Geological Setting and New Fossil Sites

The Tugen Hills in Central Kenya is a mountain range that lies within the eastern branch of the East African Rift System (EARS) (Fig. 1). The rocks exposed in the Tugen Hills originated during the formation of the Central Kenya Rift section of the EARS during the Miocene and Pliocene. Volcanic rocks testify to periods of enhanced tectonic activity, while fluvial and lacustrine sediments accumulated when volcanism had effectively ceased and only occasional ash falls occurred (Bishop and Chapman, 1970; Bishop and Pickford, 1975; Rasmussen et al., 2017). The fossils studied here were recovered from the middle to upper Miocene Ngorora Formation in the Tugen Hills. The study area is located in the Baringo District, ca. 30 km northwest of Lake Baringo (Fig. 1). The Ngorora Formation comprises volcaniclastic rocks and tuffs alternating with fluvial and lacustrine, mostly siliciclastic sediments and paleosols; it reaches a thickness of 365 m and can be subdivided, from bottom to top, into the five lithostratigraphic members A to E (Bishop and Pickford, 1975; Rasmussen et al., 2017). The rocks of the Ngorora Formation were deposited in the Ngorora Basin, which can itself be partitioned into a northern (Kabasero sub-basin, area I) and a southern area (Kapkiamu sub-basin, area II) (Bishop and Pickford, 1975; Rasmussen et al., 2017:fig. 2). Apart from abundant finds of fossil mammals, turtles, crocodiles and plants, the Ngorora Formation hosts a fossil-fish Lagerstätte, characterized by numerous assemblages of cichlid fish fossils, especially in the sediments comprising the Members C to E of the Kapkiamu sub-basin (Bishop and Pickford, 1975; van Couvering, 1982; Rasmussen et al., 2017; Altner et al., 2017; Kevrekidis et al., 2019; Penk et al., 2019).

The fossil cichlid fishes described here were collected from the Ngorora Formation during field campaigns in the Tugen Hills in 2011 (site Terenin) and 2014 (site Yatianin). Terenin (GPS coordinates 0°48.284′N, 35°48.936′E, 1842 m above sea level) is situated south west of the small village of Bartabwa in the Kabasero sub-basin, whereas Yatianin (0°43.986′N, 35°46.904′E, 1405 m above sea level) is located ca. 20 km south of Terenin in the Kapkiamu sub-basin. Terenin represents Member A of the Ngorora Formation according to Pickford et al. (2009), whereas Yatianin belongs to the uppermost part of Member C of the Ngorora Formation according to Rasmussen et al. (2017). Using the stratigraphic scheme of Rasmussen et al. (2017), the stratigraphic age of Terenin is ca. 13 Ma and that of Yatianin ca. 12 Ma. At both sites, the fossils with which we are concerned were collected from silicified, white to light grey, laminated diatomites. For a more detailed description of the Yatianin section see Rasmussen et al. (2017), for Terenin no further data are available.

Material

Comparative Extant Material—We used this set of comparative material to examine characters that are also discernible in our fossils, but whose taxonomic or systematic utility has not been previously assessed: (1) the numbers of sensory canal pores on the preopercle were determined for 231 species representing all cichlid subfamilies and all African tribes (Table S1). These data were drawn from the literature (164 species), from specimens preserved in alcohol or formaldehyde from the collection of the SNSB-Bavarian State Collection of Zoology in Munich (97 species), from bone preparations from the collections of the Bavarian State Collection of Anthropology and Paleoanatomy in Munich (15 species) and from a µCT scan of O. (Alcolapia) grahami. (2) X-ray images of 1301 formalin-fixed specimens of Pseudocrenilabrine cichlids from all tribes were inspected (Table S2) in order to assess the range of intra- and interspecies variation of two particular characters – the fusion pattern of the hypural plates, and the number of supraneurals. The number of supraneurals was recognizable in all but two of these individuals. The fusion pattern of the hypural plates was discernible in over 90% of the specimens. (3) The number of lateral-line tubules on the lacrimal was surveyed for almost all species of the tribe Oreochromini (59 out of 63, Table S3). The data were compiled from the literature (59 species), and from specimens preserved in alcohol or formaldehyde from the SNSB-Bavarian State Collection of Zoology in Munich (14 species).

Fossil Material—The material from the Yatianin site consists of remains of 23 individuals, here numbered OCO-11-1 to -23. Eleven of these are almost complete, six preserve the head and anterior portions of the body, and in the rest the caudal fin and some posterior portions of the body can be discerned. Eleven slabs were recovered from the site Terenin, which contained isolated or partially articulated bones and one articulated postcranial skeleton (numbers OCO-683-11 to OCO-692-11, OCO-773-11). Furthermore, the holotype of †Rebekkachromis ngororus (OCO-3-3a, b), which is the type species of †Rebekkachromis, was reexamined. All specimens are currently housed in the Department of Earth and Environmental Sciences at the Ludwig-Maximilians-Universität München, and will be transferred to Kipsaraman, Baringo County, Kenya, when the planned Baringo County Geopark is established.

Methods

Measurements, Meristics, and Osteology—The fossils were measured with digital sliding calipers and measurements were rounded to the nearest 0.1 mm. For each fossil specimen from Yatianin, the relative body proportions were calculated after normalization to standard length (SL). The meristic counts of vertebrae include the terminal centrum; abdominal vertebrae are characterized by the absence of a closed hemal arch. Dorsal and anal fin ray counts included every discernible ray associated with a pterygiophore; because the last two rays of the dorsal and anal fin share one pterygiophore, they were counted as one ray. Circuli were counted on the posterior lateral field of the scale. A dagger symbol (†) denotes extinct taxa. For details on the preparation and optical imaging of fossils see Supplemental Data.

µCT—All slabs bearing fossil fishes were first X-rayed (FaxitronUltraFocus, SNSB-Bavarian State Collection of Zoology, Munich) in order to determine which specimens had the highest contrast between the bones and the surrounding sediment, as well as to identify potential sources of artifacts (e.g., concretions, other bones or skeletons underlying the specimens in question). Six specimens were selected for µCT scanning with a Phoenix Nanotom m (GE Sensing & Inspection Technologies GmbH). Details on the scanning process can be found in Appendices S2 and S3.

Institutional Abbreviations—OCO, Orrorin Community Organisation; SAPM, Bavarian State Collection of Anthropology and Paleoanatomy, Munich, Germany; ZSM, SNSB-Bavarian State Collection of Zoology, Munich, Germany.

Notes on the Morphology of Oreochromis (Alcolapia)

As mentioned above, our analysis of extant taxa focused on the characters that are also discernible in our fossils. Based on the µCT data for Oreochromis (Alcolapia) grahami (Supplemental Data), the lacrimal (= first infraorbital) is followed by a small second infraorbital (io2) with two openings (Fig. 3A, B). Behind the latter, at the posteroventral corner of the eye, is a long infraorbital (io3) with three to four openings; this is in turn separated from the dermosphenotic (io4) by a small gap (Fig. 3B). The preopercle displays three sensory canal pores on the lower arm and at least two sensory canal pores on the upper branch, including the terminal pore (Fig. 3C). The urohyal has a very small dorsal spine, which is directed anteriorly (Fig. 3D) – not posteriorly as in other species of Oreochromis (Fig. 3E). The ventral process of the anguloarticular is perforated by a canal. The hyomandibula has a convex anteroventral flange.

Based on the information obtained from X-ray images and alcohol-preserved specimens of all four species of Oreochromis (Alcolapia), the orientation of the supraneural bone ranges from sharply angled relative to the vertical level (with the ventral tip facing anteriorly) to upright (see also Penk et al., 2019). The hypural plates of the caudal skeleton are not fused with the urostyle; the hypurapophysis of the parhypural is well developed. The scales on the throat and belly are minute (see also Penk et al., 2019); the scales on the nape are intermediate in size between the minute scales of the throat and those of the flank; and two longitudinal scale rows appear between the upper lateral line and the dorsal fin (Fig. 3A).

FIGURE 3.

A–D, µCT data volume rendering of Oreochromis (Alcolapia) grahami (ZSM 25618). A, whole specimen; B, infraorbitals; C, preopercle, reversed; D, urohyal; E, photograph of urohyal of Oreochromis niloticus (Linnaeus, 1758) (SAPM 01887). The photograph in E was provided by M. Altner. All bones are depicted in lateral view; numbers denote the lateral-line tubules of the lacrimal and the sensory canal pores on the preopercle, the red arrow denotes the anterior ridge. Abbreviations: io, infraorbital; ds, dermosphenotic.

Variation of Selected Characters in Extant African Cichlids

The only criteria that permit one to analyze species diversity in fossil faunas are morphological characters that are known to exhibit low intraspecific variability. Here we have evaluated the taxonomic and systematic utility of four characters that are discernible in our fossils and for which only little information on their variation was available. Based on our comparative dataset of extant cichlid species, their intraspecific and interspecific variation, and their range of variation within a tribe were assessed in order to substantiate their use for taxonomic and systematic purposes. The traits selected for this analysis are: (1) the number of sensory canal pores on the lower arm of the preopercle; (2) the fusion pattern of the hypural plates; (3) the number of supraneural bones; and (4) the number of lateral-line tubules on the lacrimal.

With regards to trait 1, virtually all members of the African subfamily Pseudocrenilabrinae have four sensory canal pores on the lower arm of the preopercle (Stiassny, 1991; Takahashi, 2002; see here Fig. 4A, B and Table S1). Oreochromis (Alcolapia) is the sole extant haplotilapiine taxon that has three sensory canal pores in this position (Fig. 3C), and only one other Pseudocrenilabrine taxon possesses this character, namely the chromidotilapiine Congochromis (see Stiassny and Schliewen, 2007; Table S1). Cichlinae have mostly three sensory canal pores on the lower arm of the preopercle (Kullander, 1986, 1998; see here Fig. 4C, D) and the examined Etroplinae and Ptychochrominae have four (Table S1).

As to the fusion pattern of the hypural plates (trait 2), the X-rayed specimens could be classified into four categories (relative frequencies based on our comparative dataset are given in parentheses): all hypurals separated (50.2%), hypurals 1 and 2 fused (2.4%), hypurals 3 and 4 fused (6.3%), hypurals fused in pairs, i.e., 1 and 2, and 3 and 4 fused (41.1%) (Table S2). Fusion between hypurals 2 and 3 may occasionally occur, but was difficult to diagnose from the X-rays and is not considered here. Within a given species, the variability was low, with most specimens falling into a single category; only occasionally were a few individuals assigned to a different category (Table S2). Intratribal diversity was also low, with specimens from a given tribe falling into one or at most two categories (Table S2).

The same set of X-rayed specimens was used to assess the stability of the number of supraneural bones (trait 3; Table S2). Here we focused on species which usually have either one supraneural or none. Only eight species (Trematocara kufferathi Poll, 1948, T. marginatum Boulenger, 1899, T. nigrifrons Boulenger, 1906, T. stigmaticum Poll, 1943, Bathybates fasciatus Boulenger, 1901, B. graueri Steindachner, 1911, B. vittatus Boulenger, 1914, Orthochromis sp. “Igamba”) show any variability in this regard, and the first four of these (for which ten or more specimens were available) have an ‘abnormal’ number of supraneurals in ≤10% of cases (Table S2).

The number of lateral-line tubules on the lacrimal (trait 4) was surveyed for almost all (59 out of 63) species of the tribe Oreochromini. Except for the alkaliphile species, all species usually have five lateral-line tubules on the lacrimal; only some specimens of Oreochromis niloticus (Linnaeus, 1758) and one of the examined specimens of Iranocichla hormuzensis Coad, 1982 have four. Oreochromis amphimelas, O. (Alcolapia) alcalicus, and O. (Alcolapia) latilabris have four lateral-line tubules, O. (Alcolapia) grahami has mostly four (exceptionally five) lateral-line tubules (see also Trewavas, 1983; Seegers and Tichy, 1999). The condition in O. (Alcolapia) ndalalani seems to be mixed. According to Seegers and Tichy (1999) there are four lateral-line tubules in this species, but two out of the three examined alcohol-preserved specimens have five lateral-line tubules and in one specimen there is a left/right asymmetry between four and five lateral-line tubules (see Table S3).

Taxonomy and Systematics

Delimitation of Fossil Cichlid Species—In cichlid paleontology, a conservative approach in delimiting taxa is unavoidable, because modern African cichlids are frequently distinguished based on color patterns or characters that are never or hardly ever fossilized (e.g., Poll, 1986; Trewavas, 1983; Greenwood, 1989; Casciotta and Arratia, 1993). Murray (2000) described five different species from the Eocene of Mahenge, Tanzania, based on differences in the shape and proportions of some bones of the head, e.g., the opercle, the anguloarticular, and the hyomandibula. Such characters have considerable taxonomic potential (Murray and Stewart, 1999; Dierickx et al., 2017), but the fossils described here show a greater degree of homogeneity in these characters. Therefore, our fossils cannot be further distinguished on this basis. Furthermore, in the case of many other characters of known taxonomic value, variation due to factors such as ontogeny and sex must be considered. An abundant set of complete skeletons is needed to justify the use of such characters, which is why we refrain from using them to distinguish additional species from the site Yatianin. In the following, we discuss examples of such characters:

The shape and distribution of the oral teeth (e.g., Trewavas, 1983; Poll, 1986; Takahashi, 2003a) are subject to change throughout ontogeny, and cichlids can change the type of their oral teeth when they are close to sexual maturity (e.g., Trewavas, 1983; Schliewen and Stiassny, 2003). Furthermore, it is known that in older, larger specimens (particularly males) of Sarotherodon, Oreochromis, and Oreochromis (Alcolapia) some or all teeth can be unicuspid, as a result of abrasion or replacement (Trewavas, 1983), whereas younger individuals have bicuspid or tricuspid teeth. Among the studied material, the specimens OCO-11-20 and OCO-11-14, which were determined as †Rebekkachromis sp., have exclusively conical unicuspid teeth (Fig. 9F; Table 2). They are also slightly larger than the rest and OCO-11-20 has the highest number of circuli on its scales among all studied specimens (Fig. 11B, OCO-11-14 does not have well preserved scales). Therefore, the dentition of these two specimens may be a result of their age.

FIGURE 9.

Teeth of fossil cichlids from the Yatianin site. A, †Rebekkachromis valyricus, sp. nov., large and small tricuspid teeth from the anterior tip of the lower oral jaw, OCO-11-19b; the arrow on the right points anteriorly. B, †Rebekkachromis sp., left: large tricuspid, right: small tricuspid teeth from the anterior tip of the lower oral jaw, OCO-11-9; C, †R. vancouveringae, sp. nov., left: large tricuspid, middle: small tricuspid, right: recurved conical unicuspid teeth from the anterior tip of the lower oral jaw, OCO-11-4b; D, †Rebekkachromis sp., left: large tricuspid, right: small, shouldered unicuspid teeth from the anterior tip of the lower oral jaw, OCO-11-13a; E, †Rebekkachromis sp., left: large tricuspid, right: small, weakly shouldered unicuspid teeth from the anterior tip of the lower oral jaw, OCO-11-21; F, †Rebekkachromis sp., left: µCT data volume rendering of the alveoli and unicuspid tooth of the anterior part of the premaxilla, OCO-11-20; large conical (middle) and smaller shouldered unicuspid (right) tooth, both from the anterior part of the upper oral jaws, OCO-11-14; G, pharyngeal teeth, left: bicuspid, middle: bevelled unicuspid, right: hooked bicuspid. Continuous lines denote definite outlines, dotted lines indicate that the tooth is fractured and its outline is uncertain.

Differences regarding the extension of the rays of the dorsal and anal fins relative to the base of the caudal fin, as noted here, might reflect sexual dimorphism. The posterior tips of the dorsal and anal fins are more pointed and longer in the males of some oreochromine cichlids, e.g., in Sarotherodon galilaeus (Linnaeus, 1758), Oreochromis aureus (Steindachner, 1864), and O. mossambicus (Peters, 1852) (see Chervinski, 1965; Trewavas, 1983; Oliveira and Almada, 1995), which use them against other males during competitive displays (Oliveira and Almada, 1995). Since all the species of †Rebekkachromis are represented by one individual each, this character cannot be used here to distinguish between the sexes.

As a result, seven characters were considered here as taxonomically relevant to discriminate among †Rebekkachromis (Table 2). However, †R. valyricus, †R. vancouveringae and each of the five specimens described separately as †Rebekkachromis sp. (OCO-11-13, OCO-11-9, OCO-11-21, OCO-11-20, and OCO-11-14) exhibits a different combination of these seven characters (Table 2). As explained above, it appears not appropriate to introduce new species for the †Rebekkachromis sp. specimens because of their incomplete preservation. Nevertheless, the number of two species from the Yatianin site should be considered as a minimum and additional well-preserved material from this site might elevate this number.

Systematics of †Rebekkachromis at the Level of Tribe —†Rebekkachromis has already been established as a haplotilapiine African cichlid, based on the possession of tricuspid teeth in the inner rows of its oral dentition (Kevrekidis et al., 2019). The newly described characters (pertaining e.g., to the urohyal, vomerine-parasphenoid suture and squamation, see ‘Systematic Discussion’ in Supplemental Data) support the previous assignment of †Rebekkachromis up to the level of the lineage of the haplotilapiines.

†Rebekkachromis was originally referred to as being “comparable to (cf.) Etiini” in Kevrekidis et al. (2019:56). However, the additional fossil specimens and the µCT data presented enable us to expand the definition †Rebekkachromis and to describe an array of previously unknown characters (e.g., six sensory canal pores on the preopercle, small scales on the nape, urohyal lacking an anterodorsal spine). As a result, its systematic placement can now be undertaken with greater confidence. A morphological phylogeny comprising all pseudocrenilabriine lineages recognized today is currently lacking, but the published information on their morphology, combined with the new data presented here, is sufficient to permit systematic inferences.

The number of lateral-line tubules on the lacrimal is an established character for the systematics of cichlids (Trewavas, 1983; Takahashi, 2003a, 2003b). Intraspecific and intrageneric variation of this character, as well as left-right asymmetry, has been previously noted (Greenwood, 1989; Trewavas, 1983; Penk et al., 2019) but seems not to occur regularly. Among the extant haplotilapiines, a lacrimal bone with four lateral-line tubules, as seen in most specimens of †Rebekkachromis, is found only in the Cyprichromini, Trematocarini, Lamprologini, Ectodini, Oreochromini, and in the haplochromine Pseudocrenilabrus-group (Takahashi, 2003b; Altner et al., 2017; Penk et al., 2019; Altner et al., 2020). In addition, a lacrimal with four lateral-line tubules has been reported for two extinct cichlid genera from the Miocene of the Tugen Hills, i.e., †Oreochromimos (see Penk et al., 2019) and †Warilochromis (see Altner et al., 2020), and is also known for a further new cichlid taxon from the same area (Altner and Reichenbacher, 2020). †Rebekkachromis is very similar to †Oreochromimos, which is why we can refer here to the systematic discussion of Penk et al. (2019) why †Oreochromimos can be assigned to the Oreochromini. In addition, some characters only or better discernible in †Rebekkachromis (e.g., processes of the anguloarticular), partially through the use of µCT imaging, add further support why †Rebekkachromis cannot belong to the following tribes: (1) Cyprichromini: according to Takahashi (2003a) characterized by a forked caudal fin (vs. subtruncate to emarginate in †Rebekkachromis) and ctenoid scales at midbody (vs. exclusively cycloid). (2) Trematocarini: according to Poll (1986) characterized by a head which is not covered by scales (vs. covered by scales in †Rebekkachromis); expanded cephalic sensory canal pores (see also Takahashi, 2003a) (vs. not expanded); exclusively unicuspid conical teeth (vs. unicuspid and tricuspid); a short upper lateral line, lower lateral line absent (vs. two ordinary lateral lines). According to Stiassny (1981), the dorsal process of the anguloarticular has a laterally expanded posterior border (vs. slender, unexpanded dorsal process). (3) Lamprologini: according to Stiassny (1997) characterized by a notched head of the hyomandibula (vs. not notched in †Rebekkachromis); more than three anal fin spines (see also Takahashi, 2003a) (vs. three anal fin spines); fusion between hypurals 3 and 4 and the urostyle (vs. no fusion between hypurals and urostyle, fusion between hypurals 3 and 4 variable); usually unicuspid inner row teeth and large, fang-like canines in the outer row (vs. unicuspid and tricuspid, no canines); a reduction in the number of infraorbitals (vs. at least three to four infraorbitals including the lacrimal); ctenoid scales (see also Lippitsch, 1998; Takahashi, 2003a) (vs. exclusively cycloid); an abrupt change to small scales above the upper lateral line (vs. gradual); a cheek lacking scales (vs. scaled cheek). (4) Ectodini: according to Greenwood (1983) characterized by a palatine whose posterior and dorsal margins form a 90° angle (vs. 120° in †Rebekkachromis); a distinct process at the posterodorsal corner of the operculum (vs. convex dorsal margin); an elongate lacrimal (see also Takahashi 2003b) (vs. rectangular); the dorsal process of the anguloarticular has a posteriorly expanded border (see also Liem, 1981) (vs. not expanded). Ectodini also possess ctenoid scales (Lippitsch, 1998; Takahashi, 2003a) (vs. exclusively cycloid). (5) The Haplochromini and the Oreochromini are morphologically very diverse tribes and it is not easy to tell them apart exclusively based on hard-part characters (see Takahashi, 2003b; Altner and Reichenbacher, 2020). The Haplochromini however, including those of the Pseudocrenilabrus-group, are characterized by the possession of, at least some, ctenoid scales (Greenwood, 1989; Lippitsch, 1993, 1997, 1998).

FIGURE 10.

Schematic drawings of the caudal fins of fossil cichlids from the Yatianin site. A, †Rebekkachromis valyricus, sp. nov., OCO-11-19; B, †R. vancouveringae, sp. nov., OCO-11-4; C, †Rebekkachromis sp., OCO-11-13; D, †Rebekkachromis sp., OCO-11-9. Continuous lines denote a definite outline, dotted lines indicate that the outline is uncertain. Abbreviations: ep, epural; H, hypural plates; hp, hypurapophysis; hs, hemal spine of preural centrum; lprcr, lower procurrent caudal rays; ns3, neural spine of preural centrum 3; ph, parhypural; plcr, principal caudal rays; pu, preural centrum; u, urostyle; un, uroneural; uprcr, upper procurrent caudal rays.

FIGURE 11.

Flank scales of fossil cichlids from the Yatianin site, anterior is to the top. A, terminology; B, scatter plot of the number of circuli on the posterior lateral field relative to standard length (SL); C, †Rebekkachromis sp., OCO-11-13, lateral view; D, †R. vancouveringae, sp. nov., OCO-11-4, medial view; E, †Rebekkachromis sp., OCO-11-20, lateral view; F, same individual, medial view. Coated with ammonium chloride.

In conclusion, according to the results of the present study and also taking into account that the probably related †Oreochromimos has been classified as a member of the Oreochromini (Penk et al., 2019), †Rebekkachromis can be attributed to this tribe as well. The main difference between †Rebekkachromis and other members of the Oreochromini is that †Rebekkachromis has one or two supraneurals (vs. one), although exceptions may occur (only one of the examined extant Oreochromini specimens had two supraneurals, see Table S2 and Kevrekidis et al., 2019).

Systematics of †Rebekkachromis within the Extant Oreochromini—Among the nine extant genera of the Oreochromini, only Oreochromis, O. (Alcolapia), and Iranocichla may have four lateral-line tubules on the lacrimal (see Penk et al. 2019:fig. 11). However, as described in the Results, a number of four lateral-line tubules on the lacrimal was regularly found in O. amphimelas, O. (Alcolapia) alcalicus, and O. (Alcolapia) latilabris, and mostly also in O. (Alcolapia) grahami. Among the further species, only some specimens of O. niloticus and one of the examined specimens of I. hormuzensis had four lateral-line tubules on the lacrimal. Penk et al. (2019:fig. 11h) noted a strongly bent and anteriorly convex supraneural of Iranocichla, which is very different from the straight or only slightly curved supraneural of Oreochromis and †Rebekkachromis (Fig. 8). Accordingly, among the extant Oreochromini †Rebekkachromis is considered here as probably most closely related to O. (Alcolapia). This assignment is reinforced by an additional line of evidence: †Rebekkachromis possesses three sensory canal pores on the lower arm of the preopercle (Figs. 4E–H, 13D; Fig. S3B–D) and the only other extant haplotilapiine group that possesses this character is O. (Alcolapia) (see Fig. 3C; Table S1).

FIGURE 12.

Skeletal elements of †Rebekkachromis sp. from the Terenin site. A, left and right dentary, OCO-688-11; B, same individual, complete recurved, shouldered unicuspid tooth, and lingually some teeth with broken crowns; C, fifth ceratobranchial (lower pharyngeal jaw), ventral view, OCO-678-11; D, bones of the hyoid and branchial complexes, dorsal view, anterior to the top (the urohyal is seen in lateral view, dorsal margin on the left), OCO-691-11; E, hyoid bar, OCO-689-11; F, caudal fin, OCO-773-11. Abbreviations: ac, anterior ceratohyal; cb, ceratobranchial; dh, dorsal hypohyal; ep, epural; H, hypural plates; hp, hypurapophysis; hs, hemal spine of preural centrum; ns3, neural spine of third preural centrum; pc, posterior ceratohyal; ph, parhypural; plcr, principal caudal rays; pu, preural centrum; u, urostyle; un, uroneural; ur, urohyal; vh, ventral hypohyal.

Comparison of †Rebekkachromis with Oreochromis (Alcolapia) and Other Oreochromis—Apart from the new data presented here on the morphology of the hard parts of O. (Alcolapia) and other species of Oreochromis, data from the literature, particularly Murray and Stewart (1999), is used to further discriminate †Rebekkachromis. Murray and Stewart (1999) examined the osteology of five Oreochromis species (O. aureus, O. mossambicus, O. niloticus, O. placidus, and O. urolepis). The molecular phylogeny of Ford et al. (2019:fig. 1), which included the latter four species, suggests that they form a paraphyletic group with regards to O. (Alcolapia) (Fig. 2). These five species are hereafter referred to as ‘Oreochromis spp.’ Though the osteology of other species of Oreochromis is not well studied, they are used here for tentative comparisons with †Rebekkachromis. They reveal that, in addition to the presence of five lateral-line tubules on the lacrimal (vs. mostly four in †Rebekkachromis and O. (Alcolapia)), and four sensory canal pores on the lower arm of the preopercle (vs. three in †Rebekkachromis and O. (Alcolapia)), the three following characters differentiate both †Rebekkachromis and O. (Alcolapia) grahami from Oreochromis spp. (sensu Murray and Stewart, 1999): (1) Oreochromis spp. has a supraoccipital crest with an enlarged posterior tip in dorsal view (Murray and Stewart, 1999:fig. 2), whereas in †Rebekkachromis and O. (Alcolapia) grahami this tip is tapered (Appendix S3);(2) Oreochromis spp. has an opercle with a posterodorsal excavation (Murray and Stewart, 1999:fig. 3b), whereas in †Rebekkachromis and O. (Alcolapia) grahami this area of the opercle is convex (Figs. 3A, 7); (3) Oreochromis spp. has an acute notch at the posteroventral edge of the dorsal plate of the cleithrum (Murray and Stewart, 1999:fig. 3c), whereas in †Rebekkachromis and O. (Alcolapia) grahami this notch is absent (Figs. 3A, 7).

In addition, three characters differentiate †Rebekkachromis from both Oreochromis spp. (sensu Murray and Stewart, 1999) and O. (Alcolapia) grahami: (1) Oreochromis spp. and O. (Alcolapia) have a dorsal process of the posttemporal with a rounded tip in dorsal view (vs. an angled tip in †Rebekkachromis) (Murray and Stewart, 1999:fig. 3f; see also this paper Appendix S3); (2) Oreochromis spp. and O. (Alcolapia) have a normative number of one supraneural (vs. one or two in †Rebekkachromis, Table S2); (3) Oreochromis spp. and O. (Alcolapia) have a urohyal with a small dorsal spine (vs. no spine in †Rebekkachromis, Figs. 3D, E, 7).

Overall, †Rebekkachromis is morphologically most similar to Oreochromis (Alcolapia), but in the absence of a phylogenetic study, the current taxonomic status of †Rebekkachromis as a separate genus is retained for now.

Comparison with Previously Described Cichlids from the Tugen Hills—The presence of complete skeletons of cichlid fishes was noted in the original description of the Ngorora Formation (Bishop and Chapman, 1970). The first cichlid from the Tugen Hills that was described in detail was introduced as a new species and named “†Sarotherodon martyni” (Van Couvering, 1972, 1982). It had been collected from the Kapkiamu shales (≈12 Ma), which represent an equivalent of the Ngorora Formation (see Van Couvering, 1982). Van Couvering (1982) suggested a close affinity of her new species with the members of the “Alcolapia” group, O. (Alcolapia) grahami, O. (Alcolapia) alcalicus, and Oreochromis amphimelas (at that time all were referred to as Sarotherodon, see Introduction). She based this conclusion on the low meristic counts of †“S.” martyni, particularly with respect to the spines of the dorsal fin, and the presence of minute scales (or absence of scales) on the chest and belly (Van Couvering, 1982). †“Sarotherodon” martyni might be attributable to the genus Oreochromis, because the species of the “Alcolapia” group it resembles were transferred to this genus (Murray and Stewart, 1999).

FIGURE 13.

†Rebekkachromis ngororus from the Rebekka site, OCO-3-3a. A, complete skeleton of the holotype; B, urohyal, reversed; C, imbricate cycloid scales, anterior on top; D, preopercle, in lateral view. C and D are coated with ammonium chloride. Numbers denote the sensory canal pores of the preopercle, dotted lines denote a fracture and the outline is uncertain.

Based on the text and the figures of the original description (Van Couvering, 1982:pl. 8, 9), several similarities between †“S.” martyni and †Rebekkachromis can be noted: the lacrimal is deeper than wide, and followed by some infraorbitals, at least one of which, at the posteroventral angle of the orbit, seems to have more than two openings. The preopercle of †“S.” martyni is described as having four sensory canal pores, “two of which open directly from the main canal and two by way of side branches” (van Couvering, 1982:84). It is possible that the two terminal sensory canal pores were not included in the count, which would mean that the total number of sensory canal pores might be six, as in †Rebekkachromis. However, †“S.” martyni differs from †Rebekkachromis by the possession of a mostly unicuspid dentition. Without a reexamination of the holotype of †“S.” martyni, which was not possible in the course of this work, it is impossible to conclude whether this taxon corresponds to Oreochromis, or to an extinct genus. Therefore we refer to it here as †“S.” martyni.

More recently, the extinct monotypic genus †Oreochromimos, represented by Om. kabchorensis, was described from the middle Miocene of the Ngorora Formation (≈12.5) Ma of the Tugen Hills (Penk et al., 2019). †Oreochromimos shares several similarities with †Rebekkachromis (Table S9), i.e., the lacrimal has four lateral-line tubules, a slender urohyal without an anterodorsal projection, the dorsal process of the anguloarticular is curved, the oral dentition comprises unicuspid and small tricuspid teeth, the meristic counts are similar, there is one supraneural, the head bears scales, the scales of the chest and belly are minute, and the squamation is cycloid (Penk et al., 2019). Penk et al. (2019) concluded that †Oreochromimos has a morphology intermediate between Oreochromis and “Alcolapia,” based on meristic counts and squamation, and on osteological characters such as the lacrimal depth, the number of lateral-line tubules on the lacrimal, and the presence/absence of a notch on the cleithrum (see Penk et al., 2019:table 1). An important difference relative to †Rebekkachromis is the presence of a single club-shaped supraneural and four sensory canal pores (vs. three) on the lower arm of the preopercle, which is also the condition in Oreochromis (Table S9).

It is worth noting that the three studies that have independently examined multiple specimens of fossil cichlids from the Tugen Hills in detail (Van Couvering, 1982; Penk et al., 2019; this study) used different material, character sets, and methods, but all arrive at similar conclusions. These fossil taxa (†Rebekkachromis, †Oreochromimos kabchorensis, †“Sarotherodon” martyni) indicate that a diverse fauna of cichlids, distinguishable from, but morphologically similar to Oreochromis (Alcolapia), were abundant in the paleolakes of the Tugen Hills during the middle Miocene.

Finally, three further extinct cichlid genera are known from the upper Miocene site Waril (10–9 Ma), i.e., the monotypic genera †Tugenchromis Altner, Schliewen, Penk, and Reichenbacher, 2017 and †Warilochromis Altner, Ruthensteiner, and Reichenbacher, 2020, as well as †Baringochromis Alter and Reichenbacher, 2020 that is represented with three species (Altner and Reichenbacher, 2020). †Tugenchromis pickfordi was described based on a single specimen and has been proposed to be a member of the ‘East African Radiation’ clade; it has a tripartite lateral line and six lateral-line tubules on its lacrimal (Altner et al., 2017). It is thus clearly different from †Rebekkachromis. The single species of †Warilochromis, W. unicuspidatus, has been assigned to the tribe Haplochromini; it is clearly distinct from †Rebekkachromis because of its fang-like dentition and the high number of vertebrae (33) (amongst others; see Altner et al., 2020). The three species of †Baringochromis differ from †Rebekkachromis in the number of supraneurals (0–1 vs. 1–2), because the lacrimal is followed by five tubular infraorbitals (vs. probably not more than three), and because none of its tubular infraorbitals has more than two openings (vs. three) (amongst others; Altner and Reichenbacher, 2020).

Comparison with other Pseudocrenilabrinae Fossil Species—A comparison of †Rebekkachromis with other Pseudocrenilabrinae fossils was given by Kevrekidis et al. (2019). Where possible, this comparison is extended here in the light of the revised diagnosis of †Rebekkachromis.

The earliest cichlid found so far in Africa is the middle Eocene (ca. 46 Ma) †Mahengechromis Murray, 2000, from Tanzania (Murray, 2000). In addition to the differences described in Kevrekidis et al. (2019), †Mahengechromis is clearly distinct from †Rebekkachromis because its supraoccipital crest is high (vs. low), the urohyal bears a dorsal spine (vs. absent), and the preopercle has seven sensory canal pores (vs. six) (see Murray, 2000, 2001).

An early fossil member of the Pseudocrenilabrinae from the lower Oligocene (ca. 32 Ma) of eastern Europe (Bulgaria) is †Rhodopotilapia gracialis Kirilova and Georgiev, 2015. This species has two supraneurals, unicuspid pharyngeal teeth and cycloid scales (Kirilova and Georgiev, 2015). The head is not well preserved, but it differs from †Rebekkachromis in having a larger number of dorsal fin rays (16 vs. 8–11 in †Rebekkachromis), and a slightly lower number of vertebrae (27 vs. 29–31) (Kirilova and Georgiev, 2015).

†Macfadyena dabanensis Van Couvering, 1982, from the Oligocene Daban Beds in Somalia is differentiated from †Rebekkachromis by its high supraoccipital crest and its bicuspid to tricuspid pharyngeal teeth, in addition to the differences already stated in Kevrekidis et al. (2019).

†Palaeofulu kuluensis Van Couvering, 1982, from the Miocene Kulu Formation in Kenya (17–15 Ma) is similar to †Rebekkachromis in terms of meristics and squamation, but its oral teeth are unicuspid and ‘leaf-shaped’ (see Kevrekidis et al., 2019). In addition, its urohyal bears a prominent dorsal process (vs. absent) and the preopercle has four sensory canal pores on its lower arm (vs. three) (see Van Couvering, 1982:plates 5, 6; fig. 23).

The Seybouse Gypsiferous Marls of the late Miocene (>7 Ma) of Algeria have yielded two species assigned to the genus †Palaeochromis Sauvage, 1907 (Sauvage, 1907, 1910; Van Couvering, 1982), which have a dentition which differs from that seen in †Rebekkachromis (see Kevrekidis et al., 2019). Furthermore, †Palaeochromis differs from †Rebekkachromis in having a smaller number of vertebrae (25 or 26 vs. 29–31) and a larger number of dorsal fin rays (9–16 vs. 8–11) (Sauvage, 1910; Van Couvering, 1982).

Finally, †Oreochromis lorenzoi Carnevale, Sorbini and Landini, 2003 from the upper Miocene (≈6 Ma) of the Gessoso-Solfifera Formation, Italy (Carnevale et al., 2003) and †O. harrisae Murray and Stewart, 1999 from the lower Pliocene of Ethiopia (Murray and Stewart, 1999) are distinguished from †Rebekkachromis based on the presence of an acute notch on their cleithrum (vs. no notch) and the possession of bicuspid teeth (vs. unicuspid and tricuspid). Furthermore, †O. lorenzoi has four anal fin spines (vs. three) and four sensory canal pores on the lower arm of its preopercle (vs. three) (see Carnevale et al., 2003).

†Rebekkachromis and its Paleoenvironment

In this section we provide some background information about the characteristics of alkaline lakes and discuss the abiotic and biotic environment in which †Rebekkachromis lived.

Geochemistry of Alkaline Lakes—Saline-alkaline (soda) lakes are found today on every continent except Antarctica, but they are particularly numerous in East Africa, especially in the eastern branch of the East African Rift System (EARS) (Grant and Sorokin, 2011; Grant and Jones, 2016; Fazi et al., 2018) (see Fig. 1). There, tectonism has created several endorheic basins, from which water is lost mostly through evaporation (e.g., Schagerl and Renaut, 2016). Evaporation may increase the salinity of a lake, but volcanic activity, past or present, is crucial for enhanced alkalinity (Pecoraino et al., 2015). The weathering of volcanic rocks results in waters that are rich in sodium (Na⁺) and bicarbonate and carbonate ions (HCO₃^–, CO₃^2–), and is responsible for the alkalinity of several lakes along the present-day eastern branch of the EARS (e.g., Pecoraino et al., 2015; Schagerl and Renaut, 2016; Fazi et al., 2018).

Analcime (NaAlSi₂O₆·H₂O) is a silicate mineral that forms under highly alkaline conditions (Hay, 1966; Surdam and Sheppard, 1978) and thus can be used as an indicator of soda conditions when found in paleolake sediments (e.g., van Couvering, 1982; Rasmussen et al., 2017). Volcanism in the central portion of the Kenya rift, where the Tugen Hills are located, began ca. 17–15 Ma ago (e.g., Hill, 2002; Macgregor, 2015). Analcime is part of the clay mineral fraction in several beds of the middle to late Miocene (13.3–9 Ma) Ngorora Formation (Van Couvering, 1982; Renaut et al., 1999; Rasmussen et al., 2017), as well as in the underlying Tambach Formation (16–14 Ma, Renaut et al., 1999). This confirms previous conclusions that the paleolakes of the Tugen Hills, in which the sediments of the Ngorora Formation were deposited, must have been highly alkaline (see Bishop and Pickford, 1975; van Couvering, 1982; Renaut et al., 1999; Rasmussen et al., 2017).

Remarks on Taphonomy—The exceptional preservation of the fish found in the Tugen Hills might be attributable to anoxic conditions at the bottom of a lake (Rasmussen et al., 2017). In modern soda lakes, anoxia can be induced by chemical or thermal stratification (Melack and MacIntyre, 2016). Soda conditions might also promote fossilization by slowing down the decomposition of a fish carcass by bacteria (Gäb et al., 2020). In general, excellent preservation is an indicator that fish specimens were fossilized in situ and have not been transported over a long distance after death.

Accompanying Flora and Fauna—The goal of this section is to demonstrate that the taphocoenoses in which †Rebekkachromis is found are congruent with those of soda lakes, rather than freshwater lakes. Despite being considered extreme environments, soda lakes are among the most productive aquatic ecosystems on Earth (Oduor and Schagerl, 2007). Most primary production is due to alkaliphilic or alkali-tolerant cyanobacteria and eukaryotic algae, which are very abundant and diverse (Grant and Jones, 2016; Krienitz and Schagerl, 2016), unlike vascular plants (Kipkemboi, 2016). Although this microflora is a rich food source for consumers, only a few groups of zooplankton, macro-invertebrates, and vertebrates are adapted to tolerate highly alkaline conditions (Kavembe et al., 2016; Mengistou, 2016; Yasindi and Taylor, 2016). Lakes with low alkalinity and salinity, e.g., Lake Turkana, may host a more diverse macroflora and fauna (Kavembe et al., 2016; Kipkemboi, 2016) than the depauperate macrodiversity that is typical of lakes with more extreme conditions, e.g., Lakes Natron and Magadi (Melack, 1996). The only vertebrates known to inhabit the latter two are extremophile cichlids of Oreochromis (Alcolapia), as well as the lesser flamingo (Kavembe et al., 2016; Krienitz et al., 2016).

The above-mentioned characteristics of modern soda lakes, particularly the most extreme ones, correspond well with the faunistic composition of the fossil sites of the Ngorora Formation. No macrofossils of plants or animals, apart from cichlids, have been recovered from Yatianin, Rebekka, Kabchore (Rasmussen et al., 2017), or Terenin. The absence of evidence for the existence of other macroorganisms is not necessarily evidence of absence, but the favorable conditions for fossilization in these sites and the autochthonous nature of the cichlid fauna indicate that it would be reasonable to expect a more diverse accompanying fauna, if one had existed there at that time. Therefore, the apparent in situ preservation of these fossils indicates a very impoverished macrofauna.

Fossil cichlids that are very different from O. (Alcolapia) and the Oreochromini have been described from the upper Miocene part of the Ngorora Formation (Altner et al., 2017, 2020), and fossils of a wider range of aquatic invertebrates and vertebrates, such as freshwater crabs, gastropods and bivalves, catfish, aquatic turtles, and crocodiles have also been reported (Bishop and Chapman, 1970; Bishop and Pickford, 1975). These localities may represent periods of permanent or seasonal high lake-water levels. The salinity and alkalinity of modern soda lakes recede when their volume increases, e.g., due to a wetter climate, making them habitable for a wider range of organisms (Oduor and Kotut, 2016). †Rebekkachromis might still have been able to survive in more moderate conditions because experiments show that O. (Alcolapia) can be conditioned to tolerate them (Wood et al., 2002).

Consequently, the absence of accompanying flora and fauna from the sites in which †Rebekkachromis has been found is wholly compatible with what is known from modern soda lakes. Therefore, stable soda conditions are likely to have been in place long before the death of these fish. Moreover, it appears more plausible that a disruption of the prevailing conditions in a soda lake (e.g., acidification brought about by ash falls from volcanic eruptions; see Rasmussen et al., 2017), rather than the induction of soda conditions, should have caused mass die-offs of fishes.

Alkaline Environment and Fish Size—Some morphological features of O. (Alcolapia) have been proposed to be related to the particular conditions of soda lakes. The ‘small’ size (80–100 mm) of fossil cichlids from the Tugen Hills has been suggested as a proxy for the alkalinity of the paleolakes (Bishop and Chapman, 1970; Bishop and Pickford, 1975). However, Trewavas (1983) rejected the notion that the small size of O. (Alcolapia) (SL≈ 40–80 mm, Trewavas, 1983, Seegers and Tichy, 1999) could be the result of the soda condition itself. Although there is no doubt that the size of O. (Alcolapia) is controlled largely by environmental factors, alkalinity and salinity are only two among several such factors. For example, when A. grahami was introduced into the soda Lake Nakuru in the 1950s and 1960s, it very quickly reached sizes up to twice those observed in its native Lake Magadi, although these two lakes have a similar pH value of about 10 (Vareschi, 1979; Trewavas, 1983). Hence, this marked change in size argues that alkalinity alone cannot determine size; other factors, e.g., temperature, or also the size of the lake, must be at work (Trewavas, 1983). Reduced predation has also been hypothesized to account for the growth of A. grahami to larger sizes (Maina et al., 2019). Therefore, the size alone of the fish inhabiting a paleolake cannot be regarded as a proxy for the alkalinity of the water.

This is additionally corroborated by fossil cichlids, possibly comparable to Oreochromis or O. (Alcolapia) grahami, which have been reported from the area around Lake Magadi. They have an early Holocene age, a time when the extreme soda conditions of today were not yet in place (White, 1953; Butzer et al., 1972; Whitehead, in Trewavas, 1983; Tichy and Seegers, 1999). These fossil cichlids were reported to reach greater sizes than modern O. (Alcolapia) (ca. 100 mm SL, Whitehead, in Trewavas, 1983:384; Tichy and Seegers, 1999) and this size is comparable to that of many cichlids from the soda paleolakes of Tugen Hills.

Alkaline Environment and Dentition—Species of O. (Alcolapia) are predominantly herbivorous and it seems that the depauperate macrodiversity of soda lakes prevents the evolution of other trophic adaptations (Ford et al., 2016). On the other hand, species of O. (Alcolapia) display an oral dentition (Tichy and Seegers, 1999) which is very different from that observed in some specialized feeders e.g., insectivorous cichlids (Fryer and Iles, 1972), but is reminiscent of the variable oral dentition of †Rebekkachromis (completely tricuspid, unicuspid, or mixed). On the other hand, there is little diversity in the pharyngeal teeth and jaw of O. (Alcolapia), presumably because once acquired, their food is of similar size and consistency and no further specialization is needed (Tichy and Seegers, 1999; Ford et al., 2016). Their pharyngeal teeth are ‘kukri’ unicuspid to hooked bicuspid (Tichy and Seegers, 1999; Seegers and Tichy, 1999) and †Rebekkachromis has a similar pharyngeal dentition. The lack of large flat molariform pharyngeal teeth that are more suitable for prey such as gastropods (Fryer and Iles, 1972) may be another indicator of the absence of such organisms from the soda paleolakes of the Ngorora Formation.

A Nascent Species Flock?

The propensity of cichlids to form species flocks, especially in lakes, has been studied extensively (e.g., Greenwood, 1984; Salzburger and Meyer, 2004), with the most iconic examples of cichlid species flocks being those of the Haplochromini of the Lakes Malawi and Victoria, with hundreds of species each. Members of the Oreochromini have also formed species flocks, especially in smaller lakes, e.g., the cichlids of the crater Lake Barombi Bo (Schliewen et al., 1994; Schliewen and Klee, 2004), or the modern alkaliphile cichlids of Lake Natron and Lake Magadi (e.g., Trewavas, 1983; Tichy and Seegers, 1999; Ford et al., 2015).

Lecointre et al. (2013) summarized the work of Ribbink (1984), Greenwood (1984) and Eastman and McCune (2000) and proposed five criteria to detect species flocks. The first three criteria (species diversity, endemicity, monophyly) were considered core characteristics of a species flock, whereas the other two criteria (habitat dominance in terms of biomass; ecological diversity) characterize a ‘full flock’ (Lecointre et al., 2013). Below, these criteria are considered for the cichlids from the Yatianin site.

Species Diversity—There are at least two †Rebekkachromis species known from Yatianin, and the discovery of more complete material would possibly allow the description of additional species. The presence of individuals which are similar but not identical to the named species indicates that the criterion about species diversity is at least partially satisfied.

Endemicity—†Rebekkachromis seems to be endemic for the middle to late Miocene of the Tugen Hills; however, the absence of sediments of the same age from other areas needs to be considered.

Monophyly—Species of †Rebekkachromis share a unique combination of features which are not usual among haplotilapiines (preopercle with three sensory canal pores on the lower arm, scales of the nape minute, urohyal lacking anterior spine), which points to a common ancestry.

Habitat Dominance—The absence of other macrofauna combined with the excellent preservation of cichlids indicate that in terms of animal or at least vertebrate biomass †Rebekkachromis was dominating its environment.

Ecological Diversity—†Rebekkachromis has a variable oral dentition (exclusively tricuspid, tricuspid + unicuspid) which might point to different food acquisition strategies. †Rebekkachromis is also diverse with regard to the fusion between hypurals of the caudal fin. The caudal fin is mainly involved in propulsion, but if and how the fusion between the hypural plates can be related to function has not yet been explored for cichlids.

In conclusion, †Rebekkachromis spp. from Yatianin could represent an early stage of differentiation and the idea that this assemblage represents a species flock in nascent state needs to be researched further. It is not always possible to establish endemicity in paleontology, but the criteria concerning monophyly and ecological diversity could be examined in future research. Species flocks “in the making” have been reported previously for killi-fishes from the upper Miocene of the Tugen Hills (Altner and Reichenbacher, 2015).

Origin and Dispersal Scenarios of †Rebekkachromis

As mentioned above (‘Geochemistry of alkaline lakes'), both tectonism and volcanism can be responsible for the genesis of alkaline lakes. Tectonism and associated volcanism proceeded in East Africa with a general north to south direction (e.g., Macgregor, 2015). Volcanism along the eastern branch of the EARS began in the Turkana region in northern Kenya during the Eocene (ca. 35–40 Ma) (Furman et al., 2006), and reached other parts of northern Kenya (Morley et al., 1992), as well as the central Ethiopian Plateau, in the Oligocene (ca. 30 Ma) (Hofmann et al., 1997). In the early-to-middle Miocene (ca. 17–15 Ma), the central part of the Kenya Rift, where the Tugen Hills are situated, became volcanically active (Hill, 2002; Macgregor, 2015), and the Ngorora Formation documents the existence of alkaline lakes shortly afterward. In northern Tanzania, volcanism began in the late Miocene (ca. 8 Ma) (Dawson, 1992), and in the area of modern-day Lake Natron and Lake Manyara volcanic activity set in about 5 Ma (Foster et al., 1997). The lower Pleistocene Humbu and Moinik formations, deposited between 1.7 and 1.2 Ma, show evidence for the existence of alkaline lakes at that time (Dawson, 1992).

Because of the direction of volcanism, a north to south direction for the migration of alkaliphile cichlids in those areas is possible. As described above, cichlid fishes in alkaline lakes were present in the Tugen Hills since the middle Miocene (Bishop and Pickford, 1975; van Couvering, 1982; Renaut et al., 1999; Rasmussen et al., 2017). Consequently, alkaliphile cichlids could have evolved several million years before the formation of the Magadi-Natron-Manyara lake basins and much farther north. As the youngest part of the Ngorora Formation, known from the site Waril (9–10 Ma), revealed a different cichlid fauna (Altner et al., 2017, 2020; Altner and Reichenbacher, 2020), it seems possible that †Rebekkachromis became extinct in the Tugen Hills about 10 Ma.