Amargasaurus

Credit: Fred Wierum.

*Amargasaurus* is an extinct genus of dicraeosaurid sauropod dinosaur that lived in Argentina during the Early Cretaceous.

## Description

*Amargasaurus* was a small sauropod, following the typical body plan (long neck and tail, barrel-shaped body, column-like legs and a small head). However, it had a shorter neck (2.4 meters (7.9 feet) long), being 136% of the dorsal vertebral column. This is nearer to *Dicraeosaurus* but longer than *Brachytrachelopan*. It is made of 13 cervicals, the opisthocoelous being convex at the front and hollow at the back, which forms a ball-and-socket joint with neighbouring vertebrae. The trunk is made of 9 dorsals and ~5 fused sacrals. The foremost dorsals are opisthocoelous, with the remaining being amphiplatyan.

Robust transverse processes indicate a strongly-developed ribcage. Along with other dicraeosaurids, *Amargasaurus* lacks pleurocoels in the dorsals. The most striking feature of the neck, and the entire animal, is the series of very tall, upwards-projecting neural spines, which stretch from the top of the neck to the anterior dorsals. From the third cervical and on, they are bifurcated along their entire length, which forms a double row. They have a circular cross section and taper nearer to the tips. The tallest spines sit on the middle of the neck, reaching a 60 centimeter (24 inch) peak on the 8th cervical.

They bow backwards on the neck, projecting above adjacent vertebrae. The last two dorsals, the hip and foremost tail had elongate spines, lacking bifurcation, but rather flaring into a paddle-shaped upper end. The pelvic area is relatively wide, which can be judged from the long, laterally-projecting transverse processes of the sacrals. The forelimbs are shorter than the hind. Most of the hands and feet were not preserved, but it probably had 5 digits (as in all sauropods).

Only the posterior of the skull is preserved, but it likely had a horse-like, broad snout with pencil-like teeth, which is drawn from similar sauropods. The nostrils sat in the posterior half of the skull, diagonally above the orbits (who were large). Like most dinosaurs, the skull had three fenestra. The supratemporal fenestra was small in dicraeosaurs. Unusually, fontanelles exist in the back of the skull, which are only seen in juvenile tetrapods. Cranial features shared with *Dicraeosaurus* but not most other sauropods include fused frontals and long basipterygoid processes.

## Paleobiology

### Vertebral spines

The function and life appearance of the double-row spines is unknown. Salgado and Bonaparte (1991) suggest they were for defense since their tips taper. They could have also functioned as display for courtship or intimidation. Some paleoart shows they supported skin sails. Gregory S. Paul (1994) considered (but stated how unlikely,) that the neck sails reduced flexion, and that a circular cross section is seen in other sail-baring animals. Instead, he finds they supported a keratinous sheath that extended the length of the spines.

They could have been for display or as a weapon for predators and of the same species, since it may have been able to direct the anterior-most spines by bending it's neck. He also hypothesizes they could have clattered together to make sound. Salgado (1999) produced a skeletal that showed such keratinous structures. A 2022 study finds no evidence for a keratinous sheath over the neck spines, but does find evidence for ligament attachment, meaning the spines were covered with soft tissue.

## Paleoecology

*Amargasaurus* is from sedimentary rock from the La Amarga Formation, Neuquén Basin of the Barremian and Late Aptian-aged Cretaceous. Most vertebrates (including *Amargasaurus*) are knwon from the lowermost of the formation, the Puesto Antigual Member, 29 meters (96 feet) thick and mainly composed sandstones deposited from braided rivers, It was recovered from a layer of sandy conglomerates.

The fauna also includes *Zapalasaurus*, *Amargatitanis* and unnamed basal titanosauroforms. This high diversity suggests sauropods exploited different food sources in order to reduce competition. *Amargasaurus* likely reached above 2.7 meter (8.9) foot heights, evidenced by neck and inner ear morphology. Other dinosaurs include Amargastegos, *Ligabueino* and a large tetanuran tooth taxon. *Vincelestes* (a mammal) and *Amargasuchus* (a crocodylomorph) are known, the latter found associated with *Amargasaurus*.

## References

* https://www.patagonia.com.ar/dinosaurs/659E\_Amargasaurus+Casuei.html
* https://museum.wa.gov.au/explore/dinosaur-discovery/amargasaurus
* https://www.thoughtco.com/amargasaurus-1092816
* https://www.nhm.ac.uk/discover/dino-directory/amargasaurus.html
* https://new.smm.org/dino-days/battle/amargasaurus
* https://en.wikipedia.org/wiki/Amargasaurus
* https://onlinelibrary.wiley.com/doi/full/10.1111/joa.13659?saml\_referrer#.Yj2XjX-oOqw.twitter