La Amarga Formation

The La Amarga Formation is a geologic formation with outcrops in the Argentine provinces of Río Negro, Neuquén, and Mendoza. It is the oldest Cretaceous terrestrial formation in the Neuquén Basin.

La Amarga Formation Stratigraphic range: Barremian-Early Aptian ~130–120 Ma PreꞒ Ꞓ O S D C P T J K Pg N
Type	Geological formation
Unit of	Rayoso Group
Sub-units	Puesto Antigual, Bañados de Caichigüe & Piedra Parada Members
Underlies	Lohan Cura Formation
Overlies	Agrio Formation
Thickness	160 m (520 ft)
Lithology
Primary	Sandstone
Other	Mudstone
Location
Coordinates	39.6°S 70.2°W
Approximate paleocoordinates	39.6°S 35.2°W
Region	Río Negro, Mendoza & Neuquén Provinces
Country	Argentina
Extent	Neuquén Basin
Type section
Named for	La Amarga Arroyo
Named by	Musacchio
Year defined	1970
La Amarga Formation (Argentina)

The type locality is La Amarga Arroyo and China Muerta Hill.[1] The La Amarga Formation unconformably overlies the marine Agrio Formation of the Mendoza Group. It is in turn overlain by the Lohan Cura Formation, separated by another unconformity.

Composition

There are three members within the La Amarga Formation.

1. The oldest is the Puesto Antigual Member, which is approximately 28.9 metres (95 ft) thick and consists mainly of sandstone deposited in the channels of a braided river system. Paleosols, or soil deposits, are well-developed.
2. The Bañados de Caichigüe Member is the next highest, approximately 20.9 metres (69 ft) thick. Alternating limestones, shales, and siltstones make up this member, indicating a lacustrine (lake) environment.
3. Youngest and thickest is the Piedra Parada Member, approximately 109.4 metres (359 ft) thick in some sections. This member consists of alternating sandstones and siltstones from an ancient alluvial plain, with some swamp and paleosol deposits.

Fossil content

Most of the tetrapod fossils found in the La Amarga come from the Puesto Antigual Member, including:

• a crocodilian (Amargasuchus)
• a dicraeosaurid sauropod (Amargasaurus)[2]
• a diplodocoid sauropod (Zapalasaurus)
• a titanosaurian sauropod
• an abelisauroid theropod (Ligabueino)
• a stegosaur (Amargastegos)
• a mammal (Vincelestes)

Dinosaurs

Dinosaurs of the La Amarga Formation
Genus	Species	Province	Member	Notes	Images
Amargasaurus[2]	A. cazaui[2]	Neuquén[2]	Puesto Antigual	"Braincase and associated postcranial skeleton."[3]
Amargastegos	A. brevicollus	Neuquén	Puesto Antigual	Considered dubious by Galton & Carpenter (2016), material referred to Stegosauria indet.[4]
Amargatitanis	A. macni	Neuquén	Puesto Antigual	"Two caudal vertebrae, an incomplete right ischium, a right femur, an incomplete right tibia, an incomplete right fibula, a right astragalus, and an incomplete right metatarsal I."
Ligabueino[2]	L. andesi[2]	Neuquén[2]	Puesto Antigual	"A cervical neural arch, a mid to posterior dorsal neural arch, a posterior dorsal centrum, the left femur, left ilium, articulated pubic shafts, and two pedal phalanges."[5]
Stegosauria indet.[6]	Indeterminate	Neuquén	Puesto Antigual	"Supraorbital, cervical and caudal vertebrae, dermal armor".[4][6]
Zapalasaurus	Z. bonapartei	Neuquén	Piedra Parada

Other tetrapods

Other tetrapods of the La Amarga Formation
Genus	Species	Province	Member	Notes	Images
Amargasuchus	A. minor	Neuquén	Puesto Antigual	A trematochampsid crocodylomorph, found in association with the holotype of Amargasaurus
Pterosauria indet.[7]	Indeterminate	Neuquén		An isolated femur
Vincelestes	V. neuquenianus	Neuquén	Puesto Antigual	A cladotherian mammal

Color key Taxon Reclassified taxon Taxon falsely reported as present Dubious taxon or junior synonym Ichnotaxon Ootaxon Morphotaxon		Notes Uncertain or tentative taxa are in small text; crossed out taxa are discredited.

See also

• List of dinosaur-bearing rock formations

References

1. Musacchio, 1970
2. Weishampel et al., 2004, "Dinosaur distribution (Early Cretaceous, South America).", pp.563-570
3. "Table 13.1," in Weishampel et al., 2004, p.264
4. Galton & Carpenter, 2016
5. Carrano et al., 2011
6. Pereda Suberbiola et al., 2012
7. Montanelli, 1987

Bibliography

• Galton, Peter M., and Kenneth Carpenter. 2016. The plated dinosaur Stegosaurus longispinus Gilmore, 1914 (Dinosauria: Ornithischia; Upper Jurassic, western USA), type species of Alcovasaurus n. gen.. Neues Jahrbuch für Geologie und Paläontologie - Abhandlungen 279. 185–208.
• Pereda Suberbiola, Xabier; Peter M. Galton; Heinrich Mallison, and Fernando Novas. 2012. A plated dinosaur (Ornithischia, Stegosauria) from the Early Cretaceous of Argentina, South America: an evaluation. Alcheringa: An Australasian Journal of Palaeontology 37. 1–14. doi:10.1080/03115518.2012.702531
• Carrano, M.T.; M.A. Loewen, and J.J.W. Sertic. 2011. New Materials of Masiakasaurus knopfleri Sampson, Carrano, and Forster, 2001, and Implications for the Morphology of the Noasauridae (Theropoda: Ceratosauria). Smithsonian Contributions to Paleobiology 95. 1–53.
• Weishampel, David B.; Peter Dodson, and Halszka Osmólska (eds.). 2004. The Dinosauria, 2nd edition, 1–880. Berkeley: University of California Press. Accessed 2019-02-21. ISBN 0-520-24209-2
• Montanelli, S. B. 1987. Presencia de Pterosauria (Reptilia) en la Formación La Amarga (Hauteriviano-Barremiano), Neuquén, Argentina. Ameghiniana 24. 109–113.
• Musacchio, E. 1970. Ostrácodos de la superfamilias Cytheraceae y Darwinulaceae de la Formación La Amarga (Cretácico inferior), provincia del Neuquén, Argentina. Ameghiniana 7. 301–318.

Further reading

• Apesteguía, Sebastián. 2007. The sauropod diversity of the La Amarga Formation (Barremian), Neuquén (Argentina). Gondwana Research 12. 533–546. Accessed 2016-02-16.
• Leanza, H.A.; S. Apesteguia; F.E. Novas, and M.S. De la Fuente. 2004. Cretaceous terrestrial beds from the Neuquén Basin (Argentina) and their tetrapod assemblages. Cretaceous Research 25. 61–87. Accessed 2019-02-16.