Monday, February 19, 2018

[PaleoIchthyology • 2017] Tugenchromis pickfordi • A Stem-group Cichlid of the ‘East African Radiation’ from the upper Miocene of central Kenya


Tugenchromis pickfordi
 Altner, Schliewen, Penk & Reichenbacher, 2017


ABSTRACT
The highly diverse tropical freshwater fish family Cichlidae is sparsely represented in the fossil record. Here we describe the new cichlid †Tugenchromis pickfordi, gen. et sp. nov., from the Upper Miocene (9–10 Ma) of central Kenya. The new taxon possesses a unique combination of characters, including six lateral line foramina on the lacrimal, three lateral line segments, cycloid scales, and a low number of vertebrae (29), dorsal fin spines (13), and dorsal soft rays (9). Its lacrimal morphology and tripartite lateral line suggest an affinity with the present-day Lake Tanganyika tribes Ectodini and Limnochromini, and thus with members of the ‘East African Radiation’ among the African cichlids. To further elucidate the relationships of †T. pickfordi, we used a comprehensive comparative data set comprising meristic data from all present-day tribes of the ‘East African Radiation.’ Principal coordinates analyses support links between the fossil and Ectodini + Limnochromini, and additionally with modern Haplochromini. We conclude that †T. pickfordi could be an extinct lineage within the ‘most ancient Tanganyika tribes,’ or a stem lineage of the ‘ancient Tanganyika mouthbrooders.’ A direct relationship to the Haplochromini is unlikely because its members do not exhibit the derived characteristics of the lacrimal as seen in †T. pickfordi. Because Lake Tanganyika is located in the western branch of the East African Rift System, †T. pickfordi from the eastern branch supports the ‘melting-pot Tanganyika hypothesis,’ which posits that the cichlids of modern Lake Tanganyika are derived from riverine lineages that had already diversified prior to the lake formation.

FIGURE 2. †Tugenchromis pickfordi, gen. et sp. nov. A1–A2, holotype in part (OCO-5-35) and counterpart (OCO-5-22); A3, right lateral view of the specimen (shading refers to ribs from the left side of the specimen);

Abbreviations: cl, cleithrum; cor, coracoid; ep, epural; hs, hemal spine; hyp, hypural plate; lac, lacrimal; nlc, neurocranial lateral line canal; ns, neural spine; o, otolith; op, operculum; ph, parhypural; pha, pharyngeal teeth; ppc, postcleithrum; ptt, posttemporal; pu, preural centrum; rad, radials; sca, scapula; scl, supracleithrum; sop, suboperculum; us, urostyle; un1, uroneural 1; = , tubular lateral line scale; °, pitted lateral line scale. 

SYSTEMATIC PALEONTOLOGY
CICHLIDAE Bonaparte, 1835
PSEUDOCRENILABRINAE Fowler, 1934

TUGENCHROMIS, nov. gen.

Generic Diagnosis: Lateral line on the trunk divided into three segments, two of which are posterior lateral lines. One posterior segment positioned ventrally, the other dorsally to the anterior lateral line segment. This is a condition not seen in any other cichlid genus.

Etymology: Tugen’ refers to the ‘Tugen Hills’ (named after the local people, i.e., the ‘Tugen,’ a subgroup of the Kalenjin ethnic group), in which the type locality of the new fossil taxon is located. The Greek word ‘Chromis’ (χρόμις) is a name used by the Ancient Greek and was applied to various fish. It is a common second element in cichlid genus names. Tugenchromis is masculine.

Type Species: Tugenchromis pickfordi, sp. nov.

TUGENCHROMIS PICKFORDI, sp. nov.


Holotype: OCO-5-22/35, partially complete skeleton in part and counterpart (Fig. 2A1–A3), approximately 60 mm total length, 33.5 mm body length.

Etymology: Species named in honor of the paleontologist Martin Pickford in recognition of his outstanding contributions to the geology and paleontology of East Africa.

Locality, Horizon, and Age: Outcrop Waril in Central Kenya; Ngorora Formation, Member E; late Miocene (9–10 Ma) (see Rasmussen et al., 2017).


CONCLUSION: 
Based on lacrimal morphology and meristic data derived from all present-day cichlids of the ‘East African Radiation,’ we propose that the newly discovered cichlid fossil from the upper Miocene of Central Kenya either represents a stem lineage of the ‘ancient Tanganyika mouthbrooders’ or an extinct lineage within the ‘most ancient Tanganyika tribes.’ This result implies that the use of a comprehensive set of comparative material derived from extant cichlids may make it possible to phylogenetically place other fossil cichlids with greater confidence in future studies.

Apart from a lower Miocene cichlid from Uganda (‘cf. Pelmatochromis spp.’), none of the previously described fossil cichlid taxa from Africa, Arabia, and Europe possess distinctive similarities to †T. pickfordi. This indicates that the Ngorora fish Lagerstätte in Central Kenya may provide an unrivalled window into the evolutionary history of African cichlids, particularly into the origin of the ‘East African Radiation,’ i.e., the megadiversity of the present-day cichlids in Lake Tanganyika, Lake Malawi, and Lake Victoria.

Furthermore, the new fossil provides additional support for the presence of an ancient east-west connection (e.g., proto-Malagarasi River) between the Central Kenya Rift and Lake Tanganyika, which is consistent with previous assumptions regarding the hydrological networks across East and Central Africa during the Miocene.


Melanie Altner, Ulrich K. Schliewen, Stefanie B. R. Penk and Bettina Reichenbacher. 2017 . †Tugenchromis pickfordi, gen. et sp. nov., from the upper Miocene—A Stem-group Cichlid of the ‘East African Radiation’. Journal of Vertebrate Paleontology. 37(2); e1297819. DOI:  10.1080/02724634.2017.1297819

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