Rukwasuchus is an extinct genus of notosuchian crocodyliforms known from the Late Cretaceous Galula Formation of southwestern Tanzania. Only a limited number of fossils are known, including parts of the skull and a few isolated teeth, however the preservation of the holotype meant that the endocast could be described in detail. Phylogenetic analysis generally recover Rukwasuchus as being a member of the clade Peirosauria, either within Peirosauridae as originally proposed or more recently as a basal member of the family Itasuchidae, the members of which are sometimes also placed within Peirosauridae as the subfamily Pepesuchinae. Rukwasuchus inhabitet what is now Tanzania during the Cenomanian to Campanian, at which point the region was covered by an extensive braided river system. While several purportedly closely related forms have been interpreted as semi-aquatic, some features of the brain of Rukwasuchus may indicate more terrestrial habits. The genus is monotypic, it contains only a single species, Rukwasuchus yajabalijekundu.
History and naming Rukwasuchus is known from its holotype, RRBP 08630, a well-preserved rear part of the skull including the cranial table, braincase, and interorbital region lacking the rostrum, the front portion of the palate, both lacrimals, jugals, and quadratojugals, as well as the mandible. RRBP 08630 was collected during 2008 at Namba 2 locality (also known as RRBP 2007-02), together with the titanosaurian Rukwatitan bisepultus which is exclusive to this locality. Material referred to Rukwasuchus includes four isolated teeth, which came from the neighboring localities RRBP 2007-01 yielding the 3 teeth RRBP 07351, 07369, 09362, and RRBP 2009-01 yielding the tooth RRBP 09367. All specimens came from approximately 25 km south of Lake Rukwa in the Galula Study Area, Rukwa Rift Basin of southwestern Tanzania, belonging to the Namba Member of the Galula Formation, which though initially interpreted as being late Aptian to early Cenomanian in age has more recently been regarded as dating to the Cenomanian to Campanian. Rukwasuchus was named by Joseph J. W. Sertich and Patrick M. O’Connor in 2014 and the type species is Rukwasuchus yajabalijekundu.
The generic name refers to Lake Rukwa and the Rukwa Rift Basin, located in southwestern Tanzania, where the holotype of Rukwasuchus and other vertebrates were collected by the Rukwa Rift Basin Project, and suchus, Latinized from the Greek souchos, an Egyptian crocodile god. The specific name yajabalijekundu is derived from the Swahili language meaning "of/from the red outcrop", in reference to the Red Sandstone Group deposits exposed at the basin.
Description Rukwasuchus is only known from comparably limited material consisting of the skull table, orbital region and braincase but lacking most of the rostrum, dermatocranial material and preserving no postcrania. The skull of Rukwasuchus has been described as taller and narrower than that of Stolokrosuchus, an animal from the Cretaceous of Niger with an extremely elongated snout, and has further been noted as most closely resembling that of Hamadasuchus. A morphometric analysis later suggested that there is a clear distinction between longirostrine peirosaurians like Roxochampsa varyingly called itasuchids or pepesuchines and the oreinorostrine peirosaurines such as Montealtosuchus. However, some forms, including Rukwasuchus, do fall within the range of variation of other notosuchians as a whole.
The prefrontal is elongated and its surface is covered by rounded crenulations and pits. Due to the broken nature of the holotype skull little is known about its relationship with other bones other than that each prefrontal contacts the anterior process of the frontal medially along a mostly straight suture. The prefrontal possesses a groove that extends into a distinct shelf that likely would have also continued onto the lacrimal bone to serve as a point of articulation for a palpebral bone stretching over the eye socket. The prefrontal's contribution to the orbital margin is elevated, with said elevation continuing onto the frontal and eventually postorbital bone to give the eye socket a moderately raised rim. The frontal itself consists of two parts, the elongated anterior process sandwiched between the prefrontals, extending approximately as far forward as these bones, and the wider posterior section located on the skull table. There the frontal forms part of the supratemporal fossae, contacting the parietal bone along a straight tranverse suture between the fossa and the postorbitals along a relatively straight, raised suture laterally.
The postorbitals from the anterior corners of the trapezoidal skull table and can broadly be divided into an anterolateral and posterior process, which form the dorsal surface, and a descending process. The anterolateral process is described as moderately depressed, comes into contact with the frontal along a relatively raised peak and forms the anterior border of the supratemporal fenestra as well as the wall of the supratemporal fossa. The fenestrae are described as ellipsoidal in overall shape, which sets them apart from the almost circular openings in the skull of Stolokrosuchus. The posterior process of the postorbital forms the lateral margin of the skull table until roughly the halfway point of the fenestra, where it meets the squamosal along a suture that initially begins transversely at the edge of the skull opening but then curves forward towards the edge of the skull table. The meeting point between anterolateral and posterior processes is marked by the presence of an anterodorsally directed depression just above the descending process, likely the posterior articular facet for the posterior palpebral. The squamosal then form the posterior corner of the skull table, the dorsal surface divided into an anterior, posterior and medial process. The anterior process is the part of the squamosal that comes into contact with the postorbital along the dorsally medially, then anterolateral suture, however in truth the squamosal extends much further, underlying the postorbital and even reaching as far as the postorbital bar. The posterior process is subtriangular and extends backwards beyond the occipital surface, ending in a rounded slightly upturned point. Finally, the medial process is the smallest of the three and comes into contact with the parietal. The squamosal also forms a portion of the posterior wall of the supratemporal fossa. The parietal forms the posterior and central parts of the skull table, with the anterior and posterior ends wider than the narrow section sandwiched between the fenestrae. It has been described as bearing both ridges towards its lateral margins as well as a sagittal crest. The parietal is widest at the back of the skull, where it is notably depressed as in some other African peirosaurians and overlies the supraoccipital entirely.
The postorbital also contributes to the side of the skull through its descending process, which itself bifurcates into two processes. Part of it joins an ascending process of the jugal to form the postorbital bar, which separates the orbit from the infratemporal fenestra. Like in other crocodyliforms the postorbital bar bears a vascular opening, however in the case of Rukwasuchus its located posterolaterally, which is otherwise only seen in Epoidesuchus and Pepesuchus. The other segment of the descending process is a well developed posteroventral process that forms the upper edge of the infratemporal fenestra, overlays part of the quadrate bone and would have likely ended as it contacted the quadratojugal. Also visible laterally beneath the skull table is a robust anterior descending lamina of the squamosal and the bony optic aperture, which is described as irregularily shaped with an oval, dorsally opening posterior section. Above otic aperture the lamina flares as it contacts both the anterior process of the quadrate and the postorbital's posteroventral process. Together the posteroventral postorbital and the anterodorsal process of both quadrate and quadratojugal form the anterior edge of the meatal chamber via a well developed concave surface, which differs strongly from the much less well developed surface seen in longirostrine peirosaurians like Pepesuchus and Stolokrosuchus. The optic aperture is overhung by the edge of the squamosal′s dorsal surface, which forms a deep optic recess. Unlike in derived neosuchians including today's crocodiles the recess is not closed posteriorly but instead opens ventrally.
As typical for crocodyliforms the quadrate consists of an anterodorsal region and a main body. The former extends anterior to contact both the postorbital and squamosal and forms both the anterior and ventral edges of the optic aperture. The main body is robust, elongated and featuring two hemicondyles, one laterally and one medially. While the quadratojugal itself is not preserved, the edges of the quadrate indicate that the two were in contact onward from the postorbital and that the quadratojugal participated in forming the articular surface of the lateral hemicondyle. Dorsally the main body of the quadrate contacts the squamosal via a not very tall but long dorsal process that effectively creates significant distance between the optic aperture and the back of the skull. The dorsal process is separated from the medial hemicondyle by a low yet prominent crest that splits the quadrate, when viewed from behind, into a posterodorsal and a posteromedial surface, the latter of which bears both the foramen aereum and contributes to the cranioquadrate passage. Also in posterior view the quadrates are subrectangular in shape and jut out from the rest of the skull outward and downward. The medial hemicondyle is described as larger with a sharply pointed ventromedial (lower and inner) edge and a main axis that is oriented from this corner dorsolaterally towards the upper and outer corner. The lateral condyle on the other hand is smaller and more rounded with an axis oriented straight down.