Warilochromis is an extinct genus of predatory cichlid fish known from the Late Miocene Ngorora Formation of Kenya. It represents one of the oldest known members of the tribe Haplochromini, and only the species Warilochromis unicuspidatusis currently recognized.
History and naming Warilochromis has been discovered at the Waril locality, which belongs to Member E of the Ngorora Formation in Kenya. The holotype, and sole known specimen, of this genus is an almost complete skeleton, only missing a handful of bones, of which solely imprints are visible. Its generic name is a combination of “Waril”, the place where it was found, and Ancient Greek χρόμις (“chromis”), which has been applied to a variety of fish, and is a common element in the scientific names of cichlids. The specific name “unicuspidatus” combines the Latin words “unus”, meaning “one”, and “cuspis”, meaning “point”, referencing the remarkable dentition of its oral jaws.
Description Warilochromis reached a total length of 8.2 cm, with a standard length of 6.9 cm, a third of which is made up by its massive skull. Its oral teeth are unicuspid, with large canines (0.6-1.1 mm) in the outer row and smaller teeth in the inner row, whereas the pharyngeal teeth are mostly slender and bicuspid, possessing a prominent cusp and shoulder, interspersed with broader bicuspid teeth, that have one prominent and minor cusp, respectively. Its jaws are probably of the same width. The body is stout, and reaches its greatest depth behind the head. Its caudal peduncle is short and narrow. The preserved fossils showcase that it had a low supraoccipital crest as well as a straight and massive parasphenoid. Only the lacrimal remains of the infraorbital series, either because the adjacent infraorbitals were reduced or not fossilized. The premaxilla is slender, and its ramus is as long as the maxilla. The head possesses a robust neurocraniad process. The lower arm of the sturdy right dentary is probably of the same length as the upper arm, although deeper. The robust palatine is bent and ventrally associated with a small, slender ectopterygoid. A robust ventral process is present in the hyomandibula, as are large, dorsally directed articulation facets. The crushed opercle was likely rather large and triangular. The sturdy ventral hypohyals bear a posteroventrally directed spine, whereas the anterior ceratohyal suddenly becomes more slender towards the midline. A triangular basihyal is visible between the ceratohyals and dentary. The vertebral column is slightly concave in the caudal region. Of the 33 vertebrae, 19 are abdominal and 14 caudal. The hourglass-shaped vertebral centra are higher than long, with the first and penultimate being the shortest ones. Meanwhile, the neural spines increase in length towards the posterior, with the spines of the last abdominal to first three caudal vertebrae being the longest, before their length decreases towards the caudal fin. Each pelvic fin has a strong spine and five branched, segmented rays, that probably don't reach the origin of the anal fin. The continuous dorsal fin has 14 spines, which increase in length posteriorly, and 10 rays which are branched as well as segmented. Meanwhile, the anal fin possesses three strong spines, that gradually increase in length, and nine rays, which reach the first third of the caudal peduncle. There are twelve pterygiophores, which decrease posteriorly, with the first two being fused. Unlike in modern cichlids, the first pterygiophore is longer than the second one. The slightly truncate caudal fin consists of 16 segmented principal fin rays, the upper and lowermost of which are unbranched. These rays are supported by the parhypural as well as the hypural plates 1 + 2 and 3 + 4. A small and slender fifth hypural plate is positioned between the hypural plates 3 + 4 and the second epural. Scales cover almost its whole body, except for the predorsal part and the head.
Classification Warilochromis is a member of the tribe Haplochromini, possibly closely related to the genus Pseudocrenilabrus, with which it shares an unusual lacrimal. However, there are some notable morphological differences between these two genera, although these may be explained by their different ecological adaptions, with Pseudocrenilabrus being an inhabitant of rivers and streams, whereas Warilochromis was a lake-dweller. Its age and geographical location, rather far removed from Lake Tanganyika, makes it useful for understanding the evolutionary history of the tribe. It makes proposals, that haplochromine originated within the last nine million years unlikely, and refutes an age of 5-6 Ma as the onset of the divergence within the tribe, as well as the ‘out of Tanganyika’ hypothesis, which proposes Lake Tanganyika as cradle of all haplochromine cichlids. Both it and Tugenochromis, which was discovered at the same locality, also suggest that the Central Kenya Rift and Lake Tanganyika were hydrologically connected during the late Miocene, which is consistent with prior geological studies, that pointed towards an east–west connection between the Malagarasi and Congo prior to the formation of Lake Tanganyika. Therefore, it is likely that the progenitors of the Lake Tanganyika Radiation were already present in the eastern branch of the East African Rift System during the Miocene. The only other possible fossil haplochromine is an unnamed species from Saudi Arabia, which would be the oldest known member of the tribe. As both it and Warilochromis are known east of Lake Tanganyika, it supports the hypothesis that haplochromines originated in this region, which is supported by molecular data. Below is a cladogram showcasing the phylogenetic relationships of Warilochromis, taken from Altner et al., 2020:
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