Chanaresuchus

Chanaresuchus is an extinct genus of proterochampsid archosauromorph. It was of modest size for a proterochampsid, 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 Anisian stage of the early Middle Triassic. Chanaresuchus appears to be one of the most common "thecodont" archosaurs from the formation due to the abundance of specimens referred to the genus. Much of the material has been found by the La Plata-Harvard expedition of 1964-65.

Description
Chanaresuchus had a low, elongate skull that is characteristic of proterochampsids. The skull is quite broad posteriorly with a narrow snout, varying in length from around 165 mm to 260 mm in the largest individuals. The nares are slit-like and positioned away from the tip of the rostrum, farther up the skull. The premaxilla is slightly down-curved. The skull table is highly ornamented in larger specimens, with the dermal bones well sculptured. The palate of Chanaresuchus has two elongate choanae. Two small openings anterior to the choanae may be anterior palatine foramina that could have been used for access to vomeronasal organs. The secondary palate formed between these two sets of openings may have been an adaptation for breathing through the snout while underwater.

Unlike other proterochampsids and early archosaurs, Chanaresuchus had little body armor. The only osteoderms found are small and scale-like, forming a single row down the neural spines. They run from the neck to the last presacral, and most likely continue on down the tail, although no specimens have been shown that suggest this. There are roughly three scales per vertebra.

The feet of Chanaresuchus differs from other related archosaurs in that there is an emphasis on an enlargement of the inner phalanges, whereas other primitive archosaurs retain a somewhat more symmetrical pattern. The first digit is reduced but quite robust. The second digit is clearly the most massive, yet the third digit is the longest, although somewhat slender in comparison. The fourth digit is very slim and the fifth consists of only a metatarsal spur.

Paleobiology
A semi-aquatic lifestyle similar to phytosaurs and modern day crocodilians has been proposed for Chanaresuchus, as is suggested by the secondary palate, dorsally facing orbits, dorsally positioned nostrils, and many other characteristics. However, some evidence, such as a lack of aquatic amphibians found from the Chañares Formation, suggests that the area was relatively dry during the time of deposition. A terrestrial lifestyle is possible for the genus, as extreme differences in the size of the forelimbs relative to the hind limbs in comparison to other "thecodonts" may be considered a characteristic of bipedalism. However, this is most likely not the case for Chanaresuchus, and the evidence still remains in favor of a semiaquatic animal.

The depositional environment of the locality from which specimens of Chanaresuchus have been found was in close proximity to an area of high volcanic activity, due to the fact that it was in an active rift basin. It is thought that all of the recovered specimens had died in a single event of mass mortality and may have been buried on a fluvial strandline. The mortality-causing event was most likely linked to regional volcanic activity.

Contemporaries
The locality from which specimens of Chanaresuchus have been found is well known for its abundance of tetrapods. Theraspids include the kannemeyeriid Dinodontosaurus, and cynodonts such as Probainognathus and Massetognathus, the latter being the most abundant taxon of the locality. However, the most common group of tetrapods present from the formation are the archosaurs. Ornithodirans include Lewisuchus, Lagerpeton, Marasuchus, and Pseudolagosuchus. Other archosaurs include Gracilisuchus and Luperosuchus. Another proterochampsid, which was named alongside Chanaresuchus in 1971, is Gualosuchus. It is very similar in appearance to Chanaresuchus, differing only in size and cranial proportions.