related sauropods. Most of the hand and foot bones were not preserved, but Amargasaurus 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 orbit (eye opening), which was proportionally large. As in most other dinosaurs, the skull featured three additional openings (fenestrae). The infratemporal fenestra, located below the orbit, was long and narrow. Behind the orbit 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. The antorbital fenestra would have been located in front of the eye opening, 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. Skull features shared with Dicraeosaurus but absent in most other sauropods included the fused frontal bones and the notably long basipterygoid processes, bony extensions connecting the braincase with the palate.
caption: Neck vertebrae of the holotype, with cross-section reconstruction of soft-tissue
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