Amargasaurus

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Amargasaurus
Temporal range:
Ma
Amargasaurus skeleton cast in the Melbourne Museum foyer.
Scientific classification Edit this classification
Domain: Eukaryota
Kingdom: Animalia
Phylum: Chordata
Clade: Dinosauria
Clade: Saurischia
Clade: †Sauropodomorpha
Clade: †Sauropoda
Superfamily: †Diplodocoidea
Family: †Dicraeosauridae
Genus: †Amargasaurus
Salgado & Bonaparte, 1991
Species:
†A. cazaui
Binomial name
†Amargasaurus cazaui
Bonaparte
, 1991

Amargasaurus (

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 9 to 13 meters (30 to 43 feet) 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 could have stuck out of the body as solitary structures that supported a keratinous sheath. An alternate hypothesis, now more 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

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 80 centimeters (31 inches) above the ground and a maximum height of 2.7 meters (8.9 feet). 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

Size comparison of the Dicraeosauridae members. Amargasaurus is green.

Amargasaurus was small for a sauropod, measuring 9 to 13 meters (30 to 43 feet) in length and weighing approximately 2.6–4 metric tons (2.9–4.4 short tons).

transverse processes (lateral projections connecting to the ribs) indicate a strongly developed rib cage.[7] The dorsal vertebrae of Amargasaurus and other dicraeosaurids lacked pleurocoels, the deep lateral excavations that were characteristic for other sauropods.[7]

Neck vertebrae of the holotype, with cross-section reconstruction of soft-tissue

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 60 centimeters (24 inches) on the 8th cervical.[2] On the neck, they were bowed backward, projecting above the adjacent vertebra.[3]: 304  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.[8] 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.[9] The pelvic region was relatively wide, judging from the long, laterally projecting transverse processes of the sacral vertebrae.[2] 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.[7]

Life restoration of a pair of displaying Amargasaurus with "cervical sails" based on Cerda et al. 2022

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.[10] 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.[7][11] As in most other dinosaurs, the skull featured three additional openings (fenestrae). The infratemporal fenestra, located below the orbit, was long and narrow.[11] 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 upward, thus being visible only in top view.[2] 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.[12][13] 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.[11]

Discovery

Skeletal casts of Amargasaurus and Carnotaurus
Casts of Amargasaurus and Carnotaurus, both discovered by the same 1984 expedition in Argentina, Natural History Museum of the University of Pisa

The only known skeleton (specimen number

arroyo in the Picún Leufú Department of Neuquén Province in northern Patagonia, 70 kilometers (43 miles) south of Zapala.[7][16] 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.[2]

Diagram of the skull
Skull reconstruction, redrawn after,[2] showing known material (brown) and the location of the various skull openings. Missing parts are reconstructed after Dicraeosaurus.

The skeleton is reasonably complete and includes a partial skull. Sauropod skull bones are rarely found,

Bernardino Rivadavia Natural Sciences Museum in Buenos Aires.[7]

The first,

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.[7] One year later, Salgado and Jorge O. Calvo published a second paper focusing on the description of the skull.[11]

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,[2] 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.[19][20][21][8] An unnamed specimen from Brazil indicates that this group persisted at least until the end of the Early Cretaceous.[2] Most analyses find Dicraeosaurus and Brachytrachelopan to be more closely related to each other than to Amargasaurus.[6][22][23] Suuwassea is generally recovered as the most basal member of the family.[20]: 17 [8][19] 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.[21]: 202, 214 

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.[10]

Size comparison
Size comparison between a human, Dicraeosaurus, Amargasaurus, and Brachytrachelopan

The following cladogram by Gallina and colleagues (2019)[8] shows the presumed relationships between members of the Dicraeosauridae:

Diplodocoidea

Paleobiology

Vertebral spines

Skeletal cast showing forwards-pointing neck spikes
Skeleton cast showing the neck spikes pointing forwards during downwards bending, Museum of Paleontology Egidio Feruglio

Both the function and the appearance in life of the extremely elongated and bifurcated vertebral spines remain elusive.

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.[24] Keratinous sheaths covering the spines were also shown in a 1999 skeletal restoration published by Salgado.[12][25]

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.[9]

Life restoration
Life restoration of Amargasaurus without the proposed "cervical sail"

Daniela Schwarz and colleagues, in 2007, concluded that the bifurcated neural spines of diplodocids and dicraeosaurids enclosed an

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.[8]

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.[27]

Senses and posture

Reconstructed skull on a mounted skeleton
Reconstructed skull

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.[28]

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.[12] 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.[28] A similar value has recently been proposed for the related Diplodocus.[23] 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).[28]

Locomotion

Skeletal mount in hind view
Hind view of skeleton, showing the pelvis and tall sacral spines

Amargasaurus was

quadrupedal (moved on four legs), and probably was unable to rear on its hind legs.[7] 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.[7] 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.[29]

Life history

In a 2021 study, Guillermo Windholz and Ignacio Cerda obtained

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.[30]

Paleoecology

Amargasaurus stems from

titanosauriforms.[32] The high diversity suggests that different sauropod species exploited different food sources in order to reduce competition. Basal Titanosauriforms showed proportionally longer necks, longer forelimbs, and broader tooth crowns than Dicraeosaurids and Rebbachisaurids, suggesting greater feeding heights.[28] Amargasaurus probably fed above ground level at heights of up to 2.7 meters (8.9 ft), as evidenced by the anatomy of its neck and inner ear. Rebbachisaurids like Zapalasaurus presumably fed at ground-level, while basal Titanosauriforms exploited food sources at higher levels.[28]

Other dinosaurs of the La Amarga Formation include an indeterminate

Crocodylomorphs are represented by the trematochampsid Amargasuchus – the holotype of this genus was found in association with the Amargasaurus bones.[16]

References

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External links

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