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 .
Skull
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 scleral 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.
Behind the eye socket, the vertical process of the jugal bone is uniquely straight. The backward process of the jugal is located underneath the forward process of the quadratojugal bone, convergent upon Erpetosuchus, Postosuchus, Polonosuchus, and the Crocodylomorpha, unlike Turfanosuchus and Yonghesuchus where they are the other way around. Furthermore, a postfrontal bone is present, with an outer process that is uniquely long and extends over the back of the eye socket. There is also a postparietal bone, which is small and triangular. At the back of the skull, the posttemporal foramen is large relative to the skull's width. Also unlike Turfanosuchus and Yonghesuchus, Gracilisuchus has four teeth in the premaxilla instead of five, like Prestosuchus, Saurosuchus, Fasolasuchus, Batrachotomus, the Rauisuchidae, and the Crocodylomorpha. There is no cutting edge, or carina, at the front of the premaxillary teeth, and they lack serrated denticles on either the front or rear edges.
Vertebrae There are eight cervical vertebrae. Unlike Turfanosuchus, Euparkeria, Fasolasuchus, Saurosuchus, and sphenosuchians, the suture between the centrum and neural arch of the axis (second cervical) bears an unrounded, triangular upward projection. There is a long, narrow longitudinal keel on the bottom of the axis, which is also seen in Riojasuchus, Saurosuchus, the aetosaur Stagonolepis, and phytosaurs. The front border of the neural spine is uniquely high and vertical, while the rear border is concave like Turfanosuchus but unlike Erpetosuchus. Similar to both, the articular process known as the postzygapophysis projects over the back of the centrum, but it is uniquely in the horizontal plane. The remaining cervicals have poorly-developed keels on their bottom surfaces, in contrast to Erpetosuchus, Nundasuchus, aetosaurs, Saurosuchus, and Riojasuchus. The sides of their centra have long depressions, like Turfanosuchus, aetosaurs, Batrachotomus, and Ticinosuchus.
In at least the fourth, sixth, and seventh cervicals, there are narrow, rounded "tables" at the base of the neural spines, like Turfanosuchus and Euparkeria. There are circular depressions at the front of the neural spines, above the neural arches, a trait shared with Turfanosuchus. The postzygapophyses are located at the same level as another set of processes, the prezygapophyses, like Turfanosuchus, Erpetosuchus, and Ornithosuchus. The top margin of the postzygapophyses are convex like Turfanosuchus. There are depressions below the postzygapophyses, which are not seen in any other archosaurs except for Stagonosuchus and Batrachotomus, where they are deeper. Another set of processes known as the parapophyses extend backwards onto longitudinal ridges, like Nundasuchus, Batrachotomus, and Postosuchus.
There are sixteen dorsal vertebrae. Like Parringtonia, Nundasuchus, and other archosaurs, the articulating surfaces of their centra are flat. The keels on their bottom surfaces are again very weak, which is unlike Riojasuchus, Erpetosuchus, Parringtonia, and Nundasuchus, but like aetosaurs. There are no "tables" on the neural spines either, unlike Turfanosuchus, Erpetosuchus, Parringtonia, aetosaurs, Nundasuchus, and Ticinosuchus. Like Turfanosuchus, Parringtonia, and Nundasuchus, the prezygapophyses are located at the same level as the top of the front of some of the centra. Some of the sideways-projecting transverse processes have somewhat deep cavities underneath, like Turfanosuchus, but they are more poorly developed than Nundasuchus, Batrachotomus, or Stagonosuchus.
Two sacral vertebrae are known. The transverse processes of the first sacral, which are nearly fused to the ribs, are separated from the neural spines by two pairs of grooves which form an acute angle. The second sacral's neural arch also bears a long depression on its top surface. The appearance of these characteristics are more similar to Turfanosuchus than Nundasuchus. Uniquely, the outer edges of the first pair of sacral ribs are longer than their inner edges. The second pair has a more prominent expansion, also seen in Turfanosuchus, Euparkeria, and Saurosuchus. The two pairs did not contact each other, like Euparkeria. There were at least 16 caudal (tail) vertebrae. Their neural spines do not have accessory processes at their front margins, like Turfanosuchus and Euparkeria but unlike other basal archosaurs. They also lack "tables", unlike Turfanosuchus and Parringtonia.
Limbs In the shoulder girdle, the end of the scapular blade is widely and asymmetrically expanded, unlike Turfanosuchus, Batrachotomus, and Ticinosuchus, but similar to the crocodylomorph Dromicosuchus. Unlike Turfanosuchus but like Terrestrisuchus and Dibothrosuchus, the articulation with the radius on the humerus is narrower than the one with the ulna. The width of the bottom end of the humerus is 2.5 times that of the shaft, like Batrachotomus (2.5), Ticinosuchus (2.7), Postosuchus (2.4), and Terrestrisuchus (2.4), but smaller than Turfanosuchus (3.75).
In the pelvis, the ilium has a weakly expanded frontal process, like Turfanosuchus, Euparkeria, Postosuchus, and Caiman. The front of the articulation with the pubis reaches further than the process, like Turfanosuchus, Euparkeria, Saurosuchus, Postosuchus, and Lagerpeton. Meanwhile, the rear process is long, like Turfanosuchus and Postosuchus. There is a shelf along the bottom of this process, most similar to those of Terrestrisuchus, Dromicosuchus, and Marasuchus. The sacral ribs articulate at this shelf, unlike Turfanosuchus where the shelf is located above. The acetabulum, or hip socket, of Gracilisuchus was larger than other archosauriforms. Like Turfanosuchus, Marasuchus, Lagerpeton, and other archosaurs, the acetabulum does not bear a perforation, and there is a buttress above the acetabulum for the femur.
Unusually, but like Tropidosuchus, Protosuchus, and Orthosuchus, the articulation with the ilium on the pubis is short. There is a small bony tongue projecting downwards from this articulation, which has only been recognized in Postosuchus. There is no prominent surface of the acetabulum on the pubis, nor is there a visible articulation with the ischium; the former is similar to Fasolasuchus and Orthosuchus, and the latter is similar to living crocodilians. An L-shaped lamina is present on the rear surface of the "apron" of the pubis. The articulation between the two halves of the ischium is characteristically close to the top of the bone, with the separated portion being only 22% the length of the bone; Ornithosuchus may have a similar condition.