Amargasaurus cazaui: The Spiny Sauropod

Article by: J. D. Dixon
Edited by: Harry T. Jones and Jack Wood

A reconstruction of Amargasaurus cazaui. Artwork by Scott Reid.

Name: Amargasaurus cazaui
Name Meaning: The Lizard of La Amarga
Age: Early Cretaceous
Diet: Herbivore
Size:  9.1 metres in length approx.
Weight: 2,500 kg approx.
Location: Neuquén Basin, Argentina

Amargasaurus cazaui was a moderately sized species of sauropod, a group of long-necked herbivorous dinosaurs that first flourished towards the end of the Jurassic. Amargasaurus was discovered in 1984 during the eighth expedition of a research program designed to understand South American Jurassic and Cretaceous tetrapods. This was the same expedition that unearthed Carnotaurus sastrei. The new sauropod was found in the lower part of the Cretaceous-aged La Amarga Formation in Argentina, and was formerly described in 1991 by Leonardo Salgado and José Bonaparte. The discovery of this species was pivotal, as it confirmed the existence of the sauropod family Dicraeosauridae in South America. Amargasaurus was a quadruped, meaning it walked on all four of its legs. While some sauropods may have been able to briefly rear onto their hind legs, it is unlikely that Amargasaurus could have done this.

The genus is easily identified by its strange neck, which features two parallel branches of elongated neural spines. These neural spines have been hypothesised as having many uses, but their exact purpose remains unknown. They have been thought to have been used for display in mating rituals, intimidation of rivals, or social hierarchy. The spines have even been interpreted as having supported a skin envelope or having housed soft tissue and ligaments. Gregory S. Paul refuted the idea of skin sails, as they would have interfered with the neck’s movement and flexure. Instead, he suggested that the circular form of the spines indicated they were likely coated with a keratin sheath, which would have enhanced their strength and size. The most agreed upon proposed use of these spines is defence, where the dinosaur may have swung its neck and stabbed at predators with these projections, similar to how modern sable antelope defend themselves from lions. The keratin sheath would have made these spines even more formidable if this was indeed their purpose.

Sauropod skulls are rarely preserved, yet the Amargasaurus holotype (MACN-N 15) was found with a near complete skull, even maintaining fragile structures such as the components which attach the braincase to the palate (called the basipterygoid processes). Studies indicate that Amargasaurus held its head neutrally in a downward-facing position, but it was capable of raising its head significantly when alert. However, the long neural spines would have inhibited further flexure of the neck.

Two skeletal reconstructions of Amargasaurus cazaui. The upper is the original reconstruction taken from Salgado and Bonaparte (1991). The lower is an updated reconstruction produced by Scott Hartman. Notice the less severe neck flexure and more neutral head position in the updated restoration.

Sauropod necks have been interpreted as a key feeding adaptation in the group, because the elongated neck can increase feeding range without the animals actually having to walk. Amargasaurus roamed across South America at the same time as two other types of sauropod. These were basal rebbachisaurids (which had relatively short necks) and titanosauriforms (which had relatively long necks). For all of these animals to co-exist, they must have eaten different foods, therefore avoiding resource competition between the families. It is thought, due to neck lengths and dental morphologies, that the titanosauriforms browsed high-level vegetation; the rebachisaurids browsed at a low level; and the dicraeosaurids (such as Amargasaurus) ate plant matter from intermediate heights. This is further evidenced by the estimated skull orientation of Amargasaurus, and would provide a mechanism by which such a variety of sauropods could survive harmoniously.

The La Amarga Formation includes many predatory and carnivorous animals which may have fed on Amargasaurus. These include the abelisaurian Ligabueino andesi and the narrow-snouted crocodilian Armagasuchus minor, the latter of which was found associated with Amargasaurus fossils, which may indicate a direct feeding relationship. There were also numerous teeth found which resemble those of basal tetanurans (the group containing the majority of predatory dinosaurs). Nevertheless, the exact species of these theropod predators remain a mystery.

Amargasaurus was a peculiar sauropod that possessed one of the most iconic features seen in any dinosaur – the extraordinary neural spines along its back. The species’ discovery helped better our understanding of sauropod diversity throughout the Early Cretaceous, and going forward, hopefully more finds can shed light on the true use of those fearsome cervical structures.

Image References
[1] A reconstruction of Amargasaurus cazaui. Artwork by Scott Reid.
[2] Two skeletal reconstructions of Amargasaurus cazaui. The upper is the original reconstruction taken from Salgado and Bonaparte (1991). The lower is an updated reconstruction produced by Scott Hartman.

Information References and Further Sources
[1] Apesteguía, S. (2007). ‘The sauropod diversity of the La Amarga Formation (Barremian), Neuquén (Argentina)’, Gondwana Research, 12 (4), pp 533-546. Accessed 5th October 2020. Click Here.
[2] Bailey, J. B. (1997). ‘Neural Spine Elongation in Dinosaurs: Sailbacks or Buffalo-Backs?’, Journal of Paleontology, 71 (6), pp. 1124-1146. Accessed 5th October 2020. Click Here.
[3] Chiappe, L. M. (1988). ‘A new trematochampsid crocodile from the Early Cretaceous of north-western Patagonia, Argentina and its palaeobiogeographical and phylogenetic implications’, Cretaceous Research, 9 (4), pp. 379-389. Accessed 5th October 2020. Click Here.
[4] Dorling Kindersley. (2009). ‘Cretaceous: Vertebrates’, in Prehistoric. Great Britain: Dorling Kindersley Limited. pp. 332.
[5] Mazzetta, G. V., Christiansen, P., and Fariña, R. A. (2004). ‘Giants and Bizarres: Body Size of Some Southern South American Cretaceous Dinosaurs’, Historic Biology, 16 (2-4), pp. 71-83. Accessed 5th October 2020. Click Here.
[6] Paul, G. S. (1994). ‘Dinosaur Art & Restoration Notes: Dicraeosaurs’, The Dinosaur Report, 8. Accessed 5th October 2020. Click Here.
[7] Paulina Carabajal, A., Carballido, J. L., and Currie, P. J. (2014). ‘Braincase, neuroanatomy, and neck posture of Amargasaurus cazaui (Sauropoda, Dicraeosauridae) and its implications for understanding head posture in sauropods’, Journal of Vertebrate Paleontology, 34 (4), pp. 870-882. Accessed 5th October 2020. Click Here.
[8] Salgado, L., and Bonaparte, J. F. (1991). ‘A New Dicraeosaurid Sauropod, Amargasaurus cazaui gen. et sp. nov., from the La Amarga Formation, Neocomian of Neuquén Province, Argentina’, Ameghiniana, 28, pp. 333-346. Accessed 4th October 2020. Click Here.
[9] Schwarz, D., Frey, E., and Meyer, C. A. (2007). ‘Pneumaticity and soft−tissue reconstructions in the neck of diplodocid and dicraeosaurid sauropods’, Acta Palaeontologica Polonica, 52 (1), pp. 167–188. Accessed 5th October 2020.
[10] Upchurch, P., Barrett, P. M., and Dodson, P. (2004). ‘Sauropoda’, in Weishampel, D. B., Dodson, P., and Osmólska, H (2nd ed.) The Dinosauria. University of California Press. pp. 259-322.