Gracilisuchus (meaning "slender crocodile")[a] is an extinct genus of tiny pseudosuchian (a group which includes the ancestors of crocodilians) from the Late Triassic of Argentina.

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.

[1] 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.

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.

The lack of complete knowledge regarding its anatomy hampered subsequent studies of Gracilisuchus, until papers from Lecuona, Desojo, and Diego Pol in 2011[3] and 2017 redescribed its remains.

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.

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.

[1] 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.

On the bottom end, the groove separating the articulations with the tibia and fibula is shallow, like Turfanosuchus, Euparkeria, Tropidosuchus, Riojasuchus, Marasuchus, and Lagerpeton.

[1] Romer considered Gracilisuchus to be, "quite clear[ly]", a relative of the Scottish Ornithosuchus, owing to similarities in skull structure and other skeletal features.

Walker considered ornithosuchids to belong to the latter group, due to strong morphological similarities between the limb girdles of Ornithosuchus, Albertosaurus, Gorgosaurus, and Antrodemus (=Allosaurus).

By the time of Romer's description of Gracilisuchus in 1972, the geographic range of ornithosuchids had expanded to include the Argentinian Venaticosuchus and Riojasuchus, which had been referred to the family by Bonaparte in 1969.

Romer noted that Gracilisuchus was the smallest and oldest known member of the group to date, and accordingly had a fairly basal morphology (notwithstanding supposedly aberrant traits such as the partial closure of the infratemporal fenestra).

However, he had reservations regarding Walker's identification of ornithosuchids as dinosaurs, noting basal archosaur traits such as the closed acetabulum, osteoderms, and crocodile-normal ankle.

In 1979, Arthur Cruickshank separated pseudosuchians ("crocodile-line" archosaurs) into two groups based on whether they bore "crocodile-normal" or "crocodile-reversed" (where the peg and socket are located on the opposite bones) ankles.

It was differentiated from the latter two by the presence of postparietals and the absence of: a pit between the basioccipital and basisphenoid bones; fusion between the atlas (first cervical) and the intercentrum, an element below the axis; accessory processes on the caudal neural spines; and osteoderms on the bottom of the tail.

In 1990, Paul Sereno and Andrea Arcucci suggested that ornithosuchids - in which they included Gracilisuchus - were actually closer to the conventional "crocodile-line" archosaurs than dinosaurs.

In a 2000 description of North American material, Paul Olsen, Hans-Dieter Sues, and Mark Norell recovered Gracilisuchus as more derived than Stagonolepis but more basal than Postosuchus, Erpetosuchus, and crocodylomorphs.

In both cases, it was more basal than the same group in Olsen and colleagues' analysis, being united by a ridge on the squamosal bone above the supratemporal fenestra and the absence of a foramen on the quadrate.

They formed a polytomy with Ornithosuchidae, which was in a basal position relative to the Suchia (defined to include Stagonolepididae, Postosuchus, and Crocodylomorpha), Fasolasuchus, and the Prestosuchidae.

In their own analysis, Gracilisuchus was the sister taxon of a group containing Erpetosuchus and the Crocodylomorpha, which along with the Aetosauria (by then renamed from the Stagonolepididae) formed one branch of the Suchia.

Although they found strong support for this grouping in the form of eight synapomorphies (shared traits), with two of them (involving the ossification and position of the perilymphatic foramen of the braincase) being unambiguous, later assessment noted that this may have resulted from poor non-rauisuchian sampling.

Lecuona and Desojo also noted that the poor development of the fourth trochanter and femoral head was shared with members of the Sphenosuchia, which allowed for the possibility that they formed a monophyletic group.

[3] In 2014, an analysis led by Richard Butler, modified from that of Nesbitt, suggested for the first time that Gracilisuchus formed a group with Turfanosuchus and Yonghesuchus, two basal suchians with similarly convoluted taxonomic histories.

Strong support was obtained for the Gracilisuchidae in the form of six unambiguous synapomorphies: a process on the back of the premaxilla that fits into a slot on outer surface of the nasal bone; the nasal bordering the top of the antorbital fenestra; the tapering frontal bone; the presence of a depression on the bottom of the calcaneal tuber; the osteoderms bending downwards at their outer edges; and the presence of a triangular process bearing a clear apex on the maxilla.

[5] 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.

Lecuona and colleagues added two synapomorphies of Gracilisuchidae to those listed by Butler: the absence of the jugal's contribution to the postorbital bar behind the eye socket, and the articulations with the fibula and astragalus forming a continuous structure on the calcaneum.

Although the hand is not preserved in Gracilisuchus, Romer noted that its forelimbs were three-fifths the lengths of the hindlimbs, like ornithosuchids; however, this interpretation was based on material which has since been reassigned.

It consists of badlands at the base of a latitudinal escarpment, with the exposed rocks being composed of feldspar and quartz grains, along with glass shards, embedded in a geology silica and clay.

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.