Amargasaurus (; "La Amarga lizard") is a genus of sauropod dinosaur from the Early Cretaceous epoch (130–120 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 known species, Amargasaurus cazaui. It was a large animal, but small for a sauropod, reaching in length. Most distinctively, it sported two parallel rows of tall spines down its neck and back, taller than in any other known sauropod. Several studies postulated that, in life, the spines stuck out of the body as individual structures that supported a horn sheath. Analysis of the bone structure suggests that the spines were connected by ligaments and could have formed a scaffold supporting a neck 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 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 above the ground and a maximum height of . Within the Sauropoda, Amargasaurus is classified as a member of the family Dicraeosauridae, which differs from other sauropods in showing shorter necks and smaller body sizes.
Description
Amargasaurus was small for a sauropod, measuring in length and weighing approximately .Paul, G.S. (2024) The Princeton Field Guide to Dinosaurs. 3nd ed. Princeton University Press p. 233 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 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 amphiplatyan (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 onward, 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 on the 8th cervical. On the neck, they were bowed backward, 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 frontward and broadened toward 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 (nostril opening) was situated in the posterior half of the skull, diagonally above the (eye opening), which was proportionally large. As in most other dinosaurs, the skull featured three additional openings (fenestrae). The , located below the orbit, was long and narrow. Behind the orbit was the , 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 upward, 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 and the notably long , bony extensions connecting the with the .
Discovery
The only known skeleton (specimen number MACN-N 15) was discovered in February 1984 by Guillermo Rougier during an expedition led by 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, 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.
The skeleton is reasonably complete and includes a partial skull. Sauropod skull bones are rarely found, and the Amargasaurus skull is only the second skull known from a member of the Dicraeosauridae. Major parts of the skeleton were found in their original anatomical position: the vertebral column of the neck and back, which consisted of 22 articulated vertebrae, was found connected to both the skull and the sacrum. Of the skull, only the temporal region and the braincase are preserved. The sacrum, despite being partly eroded prior to burial, is fairly complete. Most of the tail is missing, with three anterior, three middle, and one posterior vertebrae being preserved, along with fragments of several others. The shoulder girdle is known from the scapula (shoulder blade) and coracoid (which sits on the lower end of the scapula), while the pelvis is merely known from the ilium (the uppermost of the three pelvic bones). The limbs are equally fragmentary, with the manus (hand) and most of the pes (foot) missing. The skeleton is currently stored in the collection of the Bernardino Rivadavia Natural Sciences Museum in Buenos Aires.
The first, unofficial, mention of Amargasaurus as a new genus of dinosaur was published by Bonaparte in the 1984 Italian book Sulle Orme dei Dinosauri. Here, the species was designated as Amargasaurus groeberi, honoring Pablo Groeber, which was changed into Amargasaurus cazaui in the official description published several years later. The official description, written in Spanish, was published in 1991 by Leonardo Salgado and Bonaparte in the Argentinian scientific journal Ameghiniana. The name Amargasaurus alludes to the site of discovery, the La Amarga Arroyo. La Amarga is also the name of a nearby town, as well as the geologic formation the remains were recovered from. The word amarga is Spanish for "bitter", while sauros is Greek for "lizard". The one species (A. cazaui) is named in honor of Luis Cazau, a geologist with the YPF oil company, which at the time was state-owned. In 1983, Cazau informed Bonaparte's team about the paleontological significance of the La Amarga Formation, leading to the discovery of the skeleton. One year later, Salgado and Jorge O. Calvo published a second paper focusing on the description of the skull.
Classification Amargasaurus is classified as a member of the Dicraeosauridae, a family ranked clade within the Diplodocoidea. Currently, this clade consists of nine species belonging to eight genera. These include Lingwulong shenqi from the Early or Middle Jurassic of China and four species from the Late Jurassic: Brachytrachelopan mesai from Argentina; Suuwassea emilieae from the Morrison Formation of the United States; and Dicraeosaurus hansemanni and Dicraeosaurus sattleri from the Tendaguru beds of Tanzania. Amargasaurus was the first dicraeosaurid known from the Cretaceous, although additional dicraeosaurids from the Lower Cretaceous have been described more recently, including Pilmatueia faundezi, Amargatitanis macni, and Bajadasaurus pronuspinax, which are all from Argentina. An unnamed specimen from Brazil indicates that this group persisted at least 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 is generally 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.
Together with the Diplodocidae and the Rebbachisauridae, the Dicraeosauridae is nested inside the Diplodocoidea. All members of the Diplodocoidea are characterized by their box-shaped snout and narrow teeth restricted to the foremost portion of the jaws. Both the Dicraeosauridae and the Diplodocidae are characterized by bifurcated neural spines of the cervical and dorsal vertebra. In the Dicraeosauridae, the bifurcated neural spines were strongly elongated, a trend reaching its extreme in Amargasaurus.
The following cladogram by Gallina and colleagues (2019) shows the presumed relationships between members of the Dicraeosauridae:
Paleobiology Vertebral spines
Both the function and the appearance in life of the extremely elongated and bifurcated vertebral spines remain elusive. Salgado and Bonaparte, in 1991, suggested the spines represented defense weapons against predators, arguing that they tapered towards their tips. They also could have served for display, perhaps for courtship or to intimidate rivals. Some subsequently published life restorations showed the double row of spines supporting two parallel skin sails. Gregory Paul, in 1994, considered this possibility unlikely, noting that neck sails would have reduced neck flexion, and that the spines were circular in cross-section rather than flattened as is the case in sail-bearing animals. Instead, he found that this shape indicates that the spines supported a keratinous (horn) sheath that would have extended the length of the spines in life. The spines could have been used for display or as weapons both against predators and members of the same species, as the animal might have been able to point its most anterior spines forward by bending its neck. He also hypothesized that the spines could have been clattered together to generate sound. Keratinous sheaths covering the spines were also shown in a 1999 skeletal restoration published by Salgado.