Stolokrosuchus is an extinct genus of crocodyliforms found in the Lower Cretaceous Elrhaz Formation, Niger. It is known from a nearly complete skull displaying a narrow, highly elongated and tubular snout similar to modern gharials. The classification of Stolokrosuchus has been a matter of debate, it was originally considered to be a member of the family Peirosauridae, but later studies also recovered it as possibly closely related to mahajangasuchids or more recently as a member of the family Itasuchidae. While all of these placements would classify Stolokrosuchus as a notosuchian, other studies have proposed a closer relationship to Neosuchia, although these results have been considered to be possibly influenced by its elongated snout. Stolokrosuchus is thought to have been a semi-aquatic animal like modern crocodiles and may have fed on fish and other agile prey present in the waterways of the Paleo-Tegama River System. The type and only known species is S. lapparenti.
History and naming Stolokrosuchus was described in 2000 by Hans Larsson and Boubacar Gado on the basis of the holotype specimen MNN GDF600, an almost complete skull, alongside a right angular found alongside the skull and a left mandible found not far from the other material. The fossils come from the GAD 5 site of the Aptian to Albian Elrhaz Formation in Niger. The same site has later also yielded the remains of isolated teeth assigned to Stolokrosuchus.
The name Stolokrosuchus derives from the Greek African word "stolokros", which describes the undeveloped horns of juvenile goats that resemble the protuberances on the prefrontal of Stolokrosuchus. The second part of the genus name, "suchus", is a commonly used suffix among fossil crocodyliforms and derives from the ancient Egyptian god Sobek, typically depicted as a human with the head of a crocodile. The species name "lapparenti" comes from French paleontologist Albert-Félix de Lapparent, who was chosen as the species' namesake for his work on Cretaceous vertebrates from West Africa.
Description The skull of Stolokrosuchus is triangular in dorsal view, tubular and longirostrine, meaning it is both highly elongated and narrow, with a smooth transition from the rostrum to the temporal region. Additionally the skull is described as growing gradually taller towards the back of the head.
The nares face anterodorsolaterally, opening simultaneously somewhat forward, upward and towards the sides; and are divided by a thin septum formed by the paired nasal bones that extend forward and join with a posterior projection of the premaxillae to form a septum. The nares are furthermore surrouned by an anterodorsdal perinarial fossa visible from the side, though unlike in Pepesuchus the fossa lacks a bulge. While the premaxillae extend back by a fair distance, the majority of the rostrum's length is composed of the maxillae. Looking at the skull from above shows a marked transition between the two elements, with the premaxilla expanding prominently before leading into the much narrower anterior maxillary contribution to the rostrum. Pinheiro and colleagues describe ventral margins as remaining straight throughout the rostral length, setting Stolokrosuchus apart from most peirosaurids and itasuchids as well as mahajangasuchids and sebecids, though Geroto and Bertini note it to be slightly festooned. The nasals also contribute significantly to the rostrum, extending along the length and forming the dorsal surface until their contact with the lacrimal, prefrontal and frontal bone. Despite their notable contribution to the rostrum in top view, the almost uniformly wide nasals are barely visible when looking at the skull from the side. While many itasuchids have a nasal that is poseriorly split into distinct processes by the frontal, this is not the case in Stolokrosuchus, in which the posterior edge of the nasals is described as straight. Similarly, the nasals do not form posterior processes that extend into the space between the prefrontal and lacrimals
The narrow prefrontals extend beyond the anterior tip of the frontal and each bears a small protuberance that gives Stolokrosuchus its name, located just anterior to the dorsolaterally oriented eyesockets. Stolokrosuchus preserves a small, slit-like antorbital fenestra that is primarily formed by the lacrimal and surrounded by a shallow and smooth-surfaced fossa. The distance between the antorbital fenestra and the eyesocket is noted as being longer than in other peirosaurids, a feature shared with Epoidesuchus and Hamadasuchus. Only the anterior-most margin of the fenestra is not formed by the lacrimal and instead composed of the maxilla, which has a broad contact with the lacrimal. The flat skull table bears a pair of small subcircular supratemporal fenestra, with the contact between frontal and parietal bone being located at the anterior edge of these openings. The posterior edge of the parietal is described as depressed like in Hamadasuchus and Rukwasuchus. The postorbitals form the anterior edges of the skull table while the squamosals form the posterior corner, though the latter extend beneath the dorsal surface of the postorbitals until almost reaching the orbital margin. The supraoccipital bone is not exposed on the dorsal surface of the skull table alone.
The postorbital also contributes to the robust and cylindrical or rod-shaped postorbital bar together with the jugal, which separates the eye socket from the subtriangular infratemporal fenestra. The jugal's ascending process overlaps the descending process of the postorbital bone on the bar and the entire element is slightly inset relative to the lateral surfaces of its contributing bones. Like in Pepesuchus and some neosuchians the jugal is noted for its rostral extent, with its front-most tip extending further anterior than the prefrontal bone, and even surpassing the anterior end of the antorbital fenestra. The anterior ramus broadens into a rhomboid shape beginning just lateral to the postorbital bar.
The anatomy of the meatal chamber, which together with the overlying earflaps forms the outer ear in crocodyliforms, corresponds to the "basal notosuchian/sebecian pattern" established by Montefeltro and colleagues. This morphology is widespread among basal metasuchians and forms the basis of the ear anatomy also seen in modern crocodilians. Stolokrosuchus, like Pepesuchus, has been noted for possessing a less dorsoventrally expanded meatal chamber due to the reduced descending process of the postorbital and the reduced anterolateral process of the quadratojugal while the squamosal, due to its greater anterior extent, reaches the anterior edge of the chamber. While both taxa retain the bony otic aperture, one of two structures present within the chamber, both Stolokrosuchus and Pepesuchus lack the subtympanic foramen like is also the case in modern gharials but not in other extant crocodilians.
The craniomandibular joint is formed not only by the quadrate bones but also by the quadratojugals, though the latter's contribution, like in most itasuchids, is comparably minute. The quadrate bone bears a robust dorsal projection between its hemicondyles and is firmly sutured to the braincase and the pterygoid bone but not the laterosphenoid. The tightly sutured braincase features a vertically expanded parasphenoid that is confluent with the pterygoids and a basisphenoid that is almost invisible in ventral view as it is obscured by both the pterygoids and the basioccipital. The otoccipitals are large and short and participate in closing the craniquadrate canal against the quadrates to their sides and form large bosses located towards the basioccipital tubera.
Ventrally the incisive foramen is described as elongated and its anterior edge contacts the toothrow. The opening is mostly surrounded by the ventral surface of the premaxillae except for the posterior-most edge, which is formed by an anterior process of the maxillae that divides the premaxilla. and overlaps the vomer. The palatine bones in turn form long and pointed processes that extend into the space between the ventral surfaces of the maxillae and posteriorly form the space between the two suborbital fenestrae. The descending process of the prefrontal is also noted to come into firm contact with the palatines. The posterior margin of the choanae is formed by a deep vertical pocket formed by the fused pterygoid bones which furthermore form a thin lamina that bisects the choanal opening. The pterygoid extends ventrally to form a tall shelf alongside the basisphenoid, which comes into contact with the foramen for the median eustachian canal. The ectopterygoid contacts the postorbital bar laterally and the maxilla anteriorly, though the latter contact sits well behind the end of the toothrow. The ventral surface of the quadrate bears two elongated crests regarded to be attachment sites for the adductor musculature, designated crests A and B. Crest A runs parallel to the lateral edge of the quadrate from the ventral margin of the adductor chamber to the midpoint of the bone, while crest B is located medially and continues on from a dorsal process of the pterygoid onto the quadrate's pterygoid process. The presence and relationship between these crests is also seen in other peirosaurids as well as Araripesuchus.
Mandible The flattened mandibular symphysis of Stolokrosuchus is narrow to match the narrow elongated rostrum, lacks a constriction and is formed by both the dentary bones and the splenials, which meet along a V-shaped suture. The dentaries and splenials remain in contact across the mandibular ramus, but their suture only runs parallel to the toothrow at the level of the 25th dentary tooth. The splenial's contribution to the width of the mandibular ramus is comparable to taxa like Gasparinisuchus and Barrosasuchus, making up about 25-30 % of the mandible's entire mediolateral width. Like in numerous peirosaurids Stolokrosuchus preserves an elongated groove that runs parallel to the toothrow along the outer surface of the mandible. Among peirosaurds there is some variation regarding whether or not the space between this groove and the toothrow shares the same sculpting as the remainder of the mandible. While the region is smooth in Stolokrosuchus like in Pepesuchus and Hamadasuchus, among others, it has been noted that the remainder of the mandible is likewise unornamented The dorsal surface of the mandible is straight rather than festooned, meaning that the lower jaw of Stolokrosuchus lacked the wave-like or sinusoidal profile seen in most peirosaurids.
Dentition The upper jaw of Stolokrosuchus contains a total of 30 teeth on either side, 5 in each premaxilla and an additional 25 in each maxilla. The first two premaxillary teeth are small and closely spaced to the point of almost being confluent while the subsequent three teeth are larger and better separated from each other. Overall these teeth follow the curvature of the premaxilla and are not positioned in a straight line like the first few premaxillary teeth of the neosuchian Elosuchus. The first two teeth of the maxilla project slightly forward and after this point the toothrow exhibits slight festooning, with the maxillary teeth reaching their greatest size with the fifth and 13th teeth.