Sissokosuchus is an extinct genus of itasuchid notosuchian from the Early Cretaceous Continental intercalaire of Mali. Like other itasuchids, Sissokosuchus had relatively narrow jaws with numerous partially interlocking teeth, somewhat resembling modern crocodilians moreso than the oreinorostrine peirosaurids it's most closely related to. It has therefore been speculated that it may have been semi-aquatic, an idea that matches the fact that the Continental intercalaire has been interpreted as a downstream part of what is dubbed the Paleo-Tegama river system, which also included the fluvial systems of what is now Niger. Sissokosuchus is among the few itasuchids known from Africa and the first deeply nested members from the continent, though the exact implications of this remain somewhat poorly understood. The genus is monotypic, meaning it only includes a single species: S. maliensis.
History and naming The fossil remains of Sissokosuchus were discovered in 1999 in the poorly defined sediments of the "Continental intercalaire" of Mali, an informal name for a series of rock layers spanning the Carboniferous to Cenomanian. The specific layers where the fossils originates from have been correlated to other western African sediments that suggest a rough Aptian to Albian age. The holotype is one of several specimens found spread across a 10 square meter area and the most complete, consisting of a partial dentary tip, parts of the maxilla and several cranial elements that are almost articulated with each other. Some postcranial material, such as vertebrae, osteoderms and limb bones are also part of the holotype.
The name Sissokosuchus is an eponym, combining the last name of Dr. Famory Sissoko with the Greek word "souchos" (meaning crocodile after the Egyptian deity Sobek). The species name on the other hand is a reference to the Republic of Mali.
Description The maxilla and its toothrow flare outward following the third maxillary tooth, suggesting that Sissokosuchus had an overall broadly triangular head-shape. Towards the back the maxilla comes into contact with the jugal along a crescent-shaped suture, with the maxilla bearing several prominent neurovascular foramina just before the contact. Closer towards the midline of the skull the maxilla forms a so-called posterodorsal process, an extension of the bone that inserts itself in-between the jugal and the lacrimal bone. Notably, this process preserves part of a shallow depression that could be interpreted as part of the antorbital fossa. However, the region is poorly preserved and the presence of such an opening is inconsistent among members of Itasuchidae, being present in some taxa and absent in others. Therefore whether or not Sissokosuchus had an antorbital fenestra is not confirmed. Even closer to the midline the maxilla forms another tapering process, this time inserting itself into the space between the lacrimal and the nasal bones and even coming into contact with the prefrontal bone. Like in Barreirosuchus, this effectively prevents the lacrimal and nasal from coming into contact with one-another.
Like so much of the skull, only few parts of the nasals are preserved, among them what might be the contact with the maxilla, which appears to have been following a straight suture. More concretely understood is the contact between the nasal and the prefrontal. The prefrontal forms a prominent anterior process that essentially splits the nasal into a posterolateral and a posteromedial process. As the name suggests, the posterolateral process is oriented towards the outside of the skull, forming a narrow tapering point that extends around the outer edge of the prefrontal and is just barely prevented from contacting the lacrimal by the posterodorsal maxillary process. The posteromedial process on the other hand is broader, directed inward and though it likely would have contacted the frontal bone, damage to this part of the skull means its true extent is unclear.
The eyesockets were circular and formed by the combined lacrimal, prefrontal, frontal, postorbital and jugal bones all contributing to its margins. Though no palpebral bones were discovered, they were clearly present as evidenced by the fact that the bones surrounding the eyesocket still preserve points of articulation for them. The anterior articular facet is a semicircular depression spanning the prefrontal and the lacrimal while the posterior facet is located on the postorbital. The space between the eyesockets is composed of both the rectangular prefrontals and the singular frontal bone, divided into a narrower anterior section and a wider posterior region that forms part of the skull table and connects with the postorbitals. The contact between frontal and prefrontal is visible as a square suture that emerges from the edge of the orbit and runs transversely to the skull before taking a turn, extending forward nearly parallel to the midline to form the anterior frontal process.
The most significant part of the lower orbital margin is composed of the jugal, which can be split into three sections. The anterior process extends anterior to the maxilla and bears a trapezoid depression just below its contact with the lacrimal. This depression is split into two halves by a ridge descending from its upper margin and gradually decreasing in prominence until disappearing around halway through the depression. Towards the bottom the jugal bears a line of pits that stretches from the jugal depression, runs below the orbit and eventually ends just beneath the postorbital bar. The second major section of the jugal is the ascending process, an inset peg that rises up and joins the descending process of the postorbital to form the postorbital bar just behind the eyesocket. Like in Barreirosuchus, the postorbital bar is described as robust. Finally, the posterior jugal process is a downwards sloping and narrow element that forms much of the lower edge of the infratemporal fenestra. While already being very narrow, it is also mediolaterally compressed, meaning it is even thinner than it is tall.
The jugal is directly followed by the quadratojugal, which ascends and forms a slender process that formsthe posterior edge of the infratemporal fenestra. Most notably, the quadratojugal of Sissokosuchus contributes heavily to the formation of the craniomandibular joint, making up 20% of the joint surface of the lateral condyle. While the quadratojugal also contributes in this way in some other itasuchids, Araripesuchus and certain sebecids, none of these have as much quadratojugal participation as Sissokosuchus. The remaining 80% of the condylar surface are formed by the quadrate, including the entire medial condyle. Looking at the skull from behind shows that the surface is oval and much wider than high (the height is only about 25% of its width), with the two condyles being separated by a shallow depression.
Mandible A feature often accepted to distinguish itasuchids from peirosaurids is the shape of the mandible, which is generally described as broadly U-shaped in the former and much narrower in the latter. The mandibular symphysis appears to have been relatively straight, with the toothrows running almost parallel to each other rather than diverging from each other towards the back. The mandibular symphysis is the fused anterior region of the lower jaw and formed by the dentary and the splenial bones. The former meet along the midline of the lower jaw and remain connected until the position of the sixth dentary tooth, at which point the splenials form a tapering wedge that inserts itself between and separate the dentaries. With the splenials initially also being fused along the midline, the symphysis continues on until the 11th dentary tooth, at which point the two halves of the mandible split from each other to form the mandibular rami. While the lower surface of the dentary is ornamented by a series of deep furrows and pits, Wilberg and colleagues note that the outer surface of the splenial is smooth with the exception of three major features. These are an elongated pit located right down the midline and two step-like depressions located between the midline pit and the end of the symphysis. Looking at the mandibular symphysis from the side shows that its rather uniform in height, only tapering towards the very tip of the snout.
Dentition Given that Sissokosuchus is only known from fragmentary remains, the exact number of teeth is unknown. No premaxilla has been recovered and what is preserved of the maxilla suggests a presence of at least 13 teeth (11 alveoli starting with the third). The dentary is equally poorly understood, but appears to have contained 11 teeth on the mandibular symphysis with an unknown number of teeth behind these.
While the anterior alveoli of the maxilla are placed close to the outer edge of the bone, further towards the back the tooth sockets are inset further medially with a lateral lip running along their outer edge. Such inset posterior maxillary teeth are unique within Itasuchidae, nor are they found in peirosaurids or even mahajangasuchids, with the only exception possibly being a single specimen referred to Pepesuchus Overall the toothrow appears to flare outward following the enlarged third maxillary tooth, with the following fourth maxillary tooth slightly more laterally and the remaining teeth all positioned in a more or less straight line.
Both the upper and lower jaw show signs of reception pits, marks that indicate where the teeth of the opposing jaw came into contact with the bone. Within the upper jaw, several reception pits are clearly visible more medially to the maxillary teeth and especially prominent from the fourth to eight maxillary teeth. Here, their position suggests that the maxillary and dentary teeth might have interlocked, though the dentary teeth would have still been positioned somewhat more medially than those of the upper jaw. Meanwhile, a lateral concavity in the lower jaw, located between the seventh and eight dentary teeth, suggests that an enlarged maxillary tooth broke up the toothrow of the lower jaw. Such an enlarged tooth is actually a common feature not just in itasuchids but also in peirosaurids and mahajangasuchids.
While preservation makes observation hard, the orientation of the alveolus suggests that the first tooth in the lower jaw of Sissokosuchus was directed forward, with the second dentary tooth likewise probably having been at least slightly procument. The fourth alveolus is described as being notably larger than the rest, around twice as wide as the surrounding alveoli and slightly elevated above the rest. The presence of reception pits and gaps in the toothrow becomes especially important in the lower jaw, where spacing and tooth size are a key feature that sets itasuchids apart from peirosaurids. For instance, the large fourth dentary alveolus is separated from the fifth by a small but distinct diastema, and yet another is present at the lateral concavity following the seventh dentary alveolus. Whats even more distinct is the arrangement and size of the alveoli in this region. The two alveoli right before the diastema, six and seven, are confluent with each other and the latter is only half the diameter of the former. The reverse is true for alveoli eight and nine, in which the former is smaller and the latter larger, though both are also confluent with each other. This essentially means that the diastema is surrounded by two alveolar couplets, with the alveoli immediately adjacent to the gap being smaller than the outer ones. While the overall pattern of differing spacing and variation in tooth size is common in itasuchids, the particular coupling of alveoli is unique to Sissokosuchus.