Thilastikosuchus is an extinct genus of candidodontid notosuchian from the Early Cretaceous Quiricó Formation of Brazil. It is regarded as the oldest known notosuchian from the country and possibly the oldest known notosuchian known from South America. Thilastikosuchus is most closely related to Lavocatchampsa, Pakasuchus and Malawisuchus. The genus is monotypic, only containing the species Thilastikosuchus scutorectangularis.
History and naming The genus Thilastikosuchus was described in 2025 on the basis of specimen FUP-Pv-000019, a well preserved articulated skeleton of a juvenile individual discovered in the Sanfranciscana Basin of Minas Gerais State, Brazil. The red sandstone where the fossil originates from is assigned to the Quiricó Formation and date to the Early Cretaceous, likely the Barremian to Aptian, making Thilastikosuchus one of the oldest known notosuchians of South America. The name Thilastikosuchus is a combination of the Greek "Thilastikó" meaning mammal and the widely used suffix "suchus" which means crocodile, with the name referencing the very mammal-like dentition displayed by candidodontid notosuchians. The species name scutorectangularis meanwhile alludes to the fact that the osteoderm scutes that covered the animals back were rectangular.
Description
Skull The holotype specimen of Thilastikosuchus is missing important parts of the skull such as the premaxillae, lacrimals, much of the prefrontals, palpebrals and palatines, but what remains clearly shows that the back of its skull was rectangular and leading into a pointed snout, which would give the complete skull a somewhat triangular form when viewed from above. The animals profile would have been somewhat elliptical, with the top of the head being flat and the ventral margin arched. In addition to the missing material, the fact that the holotype specimen belongs to a juvenile also impacts its appearance in ways that do not reflect the adult morphology, notably in the size of the eyes relative to the head. Given the absence of the premaxilla, most of the snout is preserved only through the maxilla, which forms an ascending and a ventrolateral process that frame the antorbital fenestra that sits between the maxilla and the missing lacrimal. The maxilla connects to the jugal, which forms the lower edge of the eyesocket, the inset postorbital bar and the lower edge of the triangular infratemporal fenestra. The paired nasals lie flat atop the head and what little remains of the prefrontals suggest that the rims of the orbits were slightly raised compared to the surface of the frontal bone. The frontal consists of an elongated process that contacts the nasals and prefrontals and a broader section that lies on the skull table where it reaches the elliptical supratemporal fossa. Within the fossa the frontal contacts the postorbital and the laterosphenoid and towards the center of the skull table it forms a V-shaped suture with the parietal bone. The postorbitals resemble inverted Ls and form the upper half of the postorbital bar. Rather than bearing a sagittal crest as in sphagesaurids, the skull table of Thilastikosuchus is characterized by the elongated posterior processes of the squamosal bones, which extend far beyond the occipital margin of the skull. The very back of the skull table prominently features a dorsally exposed supraoccipital bone, which is rectangular and effectively bars the parietal from reaching the back edge. This is similar to what is seen in Malawisuchus and noticeably more extensive than in Pakasuchus. The supraoccipital is overlapped by both the parietal and the squamosals where these elements meet, which makes this part of the bone appear depressed relative to the rest of the skull roof. On the occipital face, the back of the head, the supraoccipital appears T-shaped due to the presence of a large vertical bulge. This bulge is bordered by depressions and the supraoccipital extends down into the space between the exoccipitals, though it does not reach the foramen magnum. The otoccipitals lie lateral to the supraoccipital on the occipital face, form most the foramen magnum and a small part of the occipital condyle, which is mostly formed by the basioccipital.
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