Gracilisuchus

Genus of fossil reptiles

Gracilisuchus (meaning "slender crocodile") is an extinct genus of tiny pseudosuchian (a group which includes the ancestors of crocodilians) from the Late Triassic of Argentina. It contains a single species, G. stipanicicorum, which is placed in the clade Suchia, close to the ancestry of crocodylomorphs. Both the genus and the species were first described by Alfred Romer in 1972.

A four month expedition spanning 1964 and 1965 in the Ischigualasto Villa Unión Basin of La Rioja Province, Argentina was conducted by Alfred Romer and his colleagues, who consisted of researchers from the Museum of Comparative Zoology (MCZ) at Harvard University. While the first two months of the expedition were unfruitful, excavations near the Chañares River and the Gualo River, soon uncovered specimens belonging to a wide variety of tetrapod groups. Among these was the skeleton of a small suchian archosaur, discovered about north of the Chañares River. The specimen has since been transferred to the Museum of Paleontology at the National University of La Rioja (PULR), where it bears the specimen number PULR 08. In 1972, Alfred Romer estimated a total length of ;

A number of characteristics of the skull can be used to distinguish Gracilisuchus. Its skull openings are relatively large, with the antorbital fenestra occupying 30 36% of the skull roof's length and the eye socket occupying 35 42% of the skull roof's length. Additionally, the supratemporal fenestra is, uniquely, wider than it is long. Within the eye socket, there is a sclerotic ring, and the ossicles (bony segments) comprising the ring contact but do not overlap each other. Unlike its closest relatives, Turfanosuchus and Yonghesuchus, but convergent upon Tropidosuchus, early theropods, and the Crocodylomorpha, the lacrimal bone is as tall as the eye socket instead of being significantly shorter. Unusually, but like Tropidosuchus, Protosuchus, and Orthosuchus, the articulation with the ilium on the pubis is short. Within the Gracilisuchidae, Butler and colleagues noted that Gracilisuchus was likely closer to Yonghesuchus than Turfanosuchus, on account of three synapomorphies: the contact between the squamosal and postorbital bones continuing backwards along much of the former's bottom surface; the jugal stopping short at its rear end of the infratemporal fenestra; and the conjunction of the basisphenoid and parasphenoid being located between plate like, triangular projections of the basioccipital known as tubera, with the basipterygoid processes at the base of the basisphenoid being at least 1.5 times longer than it is wide. However, poor support for this relationship was obtained, partially due to the incompleteness of material referred to Yonghesuchus. Unlike other Carnian (Late Triassic) formations, there are no dinosaur remains in the Chañares Formation. This has led researchers to assign a Middle Triassic age to the Chañares. Owing to shared faunal components, correlations with the Dinodontosaurus Assemblage Zone of the Santa Maria Formation in Brazil have been used to assign a Ladinian age to the Chañares. Argon–argon dating of the Ischigualasto Formation (which locally overlies the Los Rastros) in 1993 also produced an age near the Ladinian–Carnian boundary, thus constraining the Chañares to the Ladinian. The Ischigualasto was reassigned to the late Carnian, making a Carnian age possible for the Chañares. This was reaffirmed by uranium lead dating of Chañares deposits in 2016, indicating an early Carnian age of 236 to 234 million years.