Fossile Cichlidae (Perciformes, Teleostei) der Ngorora Fisch-Lagerstätte in den Tugen Hills (Kenia): Schlüssel zur Rekonstruktion der Binnengewässernetze im Miozän von Ost-Afrika
Zusammenfassung der Projektergebnisse
The middle to upper Miocene Ngorora Formation (c. 13.3-9 Ma) comprises about 200 m volcaniclastic deposits and covers an area of c. 15 km2 in the mountainous Tugen Hills in the Central Kenya Rift (East Africa). Within the project, nine fossiliferous sections of the Ngorora Formation were discovered. The sections exposed tuffaceous claystones, silts and marls and yielded a total of about 500 fish fossils; apart from fish fossils only a few fossil leaves and a single fossil insect were found. All sections were mapped, sampled for sedimentological and palynological analyses, and the fossil fish assemblages were investigated. The outcome of the sedimentological and palynological studies indicate the presence of floodplains, palaeosols and alkaline lakes in a wooded grassland or grassy woodland, with the environment becoming more dynamic and progressively drier through time. The numerous fish fossils demonstrate a unique combination of Concentration- and Conservation Lagerstätte, which has led to the introduction of the 'Ngorora fish Lagerstätte'. The taphonomic study indicates occasional mass kills of fishes that may be attributable to rapid water acidification and/or asphyxiation by episodic ash falls from the Kaption Volcano nearby. Overall, the preservation of the Ngorora fish Lagerstätte results from the interplay of tectonics, formation of alkaline lakes and explosive volcanism. The new palaeoenvironmental data is also relevant to models of human evolution in East Africa as (i) records of middle Miocene (13–12 Ma) grassy savannah in the Central Kenya Rift are rare, and (ii) catastrophic ash falls that resulted in mass killings of fishes must also have affected the earliest hominids. All fossil fishes recovered from the middle to upper Miocene Ngorora Formation belonged to the family Cichlidae, today a highly diverse group of tropical freshwater fishes, which made it difficult to precisely assign the fossils to higher systematic levels (tribes, genera). The difficulties were also rooted in the fact that even distantly related cichlid species may be morphologically very similar to each other. This explains also why only about 20 fossil cichlid species from Africa have been formally described before this project had started. To determine whether or not the newly collected fossils could be assigned to any of the extant cichlid lineages, the ‘best-fit approach’ has been developed. It included comparison of the new fossil material with all the relevant modern lineages of cichlids using microscopy, X-raying and computed microtomography (μCT). On that basis, five new fossil genera, each represented with one to four species could be recognized, and each of those could be related to an extant lineage. From the older part of the Ngorora Formation (members C and D, 12.5-10.0 Ma), two new genera of the modern tribe Oreochromini were presented (+Oreochromimos Penk, Altner, Cerwenka, Schliewen & Reichenbacher, 2019, with one species, and +Rebekkachromis Kevrekidis, Valtl & Reichenbacher, 2019, with four species). It was shown that +Oreochromimos and +Rebekkachromis reveal a high similarity to the extant cichlids Oreochromis (Alcolapia) and Oreochromis amphimelas, which are the only known modern cichlid fish found in Africa that can survive in extremely alkaline environments. This suggests that alkaliphile cichlids similar to Oreochromis (Alcolapia) were present in Central Kenya about 10-12.5 Ma ago and that the ability of African cichlid fishes to thrive in highly alkaline waters had already developed by that time. A different cichlid assemblage was identified from the youngest part of the Ngorora Formation (member E, 10.0-9 Ma). It comprises three new genera, of which one represents a member of the ‘East African Radiation’ among the African cichlids (+Tugenchromis pickfordi Altner, Schliewen, Penk & Reichenbacher, 2017), while the two other ones were assigned to the Haplochromini (+Warilochromis unicuspidatus Altner, Ruthensteiner & Reichenbacher, 2020 and +Baringochromis Altner & Reichenbacher, 2020, represented with three species). Due to its fang-like dentition and non-fusiform body, +W. unicuspidatus must have had a predatory lifestyle. The presence of haplochromine fossils in an upper Miocene palaeolake in the Central Kenya Rift indicate that Haplochromini were already an important component of freshwater drainages in East Africa at that time. In addition, the age of these fossils (9-10 Ma) is incompatible with the ‘out of Tanganyika’ hypothesis, which postulates that the divergence of the Haplochromini began only 5-6 Ma. A sample of fossil fishes comprising a total of 169 individuals, of which 77 were complete, was additionally studied from the upper Miocene Lukeino Formation in the Tugen Hills. This material was attributable to the killifish Suborder Aplocheiloidei and represents the first fossils of that group. The family +Kenyaichthyidae Altner & Reichenbacher, 2015, represented with ++Kenyaichthys kipkechi Altner & Reichenbacher, 2015 was introduced. +Kenyaichthys kipkechi displays a wide range of variability and represents a species flock in statu nascendi. It thus appears that species flocks in the fossil record, which have only rarely been recognized in previous work, may not be as exceptional as hitherto thought. Moreover, the presence of a killifish assemblage in the Lukeino Formation, while typical freshwater fish are extremely rare, indicates an environment in the Late Miocene of Eastern Africa that was influenced by seasonal aridity.
Projektbezogene Publikationen (Auswahl)
- †Kenyaichthyidae fam. nov. and †Kenyaichthys gen. nov. – First Record of a Fossil Aplocheiloid Killifish (Teleostei, Cyprinodontiformes). – PLoS ONE 10(4):e0123056.
Altner, M., Reichenbacher, B.
(Siehe online unter https://doi.org/10.1371/journal.pone.0123056) - Middle–late Miocene palaeoenvironments, palynological data and a fossil fish Lagerstätte from the Central Kenya Rift (East Africa). Geological Magazine, Vol. 154. 2017, Issue 1, pp. 24-56.
Rasmussen, C., Reichenbacher, B., Lenz, O., Altner, M., Penk, S.B.R., Prieto, J., Brüsch, D.
(Siehe online unter https://doi.org/10.1017/S0016756815000849) - † Tugenchromis pickfordi gen. et sp. nov. from the upper Miocene – a stem-group cichlid of the ‘East African Radiation’. Journal of Vertebrate Paleontology, 37: e1297819
Altner, M., Schliewen, U., Penk, S.B.R. & Reichenbacher, B.
(Siehe online unter https://doi.org/10.1080/02724634.2017.1297819) - New material from middle-upper Miocene of Central Kenya provides novel insights on the evolution of cichlid fishes. 15. Tagung der Gesellschaft für Ichthyologie, 12.–14. Oktober 2018, Zoologisches Institut, Friedrich-Schiller-Universität Jena [awarded with "best student presentation"]
Kevrekidis, C. & Reichenbacher, B.
- Evidence for the existence and diversification of a new group of cichlids in the Miocene lakes of the Tugen Hills, Kenya. XVI European Congress of Ichthyology, Lausanne, Switzerland, September 2–6, Abstracts, p. 96 [awarded with "best student presentation"]
Kevrekidis, C., Ruthensteiner, B. & Reichenbacher, B.
(Siehe online unter https://doi.org/10.3389/conf.fmars.2019.07.00074) - New fossil cichlid from the middle Miocene of East Africa revealed as oldest known member of the Oreochromini. Scientific Reports, 9: 10198
Penk, S.B.R., Altner, M., Cerwenka, A.F., Schliewen, U.K. & Reichenbacher, B.
(Siehe online unter https://doi.org/10.1038/s41598-019-46392-5) - Rebekkachromis nov. gen. from the middle-upper Miocene (11 MYA) of Central Kenya: The oldest record of a haplotilapiine cichlid fish. Hydrobiologia, 832: 39–64
Kevrekidis, C., Valtl, M., Penk, S.B.R., Altner, M. & Reichenbacher, B.
(Siehe online unter https://doi.org/10.1007/s10750-018-3715-8) - A small cichlid species flock from the Upper Miocene (9–10 MYA) of Central Kenya. Hydrobiologia (2020)
Altner, M., Reichenbacher, B.
(Siehe online unter https://doi.org/10.1007/s10750-020-04358-z) - New cichlid fossils from the middle-late Miocene alkaline lakes of Africa.
Journal of Vertebrate Paleontology , Vol. 40. 2020, Issue 4, e1805621.
Kevrekidis, C., Ruthensteiner, B., Cerwenka A.F., Penk, S.B.R. & Reichenbacher, B.
(Siehe online unter https://doi.org/10.1080/02724634.2020.1805621) - New haplochromine cichlid from the upper Miocene (9–10 MYA) of Central Kenya. BMC Evolutionary Biology, Vol. 20. 2020, Article number: 65.
Altner, M., Ruthensteiner, B., Reichenbacher, B.
(Siehe online unter https://doi.org/10.1186/s12862-020-01602-x)
