The Los Colorados Formation is a sedimentary rock formation of the Ischigualasto-Villa Unión Basin, found in the provinces of San Juan and La Rioja in Argentina. The formation dates back to the Norian age of the Late Triassic.
The up to thick formation comprises sandstones, siltstones, mudstones and conglomerates with gypsum layers deposited in a fluvial to lacustrine environment. The formation is the uppermost stratigraphic unit of the Agua de la Peña Group, overlying the Lagerstätte of the Ischigualasto Formation. Los Colorados Formation is partly covered by the Cretaceous Cerro Rajado Formation, separated by an unconformity.
The formation is known for its fossils of early dinosaurs, including the coelophysoid Zupaysaurus and the "prosauropods" Coloradisaurus, Lessemsaurus, and Riojasaurus.Weishampel et al., 2004, pp.527-528 Magnetostratigraphic analysis suggests that the Los Colorados Formation was deposited between 227 and 213 million years ago.Kent et al, 2014, p.7959
Description
Los Colorados Formation is a unit with an approximate thickness of of the Agua de la Peña Group in the Ischigualasto-Villa Unión Basin, where it is exposed in the Ischigualasto Provincial Park, a World Heritage Site in Argentina. The formation gradually overlies the Ischigualasto Formation and is unconformably overlain by the Cretaceous Cerro Rajado Formation. The formation comprises red-colored, fine- to medium-grain–size sandstones together with siltstones and ancillary floodplain mudstones with early calcisol development. The formation was deposited in a fluvial to lacustrine environment.
Fossil content Synapsids Synapsids of Los Colorados Formation Genus Species Location Material Notes Images Chaliminia C. musteloides A tritheledontid cynodont.JachaleriaJ. colorata A skull and complete jaws. A large stahleckeriiddicynodont, also found in the uppermost layers of the Ischigualasto Formation.TessellatiaT. bonapartei A partial skull, including the snout, orbital region, and lower jaws. A probainognathian cynodont.
Archosauromorphs
Non-crocodylomorph pseudosuchians Non-crocodylomorph pseudosuchians of Los Colorados Formation Genus Species Location Material Notes Images Fasolasuchus F. tenax La Rioja Province A fragmented skull and jaws. A large predatory loricatan (traditionally known as a "rauisuchian"). Neoaetosauroides N. engaeus Quebrada de los Jachaleros, El Salto (San Juan) An early desmatosuchine aetosaur with adaptations for a diet of small animals. Olkasuchus O. walasto An incomplete axial skeleton, partial pectoral and pelvic girdles, partial forelimb and hindlimb, and both articulated and isolated osteoderms. An early desmatosuchine aetosaurRiojasuchus R. tenuiscepsFour specimens, including two nearly complete skeletons An ornithosuchid, one of the last and most complete members of its family.Von Baczko & Desojo, 2016, p.4
Crocodylomorpha Crocodylomorphs of the Los Colorados Formation Genus Species Location Material Notes Images ColoradisuchusC. abelini La Rioja Province A protosuchidMartínez et al., 2018, p.1 Hemiprotosuchus H. leali Quebrada de los Jachaleros, El Salto (San Juan) A protosuchid PseudhesperosuchusP. jachaleri Quebrada de los Jachaleros, El Salto (San Juan) A "sphenosuchian" (slender early crocodylomorph)
Dinosaurs Indeterminate theropod remains present in the Provincia de la Rioja.
Dinosaurs of the Los Colorados Formation Genus Species Location Material Notes Images Coloradisaurus C. brevis La Rioja Province Skull, adult"Table 12.1," in Weishampel et al., 2004. p.234 A massospondylid sauropodomorph Lessemsaurus L. sauropoides La Rioja Province Partial cervical, dorsal, and sacral vertebrae, adult A lessemsaurid sauropodiform Prosauropoda Indeterminate La Rioja & San Juan Provinces Includes Strenusaurus procerus. Dorsals, caudals, partial fore- and hindlimb."Table 12.1," in Weishampel, et al., 2004. Page 237. Indeterminate fossils of "prosauropods" (early sauropodomorphs)Powellvenator P. podocitus Ischigualasto-Villa Unión Basin Known from previously undescribed partial hindlimbs A coelophysoid theropod Riojasaurus R. incertus La Rioja Province A sauropodomorphEzcurra, 2017 Zupaysaurus Z. rougieri La Rioja Province An early theropod
Other reptiles Other reptiles of Los Colorados Formation Genus Species Location Material Notes ImagesColoradosphenodonC. lahni La Esquina locality, La Rioja ProvinceA partial skull An early rhynchocephalianPalaeochersisP. talampayensis A testudinatan (early turtle) in the family Australochelyidae.Arcucci et al., 2004, p.5612 km NE of La Esquina at Fossilworks.org
Ichnofossils cf. Brachychirotherium sp.Río de los Tarros tracksite at Fossilworks.org Chirotherium sp.Leonardi, 1994, p.25Leonardi locality 15 at Fossilworks.org
See also List of dinosaur-bearing rock formations Quebrada del Barro Formation, contemporaneous fossiliferous formation of the Marayel-El Carrizal Basin just southeast of the Ischigualasto-Villa Unión Basin Caturrita Formation, contempeoraneous fossiliferous formation of the Paraná Basin, southeastern Brazil Chinle Formation, contemporaneous fossiliferous formation of Arizona Elliot Formation, contemporaneous fossiliferous formation of the Karoo Basin, South Africa Fremouw Formation, contemporaneous fossiliferous formation of Antarctica
References
Bibliography
Further reading
A. B. Arcucci and R. A. Coria. 2003. A new Triassic carnivorous dinosaur from Argentina. Ameghiniana 40(2):217-228 J. F. Bonaparte. 1999. Evolución de las vértebras presacras en Sauropodomorpha [Evolution of the presacral vertebrae in Sauropodomorpha]. Ameghiniana 36(2):115-187 J. F. Bonaparte and J. A. Pumares. 1995. Notas sobre el primer craneo de Riojasaurus incertus (Dinosauria, Prosauropoda, Melanorosauridae) del Triasico Superior de La Rioja, Argentina [Notes on the first skull and jaws of Riojasaurus incertus (Dinosauria, Prosauropoda, Melanorosauridae), Late Triassic of La Rioja, Argentina]. Ameghiniana 32(4):341-349 J. F. Bonaparte. 1981. Descripción de "Fasolasuchus tenax" y su significado en la sistematica y evolución de los thecodontia. Revista del Museo Argentino de Ciencias Naturales "Bernardino Rivadavia" 3(2):55-101 J. F. Bonaparte. 1978. Coloradia brevis n. g. et n. sp. (Saurischia–Prosauropoda), dinosaurio Plateosauridae de la Formacion Los Colorados, Triasico Superior de La Rioja, Argentina [Coloradia brevis n. g. et n. sp. (Saurischia–Prosauropoda), a plateosaurid from the Upper Triassic Los Colorados Formation of La Rioja, Argentina]. Ameghiniana 15(3–4):327-332 J. F. Bonaparte. 1971. Los tetrapodos del sector superior de la Formacion Los Colorados, La Rioja, Argentina (Triásico Superior) [The tetrapods of the upper part of the Los Colorados Formation, La Rioja, Argentina (Upper Triassic)]. Opera Lilloana 22:1-183 J. F. Bonaparte. 1969. Dos nuevas "faunas" de reptiles triasicos de Argentina [Two new reptilian "faunas" of the Argentine Triassic]. Gondwana Stratigraphy (IUGS Symposium, Buenos Aires) 2:283-306 J. B. Desojo and A. M. Baez. 2007. Cranial morphology of the Late Triassic South American archosaur Neoaetosauroides engaeus: evidence for aetosaurian diversity. Palaeontology 50:267-276 J. B. Desojo and A. M. Baez. 2005. El esqueleto postcraneano de Neoaetosauroides (Archosauria: Aetosauria) del Triásico Superior del centro-oeste de Argentina. Ameghiniana 42:115-126 M. D. Ezcurra and C. Apaldetti. 2012. A robust sauropodomorph specimen from the Upper Triassic of Argentina and insights on the diversity of the Los Colorados Formation. Proceedings of the Geologists' Association 123:155-164 A. G. Martinelli and G. W. Rougier. 2007. On Chaliminia musteloides (Eucynodontia: Tritheledontidae) from the Late Triassic of Argentina, and a phylogeny of Ictidosauria. Journal of Vertebrate Paleontology 27:442-460 G. W. Rougier, M. S. de la Fuente, and A. B. Arcucci. 1995. Late Triassic turtles from South America. Science 268:855-858