Stratigraphic range: Carnian
Valle Pintado, Ischigualasto Formation
|Unit of||Agua de la Peña Group|
|Sub-units||Quebrada de la Sal, Valle de la Luna, Cancha de Bochas & La Peña Members|
|Underlies||Los Colorados Formation|
|Overlies||Los Rastros Formation|
|Thickness||Up to 900 m (3,000 ft)|
|Region||La Rioja Province & San Juan Provinces|
|Extent||Ischigualasto-Villa Unión Basin|
|Named for||Cacán: "Place where the moon alights"|
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.
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". The genus Ischigualastia and the species Herrerasaurus ischigualastensis, Pseudochampsa ischigualastensis, Pelorocephalus ischigualastensis and Protojuniperoxylon ischigualastianus were named after the formation.
The formation represents the second syn-rift period in the Ischigualasto-Villa Unión Basin, where the total thickness of Triassic sediments amounts to 3.5 kilometres (11,000 ft). 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. 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. The total thickness amounts to 900 metres (3,000 ft).
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.
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.
The Ischigualasto Formation is subdivided into four members, from top to bottom:
- 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)
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), 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.
Rhynchosaurs and cynodonts (especially rhynchosaur Hyperodapedon and cynodont Exaeretodon) 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; 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. 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.
Uncertain or tentative taxa are in small text;
|Dinosaurs of the Ischigualasto Formation|
|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.|
|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.|
|Eoraptor||E. lunensis||San Juan||Cancha de Bochas||Two nearly complete skeletons||An omnivorous, lightly-built, basal eusaurischian, close to the ancestry of theropods and sauropodomorphs. 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.|
|Herrerasaurus||H. ischigualastensis||San Juan||Cancha de Bochas||"Various partial skeletons, including a complete skull and mandible."||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.|
|Panphagia||P. protos||San Juan||La Peña||A guaibasaurid that is one of the most basal known genera of sauropodomorphs. 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.|
|Sanjuansaurus||S. gordilloi||San Juan||Cancha de Bochas
|An incomplete skeleton||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.|
|An unnamed herrerasaur||Unnamed||Specimen MACN-PV 18649a||A herrerasaur distinct from Herrerasaurus, Staurikosaurus and Sanjuansaurus.|
|Non-dinosaurian archosauromorphs of the Ischigualasto Formation|
|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.|
|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.|
|Ignotosaurus||I. fragilis||San Juan||Cancha de Bochas||Right ilium||A little-known silesaur|
|Pisanosaurus||P. mertii||La Rioja||"Fragmentary skull and skeleton"||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).|
|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).|
|Pseudochampsa||P. ischigualastensis||San Juan||Cancha de Bochas||An articulated incomplete skeleton||A proterochampsid originally described a species of Chanaresuchus, subsequently made the type species of a separate genus.|
|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.|
|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.|
|Trialestes||T. romeri||La Rioja
|Cancha de Bochas||The earliest known crocodylomorph, once believed to be a primitive dinosaur|
|Venaticosuchus||V. rusconii||La Rioja||A medium-sized ornithosuchid, reaching up to 2 m (6.6 ft) in length.|
|An unnamed lagerpetid||Unnamed||San Juan||Distal end of the left femur||A basal dinosauromorph|
|Synapsids of the Ischigualasto Formation|
|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|
|Diegocanis||D. elegans||San Juan||Cancha de Bochas||Partial skull, represented by the snout and the orbital region, with partially preserved upper dentition||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.|
|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.|
|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|
|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.|
|An unnamed eucynodont||Unnamed||Specimen PVSJ 882 (a cranium)||A probainognathian cynodont closely related to tritylodontids|
|Temnospondyls of the Ischigualasto Formation|
|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.|
|Promastodonsaurus||P. bellmanni||A little-known mastodonsaur|
- Quebrada del Barro Formation, contemporaneous fossiliferous formation of the Marrayel-El Carrizal Basin
- Candelária Formation, contemporaneous fossiliferous formation of the Paraná Basin in southeastern Brazil
- Molteno Formation, contemporaneous fossiliferous formation of the Karoo Basin in southern Africa
- Fremouw Formation, contemporaneous fossiliferous formation of Antarctica
- Denmark Hill Insect Bed, contemporaneous fossiliferous unit of Queensland, Australia
- (in Spanish) El lugar donde se posa la luna
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