Amargasaurus
Amargasaurus (/əˌmɑrɡəˈsɔrəs/; "La Amarga lizard") is a genus of dicraeosaurid sauropod dinosaur from the Early Cretaceous Period (130–125 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.
Description[]
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.
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.
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 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.
Jack Bailey, in 1997, argued that the spines resembled those of sail-bearing pelycosaurs like Dimetrodon. According to this author, Amargasaurus might also have possessed such a sail, which might have been used for display. Unlike those of pelycosaurs, the neural spines of Amargasaurus were bifurcated, forming a double row along the neck and back. As the space between both rows was merely 3 to 7 cm (1.2 to 2.8 inches), the existence of two parallel sails seems unlikely. Instead, Bailey suggested the spines represented a scaffold which was completely enveloped by a single skin. Neural spines from the penultimate dorsal vertebra to the foremost tail vertebrae also were strongly elongated, but different in structure, forming a single row of paddle-shaped projections. According to Bailey, these projections resembled those of modern humped ungulates such as the bison, indicating the presence of a fleshy hump above the hips. Bailey suggested similar humps for other dinosaurs with strongly elongated neural spines, such as Spinosaurus and Ouranosaurus.
Daniela Schwarz and colleagues, in 2007, concluded that the bifurcated neural spines of diplodocids and dicraeosaurids enclosed an air sac, which would have been connected to the lungs as part of the respiratory system. In Dicraeosaurus, this air sac (the so-called supravertebral diverticulum) would have rested on top of the neural arch and filled the entire space between the spines. In Amargasaurus the upper two-thirds of the spines would have been covered by a sheath of keratin, restricting the air sac to the space between the lower one-third of the spines. A cover of either keratin or skin is indicated striations on the surface of the spines similar to those of bony horn cores of today's bovids. In 2016, Mark Hallett and Mathew Wedel suggested that the backwards-directed spines might have been able to skewer predators when the neck was abruptly drawn backwards during an attack. A similar defense strategy is found in today's giant sable antelope and Arabian oryx, which can use their long, backwards directed horns to stab attacking lions. Apart from the possible function in defense, the spines may have been used for display, either for the intimidation of rivaling individuals or for courtship. Hallett and Wedel also hypothesized that rival males might have interlocked their spines for neck wrestling. Pablo Gallina and colleagues (2019) described the closely related Bajadasaurus, which had neural spines similar to those of Amargasaurus, and suggested that both genera employed them for defense. A defense function would have been especially effective in Bajadasaurus as the spines were directed forwards and would have reached past the tip of the snout, deterring predators. The keratinous sheath that likely covered the spines might have extended their length by 50%, as seen in some modern even-toed ungulates. Such extended sheaths would have made the delicate spines more resistant to damage—likely a critical threat, as the bases of the spines form the roof of the spinal cord.
In 2022, a detailed study by Ignacio A. Cerda and colleagues analyzing the structure, morphology, and microanatomy of the vertebral spines of Amargasaurus and an indeterminate dicraeosaurid (also from the La Amarga Formation) suggested that the spines were not covered in a keratinous sheath as previously believed. Osteohistology of the spines suggests that they were likely, if not exclusively, covered in a sail of skin. The spines are also highly vascularized and bear cyclical growth marks, adding credence to this theory.
Senses and posture[]
Paulina Carabajal and colleagues, in 2014, CT-scanned the skull, allowing for the generation of three-dimensional models of both the cranial endocast (the cast of the brain cavity) and the inner ear. Using these models, the cranial endocast was shown to encompass 94 to 98 milliliters (0.20 to 0.21 U.S. pints) in volume. The inner ear was 30 millimeters (1.2 inches) tall and 22 mm (0.87 inches) wide. The lagena, the part containing the hair cells for hearing, was rather short, indicating that the sense of hearing would have been poorer in Amargasaurus than in other sauropods for which inner ears have been studied.
The first skeletal reconstructions show the skull in a near-horizontal posture. Salgado, in 1999, argued that such a posture would have been anatomically impossible due to the elongated neural spines of the neck vertebrae. Instead, he envisaged the head in a nearly vertical orientation. The habitual orientation of the head is usually reflected by the orientation of the semicircular canals of the inner ear, which housed the sense of balance (vestibular system). Using their three-dimensional model of the inner ear, Carabajal and colleagues suggested that the snout faced downwards at an angle of roughly 65° relative to the horizontal. A similar value has recently been proposed for the related Diplodocus. The neutral posture of the neck can be approximated based on how the cervical vertebrae attached to each other. According to Carabajal and colleagues, the neck was gently sloping downwards, so that the snout would have rested 80 cm (2.6 ft) above the ground in neutral posture. In reality, neck posture would have varied according to the respective activities of the animals. Raising of the neck, e.g. for reaching an alert position, would have been constricted by the elongated neural spines, not permitting heights greater than 270 cm (8.9 ft).
Locomotion[]
Amargasaurus was quadrupedal (moved on four legs), and probably was unable to rear on its hind legs. Salgado and Bonaparte, in 1991, suggested that Amargasaurus was a slow walker, as both the forearms and lower legs were proportionally short, as a feature common to slow-moving animals. This was contradicted by Gerardo Mazzetta and Richard Fariña in 1999, who argued that Amargasaurus was capable of rapid locomotion. During locomotion, leg bones are strongly affected by bending moments, representing a limiting factor for the maximum speed of an animal. The leg bones of Amargasaurus were even more sturdy than those of today's white rhinoceros, which is adapted to galloping.
Life history[]
In a 2021 study, Guillermo Windholz and Ignacio Cerda obtained thin sections of the humerus, femur, and a rib of the Amargasaurus specimen in order to determine the number and spacing of lines of arrested growth (analogous to the growth rings of a tree). The rib showed the most complete record of lines of arrested growth, indicating that the Amargasaurus holotype individual was at least ten years old. In sauropodomorps, sexual maturity occurred long before adult size was reached. In the outer cortex (the most external layer of the bone when seen in cross section) of the Amargasaurus individual, lines of arrested growth are more abundant, indicating sexual maturity. However, an external fundamental system (a layer containing very closely spaced lines of arrested growth) is missing, possibly indicating that the individual was not yet fully grown, although it cannot be excluded that the external fundamental system was originally present but has since been eroded.
Gallery[]
