Ischigualasto Formation

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Ischigualasto Formation
Stratigraphic range: Carnian
~231.7–225 Ma
Valle Pintado 1 - Parque Provincial Ischigualasto.jpg
Valle Pintado, Ischigualasto Formation
TypeGeological formation
Unit ofAgua de la Peña Group
Sub-unitsQuebrada de la Sal, Valle de la Luna, Cancha de Bochas & La Peña Members
UnderliesLos Colorados Formation
OverliesLos Rastros Formation
ThicknessUp to 900 m (3,000 ft)
Lithology
PrimarySandstone, mudstone
OtherTuff, conglomerate
Location
Coordinates29°36′S 68°06′W / 29.6°S 68.1°W / -29.6; -68.1Coordinates: 29°36′S 68°06′W / 29.6°S 68.1°W / -29.6; -68.1
Approximate paleocoordinates46°00′S 40°12′W / 46.0°S 40.2°W / -46.0; -40.2
RegionLa Rioja Province & San Juan Provinces
Country Argentina
ExtentIschigualasto-Villa Unión Basin
Type section
Named forCacán: "Place where the moon alights"
Ischigualasto Formation is located in Argentina
Ischigualasto Formation
Ischigualasto Formation (Argentina)

The Ischigualasto Formation is a Late Triassic fossiliferous formation and Lagerstätte in the Ischigualasto-Villa Unión Basin of the southwestern La Rioja Province and northeastern San Juan Province in northwestern Argentina. The formation dates to the Carnian age and ranges between 231.7 and 225 Ma, based on ash bed dating.

The up to 900 metres (3,000 ft) thick formation is part of the Agua de la Peña Group, overlies Los Rastros Formation 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 sandstones, mudstones, conglomerates and tuffs of the formation were deposited in a humid alluvial to fluvial floodplain 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 dinosaurs, other archosaurs, synapsids, and temnospondyls of the Late Triassic. Coprolites 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 World Heritage Site in 2000.

Etymology[edit]

The name Ischigualasto is derived from the extinct Cacán language, spoken by an indigenous group referred to as the Diaguita by the Spanish conquistadors and means "place where the moon alights".[1] The genus Ischigualastia and the species Herrerasaurus ischigualastensis, Pseudochampsa ischigualastensis, Pelorocephalus ischigualastensis and Protojuniperoxylon ischigualastianus were named after the formation.

Description[edit]

Extent of the formation in the Ischigualasto-Villa Unión Basin

The formation represents the second syn-rift period in the Ischigualasto-Villa Unión Basin,[2] where the total thickness of Triassic sediments amounts to 3.5 kilometres (11,000 ft).[3] The formation is exposed in the Ischigualasto Provincial Park of La Rioja and San Juan Provinces. In the neighboring Talampaya National Park, the formation is thin and covered by recent sediments.[4] The formation, part of the Agua de la Peña Group, overlies Los Rastros Formation and is overlain by Los Colorados Formation. The Ischigualasto Formation strongly contrasts with the bounding formation in color.[5] The total thickness amounts to 900 metres (3,000 ft).[2]

The Ischigualasto Formation comprises a sequence of fluvial channel sandstones with well-drained floodplain 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 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.[6]

The formation is approximately coeval with the upper Santa Maria Formation of the Paraná Basin in southeastern Brazil, the Pekin Formation of the United States and the lower Maleri Formation of India.[7]

Subdivision[edit]

The Ischigualasto Formation is subdivided into four members, from top to bottom:[8]

  • Quebrada de la Sal ~60 metres (200 ft)
  • Valle de la Luna ~450 metres (1,480 ft)
  • Cancha de Bochas ~130 metres (430 ft)
  • La Peña ~50 metres (160 ft)

Paleontological significance[edit]

Fauna of the Ischigualasto Formation

The Ischigualasto Formation is highly fossiliferous and its unique paleontological characteristics made it a Lagerstätte; a stratigraphic unit containing a diverse faunal assemblage. The paleontological importance led to the establishment of the Ischigualasto Provincial Park, a UNESCO World Heritage Site since 2000.

The Ischigualasto Formation contains Late Triassic (Carnian) deposits (231.4 -225.9 million years before the present[9]), with some of the oldest known dinosaur remains, which are the world's foremost with regards to quality, number and importance. This allows for the study of the transition between dinosaurs and ancient mammals; research is ongoing.

Interpretation of Saurosuchus and Hyperodapedon

Rhynchosaurs and cynodonts (especially rhynchosaur Hyperodapedon and cynodont Exaeretodon[9]) are by far the predominant findings among the tetrapod fossils in the park. A study from 1993 found dinosaur specimens to comprise only 6% of the total tetrapod sample;[10] subsequent discoveries increased this number to approximately 11% of all findings.[9] Carnivorous dinosaurs are the most common terrestrial carnivores of the Ischigualasto Formation, with herrerasaurids comprising 72% of all recovered terrestrial carnivores.[9] Dinosaurs of Ischigualasto Formation include early samples of the two major lineages of dinosaurs (ornithischians and saurischians). The carnivorous archosaur Herrerasaurus 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 40 m (130 ft) tall attest to a rich vegetation at that time. Fossil ferns and horsetails have also been found in the formation.

Coprolites 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.[11]

Fossil content[edit]

Dinosaurs[edit]

The fossils of an undescribed species of theropod are present in San Juan Province.[12]

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.
Dinosaurs of the Ischigualasto Formation
Genus Species Province Member Material Notes Image
Chromogisaurus C. novasi San Juan Cancha de Bochas Partial skeleton including limb bones, pelvic bones and caudal vertebrae A 2 metres (6.6 ft) long saturnaliine guaibasaurid known from a partial skeleton lacking the skull. It includes elements of the front and hind limbs; the pelvis and two caudal vertebrae.[13]
Eodromaeus E. murphi San Juan Valle de la Luna A nearly complete skeleton and another partial skeleton A basal theropod with a total length of about 1.2 meters (3.9 feet) from nose to tail, and a weight of about 5 kilograms (11 pounds). The trunk was long and slender. It is unknown how fast Eodromaeus could run, but it has been suggested to about 30 kilometres per hour (19 mph). The forelimbs were much shorter than the hind limbs, ending in hands with 5 digits. Digits IV and V (the ring finger and little finger in humans) were very reduced in size.[9]
Eodromaeus 400 AJV.png
Eoraptor E. lunensis San Juan Cancha de Bochas Two nearly complete skeletons[14] An omnivorous, lightly-built, basal eusaurischian, close to the ancestry of theropods[13] and sauropodomorphs.[9] Eoraptor had a slender body that grew to about 1 meter (3.3 feet) in length, with an estimated weight of about 10 kilograms (22 pounds). It has a lightly built skull with a slightly enlarged external naris. Like the coelophysoids which would appear millions of years later, Eoraptor has a kink in its upper jaws, between the maxilla and the premaxilla.[12]
Eoraptor NT small.jpg
Herrerasaurus H. ischigualastensis San Juan Cancha de Bochas "Various partial skeletons, including a complete skull and mandible."[14] A herrerasaur with a length estimated at 3 to 6 meters (9.8 to 19.7 ft), and its hip height at more than 1.1 meters (3.6 feet). It may have weighed around 210–350 kg (460–770 lb). In a large specimen, at first thought to belong to a separate (now discredited) genus, Frenguellisaurus, the skull measured 56 cm (22 in) in length. Smaller specimens had skulls about 30 centimetres (12 in) long. Its size indicates it would have preyed upon small and medium-sized plant-eaters. Herrerasaurus itself may have been preyed upon by giant rauisuchids; puncture wounds were found in one skull.[12]
Herrerasaurus ischigualastensis Illustration.jpg
Panphagia P. protos San Juan La Peña A guaibasaurid that is one of the most basal known genera of sauropodomorphs.[13][15][16] Panphagia is currently known from the disarticulated remains of one partially grown individual of about 1.30 meters (4.3 feet) long. Portions of the skull, vertebrae, pectoral girdle, pelvic girdle, and hind limb bones have been recovered. The russet-colored fossils were embedded in a greenish sandstone matrix and took several years to prepare and describe.[15][12]
Panphagia NT.jpg
Sanjuansaurus S. gordilloi San Juan Cancha de Bochas
La Peña
An incomplete skeleton[17] A herrerasaur comparable in size to a medium-sized Herrerasaurus, with a thigh bone that was 395 millimeters (15.6 inches) long and a tibia that is 260 millimeters (10 inches) in length.[17]
Sanjuansaurus NT.jpg
An unnamed herrerasaur[13] Unnamed Specimen MACN-PV 18649a[13] A herrerasaur distinct from Herrerasaurus, Staurikosaurus and Sanjuansaurus.[13]

Other archosauromorphs[edit]

Non-dinosaurian archosauromorphs of the Ischigualasto Formation[13][18]
Genus Species Province Member Material Notes Images
Aetosauroides A. scagliai San Juan Cancha de Bochas An aetosaur that is one of four aetosaurs known from South America, the others being Neoaetosauroides, Chilenosuchus and Aetobarbakinoides. It was once proposed to be synonymous with Stagonolepis.
Aetosauroides scagliai life restoration.jpg
Hyperodapedon H. sanjuanensis San Juan A heavily-built, stocky hyperodapedontine hyperodapedont around 1.3 meters (4.3 feet) in length. Apart from its beak, this rhynchosaur had several rows of heavy teeth on each side of the upper jaw, and a single row on each side of the lower jaw, creating a powerful chopping action when it ate. It is believed to have been herbivorous, feeding mainly on seed ferns.
Hyperodapedon NT small.jpg
Ignotosaurus I. fragilis San Juan Cancha de Bochas Right ilium[18] A little-known silesaur[18]
Ignotosaurus illium.png
Pisanosaurus P. mertii La Rioja "Fragmentary skull and skeleton"[19] A non-dinosaurian dinosauriform known from a single partial skeleton. Pisanosaurus was a small, lightly-built, ground-dwelling herbivore approximately 1 meter (3 feet 3 inches) in length. Its weight was between 2.27 and 9.1 kg (5.0 and 20.1 lb). These estimates vary due to the incompleteness of the holotype specimen PVL 2577. It was originally believed to be an early species of ornithischian dinosaur, but recent studies have proven it to be a non-dinosaurian dinosauriform closely related to the silesaurs (possibly a silesaur itself).[12]
Pisanosaurus NT small.jpg
Proterochampsa P. barrionuevoi San Juan Cancha de Bochas A proterochampsid known from a 44 centimetres (17 in) skull. It could have grown up to 3.5 m (11 ft).
Proterochampsa BW.jpg
Pseudochampsa P. ischigualastensis San Juan Cancha de Bochas An articulated incomplete skeleton[20] A proterochampsid originally described a species of Chanaresuchus,[20] subsequently made the type species of a separate genus.[21]
Chanaresuchus NT small.jpg
Saurosuchus S. galilei San Juan Cancha de Bochas A prestosuchid with a length of around 6 to 9 meters (20 to 30 ft) in total body length. Dorsal osteoderms run along the back of Saurosuchus. There are two rows to either side of the midline, with each leaf-shaped osteoderm joining tightly with the ones in front of and behind it. It has a deep, laterally compressed skull. The teeth are large, recurved, and serrated. The skull is wide at its back and narrows in front of the eyes.
Saurosuchus BW.jpg
Sillosuchus S. longicervix San Juan Cancha de Bochas A shuvosaurid which could grow to large sizes. It is the only shuvosaurid currently known from outside North America.[22]
Sillosuchus longicervix life restoration.jpg
Trialestes T. romeri La Rioja
San Juan
Cancha de Bochas The earliest known crocodylomorph, once believed to be a primitive dinosaur
Trialestes.jpg
Venaticosuchus V. rusconii La Rioja A medium-sized ornithosuchid, reaching up to 2 m (6.6 ft) in length.[18]
Venaticosuchus BW.jpg
An unnamed lagerpetid Unnamed San Juan Distal end of the left femur[18] A basal dinosauromorph[23][18]

Synapsids[edit]

Synapsids of the Ischigualasto Formation[13][18]
Genus Species Province Member Material Notes Images
Chiniquodon C. sanjuanensis, C. cf. theotonicus San Juan Cancha de Bochas A carnivorous, dog-sized chiniquodontid that was a predatory cynodont being similar in ecological niche as the predatory dinosaurs it coexisted with[24][25]
Chiniquodon NT small.jpg
Diegocanis D. elegans San Juan Cancha de Bochas Partial skull, represented by the snout and the orbital region, with partially preserved upper dentition[24] A little-known ecteniniid
Ecteninion E. lunensis San Juan Cancha de Bochas A carnivorous ecteniniid known from a nearly complete skull of about 11 centimetres (4.3 in) in length
Exaeretodon E. argentinus San Juan Cancha de Bochas A gomphodontosuchine traversodont up to 1.8 meters (5.9 feet) long, with a specialized grinding action when feeding. Another point of interest is that these cynodonts had deciduous teeth, which is a characteristic of mammals and means that babies could not chew, and required specialized parental care. Only older juveniles had permanent teeth.
Exaeretodon.jpg
Ischigualastia I. jenseni San Juan Cancha de Bochas A stahleckeriine stahleckeriid that was an enormous dicynodont with a short, high skull, and lacking tusks. It is regarded as larger than its later, more famous relative Placerias.
Ischigualastia NT small.jpg
Jachaleria J. colorata A large dicynodont perhaps 3 metres (9.8 ft) in length and with an estimated mass of 300 kilograms (660 lb), making it close in size to Dinodontosaurus[18]
cf. Probainognathus Indeterminate A small probainognathid that had an incipient squamosal-dentary jaw-cranium joint, which is a clearly mammalian anatomical feature. Known from about three dozen specimens, this creature was only about 10 centimetres (3.9 in) long.
Probainognathus jenseni life restoration.jpg
An unnamed eucynodont[18] Unnamed[18] Specimen PVSJ 882 (a cranium)[18][26] A probainognathian cynodont closely related to tritylodontids[26]

Temnospondyls[edit]

Temnospondyls of the Ischigualasto Formation[13][18]
Genus Species Province Member Material Notes Images
Pelorocephalus P. ischigualastensis A chigutisaurid based on too little material. The largest individuals are estimated to have been over 107 centimetres (42 in) in length.
Pelorocephalus1DB.jpg
Promastodonsaurus P. bellmanni A little-known mastodonsaur

See also[edit]

References[edit]

  1. ^ (in Spanish) El lugar donde se posa la luna
  2. ^ a b Aceituno Cieri et al., 2015, p.60
  3. ^ Schencman, 2015, p.220
  4. ^ Balabusic et al., 2001, p.26
  5. ^ Monetta et al., 2000, p.644
  6. ^ Wallace, 2018, p.6
  7. ^ Lecuona et al., 2016, p.585
  8. ^ Martínez et al. 2013b, p. 16.
  9. ^ a b c d e f Martínez et al. 2011.
  10. ^ Rogers et al. 1993.
  11. ^ Hollocher et al., 2005, p.62
  12. ^ a b c d e Weishampel et al. 2004, pp. 527–528.
  13. ^ a b c d e f g h i Ezcurra 2010.
  14. ^ a b Weishampel et al. 2004, p. 26, Table 2.1.
  15. ^ a b Martínez et al. 2009.
  16. ^ Cabreira et al. 2011.
  17. ^ a b Alcober et al. 2010.
  18. ^ a b c d e f g h i j k l Martínez et al. 2013b.
  19. ^ Weishampel et al. 2004, p. 326, Table 14.1.
  20. ^ a b Trotteyn et al. 2012.
  21. ^ Trotteyn & Ezcurra 2014.
  22. ^ Alcober & Parrish, 2010, p.548
  23. ^ Langer et al. 2013.
  24. ^ a b Martínez et al. 2013.
  25. ^ Martínez & Forster, 1996, p.285
  26. ^ a b Wallace, 2018, p.10

Bibliography[edit]

Geology
Paleontology

Books[edit]

  • Langer, Max C.; Sterling J. Nesbitt; Jonathas S. Bittencourt, and Randall B. Irmis. 2013. Anatomy, phylogeny and palaeobiology of early archosaurs and their kin. Geological Society, London, Special Publications 379. 157–186.
  • 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

Further reading[edit]

  • F. Abdala. 2000. Catalogue of non-mammalian cynodonts in the Vertebrate Paleontology Collection of the Instituto Miguel Lillo, Universidad Nacional de Tucuman, with comments on species. Ameghiniana 37(4):463-475
  • Bonaparte, J.F. 1978. El Mesozóico de América de Sur y sus Tetrapodos - The Mesozoic of South America and its tetrapods. Opera Lilloana 26. 1–596.
  • R. M. Casamiquela. 1967. Un nuevo dinosaurio ornitisquio triasico (Pisanosaurus mertii; Ornithopoda) de la Formación Ischigualasto, Argentina [A new Triassic ornithischian dinosaur (Pisanosaurus mertii; Ornithopoda) from the Ischigualasto Formation, Argentina]. Ameghiniana 4(2):47-64
  • V.H. Contreras. 1981. Datos preliminares sobre un nuevo rincosaurio (Reptilia, Rhynchosauria) del Triasico Superior de Argentina. Anais II Congresso Latino-Americano Paleontologia, Porto Alegre 2:289-294
  • C. B. Cox. 1965. New Triassic dicynodonts from South America, their origins and relationships. Philosophical Transactions of the Royal Society of London, Series B 248(753):457-516
  • F. E. Novas. 1986. Un probable teropodo (Saurischia) de la Formacion Ischigualasto (Triasico Superior), San Juan, Argentina [A probable theropod (Saurischia) from the Ischigualasto Formation (Upper Triassic), San Juan, Argentina]. IV Congreso Argentino de Paleontologia y Bioestratigrafia 1:1-6
  • O. A. Reig. 1963. La presencia de dinosaurios saurisquios en los "Estratos de Ischigualasto" (Mesotriasico Superior) de las provincias de San Juan y La Rioja (República Argentina) [The presence of saurischian dinosaurs in the "Ischigualasto beds" (upper Middle Triassic) of San Juan and La Rioja Provinces (Argentine Republic)]. Ameghiniana 3(1):3-20
  • P. C. Sereno, C. A. Forster, R. R. Rogers and A. M. Monetta. 1993. Primitive dinosaur skeleton from Argentina and the early evolution of Dinosauria. Nature 361:64-66