Rukwasuchus

Rukwasuchus Temporal range: middle Cretaceous,
Scientific classification
Kingdom:	Animalia
Phylum:	Chordata
Class:	Reptilia
Clade:	Archosauria
Clade:	Pseudosuchia
Clade:	Crocodylomorpha
Clade:	†Notosuchia
Family:	†Peirosauridae
Genus:	†Rukwasuchus Sertich & O’Connor, 2014
Type species
†Rukwasuchus yajabalijekundu Sertich & O’Connor, 2014

Rukwasuchus is an extinct genus of peirosaurid crocodyliforms known from the middle Cretaceous Galula Formation of southwestern Tanzania. It contains a single species, Rukwasuchus yajabalijekundu.^[1]

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]

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. The morphology of the skull and isolated teeth suggests close relations to the peirosaurid Hamadasuchus rebouli from the middle Cretaceous Kem Kem Beds of Morocco.^[1]

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]

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]

A phylogenetic analysis of Crocodyliformes supports a close relationship between Rukwasuchus and other African members of Trematochampsidae, e.g. Hamadasuchus and Trematochampsa, that is positioned within a larger clade of Gondwanan peirosaurids. The following cladogram is simplified after the analysis of Sertich & O’Connor (2014); the relationships within Sebecosuchia and advanced notosuchians are not shown.^[1]