The Ischigualasto Formation is a

Late Triassic The Late Triassic is the third and final epoch (geology), epoch of the Triassic geologic time scale, Period in the geologic time scale, spanning the time between annum, Ma and Ma (million years ago). It is preceded by the Middle Triassic Epoch ...

formation Formation may refer to: Linguistics * Back-formation, the process of creating a new lexeme by removing or affixes * Word formation, the creation of a new word by adding affixes Mathematics and science * Cave formation or speleothem, a secondary ...

and

Lagerstätte A Lagerstätte (, from ''Lager'' 'storage, lair' '' Stätte'' 'place'; plural ''Lagerstätten'') is a sedimentary deposit that exhibits extraordinary fossils with exceptional preservation—sometimes including preserved soft tissues. These for ...

in the

Ischigualasto-Villa Unión Basin The Ischigualasto-Villa Unión Basin ( es, Cuenca de Ischigualasto-Villa Unión) is a small sedimentary basin located in the Argentine Northwest, Argentina. It is located in the southwestern part of La Rioja Province and the northeastern part o ...

of the southwestern La Rioja Province and northeastern San Juan Province in northwestern

Argentina Argentina (), officially the Argentine Republic ( es, link=no, República Argentina), is a country in the southern half of South America. Argentina covers an area of , making it the second-largest country in South America after Brazil, t ...

. The formation dates to the

Carnian The Carnian (less commonly, Karnian) is the lowermost stage of the Upper Triassic Series (or earliest age of the Late Triassic Epoch). It lasted from 237 to 227 million years ago (Ma). The Carnian is preceded by the Ladinian and is followe ...

and Norian ages and ranges between 231.7 and 225 Ma, based on ash bed dating. The up to thick formation is part of the

Agua de la Peña Group Agua means water in Spanish. Agua may also refer to: Places * ''Agua de Dios'' (God's water), a municipality in Colombia * Volcán de Agua, a stratovolcano located in Guatemala Arts, entertainment, and media * ''Agua'' (film), a 2006 Argentin ...

, overlies

Los Rastros Formation The Los Rastros Formation is a Ladinian to Carnian fossiliferous formation of the Agua de la Peña Group in the Ischigualasto-Villa Unión Basin in northwestern Argentina. Fossil theropod tracks,Weishampel et al., 2004, pp. 517-607 as well as many ...

and is overlain by Los Colorados Formation. The formation is subdivided into four members, from old to young; La Peña, Cancha de Bochas, Valle de la Luna and Quebrada de la Sal. The

sandstone Sandstone is a clastic sedimentary rock composed mainly of sand-sized (0.0625 to 2 mm) silicate grains. Sandstones comprise about 20–25% of all sedimentary rocks. Most sandstone is composed of quartz or feldspar (both silicates ...

s, mudstones, conglomerates and

tuff Tuff is a type of rock made of volcanic ash ejected from a vent during a volcanic eruption. Following ejection and deposition, the ash is lithified into a solid rock. Rock that contains greater than 75% ash is considered tuff, while rock ...

s of the formation were deposited in a humid

alluvial Alluvium (from Latin ''alluvius'', from ''alluere'' 'to wash against') is loose clay, silt, sand, or gravel that has been deposited by running water in a stream bed, on a floodplain, in an alluvial fan or beach, or in similar settings. Al ...

fluvial In geography and geology, fluvial processes are associated with rivers and streams and the deposits and landforms created by them. When the stream or rivers are associated with glaciers, ice sheets, or ice caps, the term glaciofluvial or fluv ...

floodplain A floodplain or flood plain or bottomlands is an area of land adjacent to a river which stretches from the banks of its channel to the base of the enclosing valley walls, and which experiences flooding during periods of high discharge.Goudi ...

environment Environment most often refers to: __NOTOC__ * Natural environment, all living and non-living things occurring naturally * Biophysical environment, the physical and biological factors along with their chemical interactions that affect an organism or ...

, characterized by strongly seasonal rainfall. The Ischigualasto Formation is an important paleontological unit and considered a Lagerstätte, as it preserves several genera of early

dinosaur Dinosaurs are a diverse group of reptiles of the clade Dinosauria. They first appeared during the Triassic period, between 243 and 233.23 million years ago (mya), although the exact origin and timing of the evolution of dinosaurs is t ...

s, other

archosaur Archosauria () is a clade of diapsids, with birds and crocodilians as the only living representatives. Archosaurs are broadly classified as reptiles, in the cladistic sense of the term which includes birds. Extinct archosaurs include non-avi ...

synapsid Synapsids + (, 'arch') > () "having a fused arch"; synonymous with ''theropsids'' (Greek, "beast-face") are one of the two major groups of animals that evolved from basal amniotes, the other being the sauropsids, the group that includes rep ...

s, and

temnospondyl Temnospondyli (from Greek τέμνειν, ''temnein'' 'to cut' and σπόνδυλος, ''spondylos'' 'vertebra') is a diverse order of small to giant tetrapods—often considered primitive amphibians—that flourished worldwide during the Carb ...

s of the Late Triassic.

Coprolite A coprolite (also known as a coprolith) is fossilized feces. Coprolites are classified as trace fossils as opposed to body fossils, as they give evidence for the animal's behaviour (in this case, diet) rather than morphology. The name is de ...

s and fossil wood also have been found in the formation. The formation crops out in the in 1967 established

Ischigualasto Provincial Park Ischigualasto Provincial Park ( es, Parque Provincial Ischigualasto), also called ''Valle de la Luna'' ("Valley of the Moon" or "Moon Valley"), due to its moon like appearance, is a provincial protected area in the north-east of San Juan Provinc ...

, which was designated a

UNESCO The United Nations Educational, Scientific and Cultural Organization is a specialized agency of the United Nations (UN) aimed at promoting world peace and security through international cooperation in education, arts, sciences and culture. I ...

World Heritage Site A World Heritage Site is a landmark or area with legal protection by an international convention administered by the United Nations Educational, Scientific and Cultural Organization (UNESCO). World Heritage Sites are designated by UNESCO for ...

in 2000.

Etymology

The name Ischigualasto is derived from the extinct Cacán language, spoken by an indigenous group referred to as the

Diaguita The Diaguita people are a group of South American indigenous people native to the Chilean Norte Chico and the Argentine Northwest. Western or Chilean Diaguitas lived mainly in the Transverse Valleys which incised in a semi-arid environment. ...

by the

Spanish Spanish might refer to: * Items from or related to Spain: ** Spaniards are a nation and ethnic group indigenous to Spain **Spanish language, spoken in Spain and many Latin American countries **Spanish cuisine Other places * Spanish, Ontario, Ca ...

conquistadors and means "place where the moon alights".El lugar donde se posa la luna
The genus '' Ischigualastia'' and the species '' Herrerasaurus ischigualastensis'', ''

Pseudochampsa ischigualastensis ''Pseudochampsa'' is an extinct genus of proterochampsid archosauriform known from the Late Triassic (Carnian) Cancha de Bochas Member of the Ischigualasto Formation of San Juan Province, Ischigualasto-Villa Unión Basin in northwestern Argenti ...

'', '' Pelorocephalus ischigualastensis'' and '' Protojuniperoxylon ischigualastianus'' were named after the formation.

Geology

Ischigualasto-Villa Unión Basin Map detail

The formation represents the second syn-rift period in the

, where the total thickness of Triassic sediments amounts to .Schencman, 2015, p.220 The formation is exposed in the

La Rioja La Rioja () is an autonomous community and province in Spain, in the north of the Iberian Peninsula. Its capital is Logroño. Other cities and towns in the province include Calahorra, Arnedo, Alfaro, Haro, Santo Domingo de la Calzada, a ...

and San Juan Provinces. In the neighboring Talampaya National Park, the formation is thin and covered by recent sediments.Balabusic et al., 2001, p.26 The formation, part of the

, overlies

and is overlain by Los Colorados Formation. The Ischigualasto Formation strongly contrasts with the bounding formation in color.Monetta et al., 2000, p.644 The total thickness amounts to .Aceituno Cieri et al., 2015, p.60 The Ischigualasto Formation comprises a sequence of

channel

s with well-drained

sandstones and mudstones, dominated by rivers and strongly seasonal rainfall has been estimated at time of deposition. The formation dates to the

Carnian Pluvial Event The Carnian pluvial episode (CPE), often called the Carnian pluvial event, was an interval of major change in global climate synchronous with significant changes in Earth's biota both in the sea and on land. It occurred during the latter part of ...

. Interlayered

volcanic ash Volcanic ash consists of fragments of rock, mineral crystals, and volcanic glass, created during volcanic eruptions and measuring less than 2 mm (0.079 inches) in diameter. The term volcanic ash is also often loosely used to refer ...

layers above the base and below the top of the formation provide chronostratigraphic control and have yielded ages of 231.4 ± 0.3 Ma and 225.9 ± 0.9 Ma respectively.Wallace, 2018, p.6 The formation is approximately coeval with the upper

Santa Maria Formation The Santa Maria Formation is a sedimentary rock formation found in Rio Grande do Sul, Brazil. It is primarily Carnian in age ( Late Triassic), and is notable for its fossils of cynodonts, " rauisuchian" pseudosuchians, and early dinosaurs an ...

of the

Paraná Basin The Paraná Basin ( pt, Bacia do Paraná, es, Cuenca del Paraná) is a large cratonic sedimentary basin situated in the central-eastern part of South America. About 75% of its areal distribution occurs in Brazil, from Mato Grosso to Rio Grande ...

in southeastern Brazil, the Pekin Formation of the United States and the lower Maleri Formation of India.Lecuona et al., 2016, p.585

Subdivision

The Ischigualasto Formation is subdivided into four members, from top to bottom: * Quebrada de la Sal ~ * Valle de la Luna ~ * Cancha de Bochas ~ * La Peña ~

Paleontological significance

The Ischigualasto Formation is highly fossiliferous and its unique paleontological characteristics made it a

; a stratigraphic unit containing a diverse faunal assemblage. The paleontological importance led to the establishment of the

, a

since 2000. The Ischigualasto Formation contains Late

Triassic The Triassic ( ) is a geologic period and system which spans 50.6 million years from the end of the Permian Period 251.902 million years ago (Mya), to the beginning of the Jurassic Period 201.36 Mya. The Triassic is the first and shortest period ...

(

) deposits (231.4 -225.9 million years before the present), with some of the oldest known

remains, which are the world's foremost with regards to quality, number and importance. Saurosuchus and hyperodapedon

Rhynchosaur Rhynchosaurs are a group of extinct herbivorous Triassic archosauromorph reptiles, belonging to the order Rhynchosauria. Members of the group are distinguished by their triangular skulls and elongated, beak like premaxillary bones. Rhynchosaur ...

s and cynodonts (especially

rhynchosaur Rhynchosaurs are a group of extinct herbivorous Triassic archosauromorph reptiles, belonging to the order Rhynchosauria. Members of the group are distinguished by their triangular skulls and elongated, beak like premaxillary bones. Rhynchosaur ...

'' Hyperodapedon'' and cynodont ''Exaeretodon'') are by far the predominant findings among the

tetrapod Tetrapods (; ) are four-limb (anatomy), limbed vertebrate animals constituting the superclass Tetrapoda (). It includes extant taxon, extant and extinct amphibians, sauropsids (reptiles, including dinosaurs and therefore birds) and synapsids (p ...

fossils A fossil (from Classical Latin , ) is any preserved remains, impression, or trace of any once-living thing from a past geological age. Examples include bones, shells, exoskeletons, stone imprints of animals or microbes, objects preserved ...

in the park. A study from 1993 found

specimens to comprise only 6% of the total tetrapod sample; subsequent discoveries increased this number to approximately 11% of all findings. Carnivorous dinosaurs are the most common terrestrial carnivores of the Ischigualasto Formation, with

herrerasaurids Herrerasauridae is a Family (biology), family of carnivore, carnivorous dinosaurs, possibly basal (phylogenetics), basal to either theropods or even all of saurischians, or even their own branching from dracohors, separate from dinosauria altoget ...

comprising 72% of all recovered terrestrial carnivores. The carnivorous

Herrerasaurus ''Herrerasaurus'' is a genus of saurischian dinosaur from the Late Triassic period. This genus was one of the earliest dinosaurs from the fossil record. Its name means "Herrera's lizard", after the rancher who discovered the first specimen in ...

'' is the most numerous of these dinosaur fossils. Another important putative dinosaur with primitive characteristics is '' Eoraptor lunensis'', found in Ischigualasto in the early 1990s. Petrified tree trunks of '' Protojuniperoxylon ischigualastianus'' of more than tall attest to a rich vegetation at that time. Fossil

fern A fern (Polypodiopsida or Polypodiophyta ) is a member of a group of vascular plants (plants with xylem and phloem) that reproduce via spores and have neither seeds nor flowers. The polypodiophytes include all living pteridophytes except ...

s and

horsetail ''Equisetum'' (; horsetail, snake grass, puzzlegrass) is the only living genus in Equisetaceae, a family of ferns, which reproduce by spores rather than seeds. ''Equisetum'' is a " living fossil", the only living genus of the entire subclass ...

s have also been found in the formation.

s were found in Valle Pintado in the upper part of the formation. Analysis of the coprolites revealed that plant remains were absent and bone material and

apatite Apatite is a group of phosphate minerals, usually hydroxyapatite, fluorapatite and chlorapatite, with high concentrations of OH−, F− and Cl− ions, respectively, in the crystal. The formula of the admixture of the three most common ...

were sparse. The most likely candidate to have produced these fossil feces has been suggest as the most common reptile in the formation; ''

''.Hollocher et al., 2005, p.62

Fossil content

Dinosaurs

The fossils of an undescribed species of

theropod Theropoda (; ), whose members are known as theropods, is a dinosaur clade that is characterized by hollow bones and three toes and claws on each limb. Theropods are generally classed as a group of saurischian dinosaurs. They were ancestrally ...

Etymology

Geology

Subdivision

Paleontological significance

Fossil content

Dinosaurs

Other reptiles

Synapsids

Temnospondyls

Plants

See also

References

Bibliography

Books

Further reading