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.
Discovery
Discovery of the holotype
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 skeleton was stored at the La Plata Museum (MLP), which had supported the expedition, under the specimen number MLP 64-XI-14-11. 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.
This specimen, which would become the holotype of Gracilisuchus, consists of a partial skull, an incomplete vertebral column, parts of the scapula and humerus, gastralia, and several associated osteoderms. Several other specimens are mixed in with these remains on the same slab. A vertebral series ("Series A"), originally identified as the tail of Gracilisuchus, has been reassigned to Tropidosuchus. Another series of cervical vertebrae ("Series B") underwent a similar reassignment. A scapula and coracoid overlapping the holotype's limb bones likely belongs to a second Gracilisuchus. There are three ilia, none of which belong to the holotype; one belongs to a specimen of Lagosuchus, another to Tropidosuchus. A right hindlimb and a left femur and tibia have been assigned to Tropidosuchus, and another one has been assigned to Lagosuchus. The right hindlimb's foot was originally assigned to Gracilisuchus.
Additional specimens and description
Three other specimens, stored at the MCZ, were found by Romer and his team in the same locality as the holotype. They are MCZ 1147, a nearly complete skull with jaws; MCZ 4118, parts of the skull, a well-preserved neck, and other elements; and MCZ 4116, a crushed skull and jaw with material from the rest of the body, some of it pertaining to a smaller specimen. José Bonaparte from the Miguel Lillo Institute (PVL) later collected additional material from the site between 1970 and 1972, including two new specimens: PVL 4597, the "Tucuman specimen", which includes a nearly complete skull and jaws, nearly complete cervical and dorsal vertebrae, partial sacral and caudal vertebrae, the hip girdle, most of the left hindlimb, and part of the hindlimb, which is 20% larger than the holotype; and PVL 4612, a nearly complete skull and jaws.
In 1972, Romer described the specimens his team had discovered in the journal Breviora. He named a new genus for the specimens, Gracilisuchus, with the prefix Gracili- referring to "the obviously graceful build of the little reptile". Furthermore, he placed them in the type and only species G. stipanicicorum, which honors the work of Pedro and Maria Stipanicic in the stratigraphy and paleobotany of the Triassic period. In this paper, Romer also provided a skeletal reconstruction of Gracilisuchus, due to his impression that nearly the entire skeleton was available. However, several elements had been erroneously referred by him. The lack of complete knowledge regarding its anatomy hampered subsequent studies of Gracilisuchus, until papers from Lecuona, Desojo, and Diego Pol in 2011 and 2017 redescribed its remains. The first of these focused on PVL 4597, while the second reviewed all of the known specimens.
Description
Gracilisuchus was a small member of the Pseudosuchia. The largest skull has a length of slightly over , and the largest femur has a length of roughly . In 1972, Alfred Romer estimated a total length of ; in 2014, Agustinia Lecuona and Julia Desojo estimated a length of for the body excluding the hip and tail. The weight of Gracilisuchus has been estimated at .