The Chañares Formation in La Rioja Provence

La Rioja Province (Argentina)

La Rioja is a one of the provinces of Argentina and is located in the west of the country. Neighboring provinces are from the north clockwise Catamarca, Córdoba, San Luis and San Juan.-History:...

, Argentina

Argentina

Argentina , officially the Argentine Republic , is the second largest country in South America by land area, after Brazil. It is constituted as a federation of 23 provinces and an autonomous city, Buenos Aires...

, dates to the Anisian

Anisian

In the geologic timescale, the Anisian is the lower stage or earliest age of the Middle Triassic series or epoch and lasted from 245 million years ago until 237 million years ago, approximately...

 stage of the early Middle Triassic

Middle Triassic

In the geologic timescale, the Middle Triassic is the second of three epochs of the Triassic period or the middle of three series in which the Triassic system is divided. It spans the time between 245 ± 1.5 Ma and 228 ± 2 Ma...

.

Some localities are well known for their abundance of tetrapods. Theraspids

Therapsida

Therapsida is a group of the most advanced synapsids, and include the ancestors of mammals. Many of the traits today seen as unique to mammals had their origin within early therapsids, including hair, lactation, and an erect posture. The earliest fossil attributed to Therapsida is believed to be...

 include the kannemeyeriid

Kannemeyeriidae

Kannemeyeriidae is a family of large, stocky, beaked and sometimes tusked dicynodonts. They were the dominant large terrestrial herbivores through most of the Triassic period...

 Dinodontosaurus

Dinodontosaurus

Dinodontosaurus is a genus of dicynodont therapsid. It was one of the largest herbivores of the Triassic and had a beak corneum...

, and cynodonts such as Probainognathus

Probainognathus

Probainognathus is a genus of meat-eating mammal-like reptile that lived during the lower Upper Triassic of South America. This creature had an incipient squamosal-dentary jaw-cranium joint, which is a clearly mammalian anatomical feature. It was at the very least closely related to the family of...

and Massetognathus

Massetognathus

A relative of Cynognathus, Massetognathus was a plant-eating cynodont belonging to the Traversodontid family. This cynodont lived in what is now South America, in Brazil and Argentina during the Middle Triassic period .Massetognathus was about long. It had cheek teeth specially adapted to...

, the latter being the more abundant taxon. However, the most common group of tetrapods present from the formation are the archosaurs. Ornithodirans include Lewisuchus

Lewisuchus

Lewisuchus is a genus of archosaur that lived during the Middle Triassic ; it was a silesaurid dinosauriform, a member of the group of reptiles which lead to the dinosaurs. Lewisuchus was about three feet long. Fossils have been found in Argentina...

, Lagerpeton

Lagerpeton

Lagerpeton is a genus of basal dinosauromorph from the Ladinian . Lagerpeton is known from several specimens of hindlimbs, hips, vertebrae, and feet. It was about 0.7 metres long and was found in the Chañares Formation of Argentina. It has unique feet, with an unusually long fourth toe...

, Marasuchus

Marasuchus

Marasuchus is a genus of dinosaur-like ornithodiran from the middle Triassic Chañares Formation of Argentina. The species Marasuchus lilloensis was originally described as a second species of Lagosuchus, L. lilloensis...

, and Pseudolagosuchus

Pseudolagosuchus

Pseudolagosuchus is a genus of dinosauromorph from the Middle Triassic Chañares Formation of Argentina. It may be a junior synonym of Lewisuchus, but there is very little overlapping material...

. Other archosaurs include Gracilisuchus

Gracilisuchus

Gracilisuchus is an extinct genus of tiny crurotarsan from the Middle Triassic. It was a suchian close to the ancestry of crocodiles, and was at one time thought to be a dinosaur, but this hypothesis has since been rejected...

and Luperosuchus

Luperosuchus

Luperosuchus is an extinct genus of crurotarsan, a group which includes crocodilians and their ancestors. It lived during the Anisian stage of the Middle Triassic, and is known from Argentina.-References:* in the Paleobiology Database...

. Two proterochampsid

Proterochampsidae

Proterochampsidae is a family of archosauriforms. Proterochampsids may have filled an ecological niche similar to modern crocodiles, and had a general crocodile-like appearance. They lived in what is now South America in the Middle and Late Triassic....

s are known from the formation, Chanaresuchus

Chanaresuchus

Chanaresuchus is an extinct genus of proterochampsian archosauriform. It was of modest size for a proterochampsian, being on average just over a meter in length. Fossils have been found from the Chañares Formation in La Rioja Provence, Argentina, dating back to the Ladinian stage of the Middle...

and Gualosuchus. They are very similar in appearance, differing only in size and cranial proportions.