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 dates to the late Aptian or possibly early Cenomanian stage of the middle Cretaceous, approximately 100 mya. Rukwasuchus was named by Joseph J. W. Sertich and Patrick M. O’Connor in 2014 and the type species is Rukwasuchus yajabalijekundu. [1]
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. [1]
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 possesses several autapomorphies which distinguished it from other crocodyliforms. These include the presence of a mediolaterally narrow, elongate, and septate internal narial fenestra located anteriorly on the pterygoid, the presence of a markedly depressed posterior border of the parietal that excludes the supraoccipital from the dorsal cranial table, and a ventrally directed descending process of the postorbital with a well-developed posteroventral process. [1]
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 eyesocket. The prefrontal's contribution to the orbital margin is elevated, with said elevation continuing ont the frontal and eventually postorbital bone to give the eyesocket a moderately raised rim. The frontal itself cosists of two parts, the longated anterior process sandwhiched between the prefrontals 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 raised suture laterally. [1]
The postorbitals from the anterior corners of the 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 and forms the anterior border of the supratemporal fenestra as well as the wall of the supratemporal fossa. The posterior process 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 tranversely 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 sandwhiched between the fenestrae. The parietal is widest at the back of the skull, where it is notably depressed and overlies the supraoccipital entirely. [1]
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. 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. Above the irregularily-shaped otic aperture the lamina flares as it contacts both the anterior process of the quadrate and the postorbital's posteroventral process. At the same time the otic aperture is overhung by the edge of the squamosal′s dorsal surface, which forms a deep otic recess. Unlike in derived neosuchians including today's crocodiles the recess is not closed posteriorly but instead opens ventrally. [1]
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 otic 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 significanta distance between the otic 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. [1]
The occipital region is well preserved in the holotype of Rukwasuchus. The supraoccipital is exclusively visible in posterior view as it is fully overlain by the parietal on the skull table, which furthermore forms a ventral deflection that gives this bone a pronounced U-shape. To either side of the indented upper edge lie the postoccipital processes, which are underlain by a raised, rounded ridge and themselves lie medial to the posttemporal fenestrae. Ventrally the supraoccipital bears a prominent bulge flanked by depressions, but the bone does not contact the foramen magnum due to the fact that the otoccipitals contact each other along the midline. The otoccipitals dominate the occipital surface and form most of the rim of the foramen magnum. The occipital condyle is missing, but the otoccipitals likely formed the elements′ dorsolateral edges while the rest was probably formed by the basioccipital. [1]
Dentition
While the holotype of Rukwasuchus preserves neither teeth nor alveoli, several teeth from the Galula Formation have nonetheless been tentatively referred to this taxon. The teeth are described as subconical with a weak construction between root and crown and moderate labiolingual (side-to-side) compression. The mesial and distal cutting edges or carinae of the teeth bear distinct denticles and the enamel furthermore possess weak crenulations and striations that run down the surface longitudinally. While not directly associated with the cranial remains, these teeth were assigned to Rukwasuchus due to the fact that they resemble the typical peirosaurid morphology while also being clearly distinct from the small peg-like and multicuspid teeth of the other Galula crocodylomorph, Pakasuchus. [1]
Endocast
Thanks to the preservation of the braincase CT scans reveal large parts of the endocast of Rukwasuchus with only some parts of the internal structure obscured by damage or dense matrix still adhering to the fossil. The overall shape resembles modern gharials and freshwater crocodiles, with the two hemispheres of the cerebrum broad and tapering both towards the back and front, forming an element that is spade-shaped and modestly high. The anterior cerebrum connects to the narrow olfactory tract, which ends in a moderately broad olfactory bulb. However the olfactory bulb differs in being very well developed and also more downturned compared to other crocodyliforms. Another distinct feature of the endocast of Rukwasuchus noted in the 2014 description is a prominent indentation just behind the cerebrum while the rest of the dorsal endocast surface is more typical in its morphology. The CT scans also show the main dorsal part of the dural venous sinus including the dorsal sinus stretching over the olfactory tract and the cerebrum as well as the occipital sinus that overlies the brainstem (tectum and medulla) as well as the hindbrain (cerebellum). The underside of the endocast features a prominent hypophyseal fossa and a pair of cavernous dural venous sinuses, the latter of which are located behind the hemispheres of the cerebrum, the postpituitary notch and the ventral longitudinal dural venous sinus on the hindbrain. The longitudinal sinus on top of the endocast is connected to the ventral cavernous sinus via the sphenoparietal dural venous sinus, which passes transversely just behind the cerebrum. Some regions, though not fully preserved, can be inferred in their location. For instance the flocculus was likely located just before where the opistotics projects into the skull cavity, pinching the endocast, as is the common placement. Passing through the anterior edge of the flocculus was most likely the transverse dural venous sinus. [1]
