The Ischigualasto Formation is a

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

fossiliferous 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 ...

formation 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 f ...

in the Ischigualasto-Villa Unión Basin 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, th ...

. The formation dates to the

Carnian The Carnian (less commonly, Karnian) is the lowermost stage (stratigraphy), stage of the Upper Triassic series (stratigraphy), Series (or earliest age (geology), age of the Late Triassic Epoch (reference date), Epoch). It lasted from 237 to 227 m ...

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, overlies Los Rastros Formation and is overlain by

Los Colorados Formation 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 f ...

. 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. All ...

to fluvial

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, 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 archosaurs,

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 reptil ...

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 Carbo ...

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, 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. It ...

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 h ...

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. Ea ...

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, Can ...

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

Herrerasaurus ischigualastensis ''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 ...

'', '' Pseudochampsa ischigualastensis'', '' 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 Ischigualasto-Villa Unión Basin, where the total thickness of Triassic sediments amounts to .Schencman, 2015, p.220 The formation is exposed in the Ischigualasto Provincial Park of

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, an ...

and San Juan Provinces. In the neighboring

Talampaya National Park Talampaya National Park ( es, Parque Nacional Talampaya) is a national park located in the east/centre of La Rioja Province, Argentina. It was designated a provincial reserve in 1975, a national park in 1997, and a UNESCO World Heritage Site in 2 ...

, the formation is thin and covered by recent sediments.Balabusic et al., 2001, p.26 The formation, part of the Agua de la Peña Group, overlies Los Rastros Formation and is overlain by

. 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 fluvial 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. 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 and other ...

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 d ...

in southeastern Brazil, the

Pekin Formation The Pekin Formation is a Late Triassic (Carnian) geological formation in North Carolina. The Pekin Formation is specific to the Sanford Sub-Basin of the Deep River Basin of North Carolina, although it may be equivalent to the Stockton Formation o ...

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 Ischigualasto Provincial Park, a

since 2000. The Ischigualasto Formation contains Late

Triassic The Triassic ( ) is a geologic period and system (stratigraphy), system which spans 50.6 million years from the end of the Permian Period 251.902 million years ago (Year#Abbreviations yr and ya, Mya), to the beginning of the Jurassic Period 201.36 ...

(

) 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. Rhynchosaurs ...

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. Rhynchosaurs ...

Hyperodapedon ''Hyperodapedon'' is a genus of rhynchosaurs (beaked, archosaur-like reptiles) from the Late Triassic period (Carnian stage). Fossils of the genus have been found in Africa, Asia, Europe and North and South America. Its first discovery and namin ...

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

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

fossils 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 comprising 72% of all recovered terrestrial carnivores. The carnivorous archosaur '' Herrerasaurus'' is the most numerous of these dinosaur fossils. Another important putative dinosaur with primitive characteristics is ''

Eoraptor lunensis ''Eoraptor'' () is a genus of small, lightly built, basal sauropodomorph. One of the earliest-known dinosaurs, it lived approximately 231 to 228 million years ago, during the Late Triassic in Western Gondwana, in the region that is now northwes ...

'', found in Ischigualasto in the early 1990s.

Petrified tree Petrified wood, also known as petrified tree (from Ancient Greek meaning 'rock' or 'stone'; literally 'wood turned into stone'), is the name given to a special type of '' fossilized wood'', the fossilized remains of terrestrial vegetation. ' ...

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 exce ...

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

Etymology

Geology

Subdivision

Paleontological significance

Fossil content

Dinosaurs

Other reptiles

Synapsids

Temnospondyls

Plants

See also

References

Bibliography

Books

Further reading