Evidence for the existence and diversification of a new group of cichlids in the Miocene lakes of the Tugen Hills, Kenya
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1
Department of Earth and Environmental Sciences, Faculty of Geosciences, Ludwig Maximilian University of Munich, Germany
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2
Bavarian State Collection of Zoology, Germany
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3
GeoBio-Center, Ludwig Maximilian University of Munich, Germany
Cichlid fishes have produced some of the largest known evolutionary radiations of vertebrates, with more than 1700 species having been identified to date. Most cichlid species are found in freshwater habitats in the tropics, and are thought to have originated in the Cretaceous Period. They are characterized by an extraordinary propensity for rapid, adaptive radiation, especially when they find themselves in lakes. The cichlid radiations in the great lakes of East Africa, i.e. Tanganyika, Malawi and Victoria, are iconic examples of this phenomenon.
However, all the known examples of major evolutionary radiations in cichlids have been identified on the basis of modern faunas. Little is known about their past diversity, especially in Africa. This is largely due to the paucity of suitable sediments on the African continent, e.g. from lakes in which the delicate skeletons of cichlids can be preserved as fossils.
In order to tackle this issue, three excavation campaigns were undertaken in 2011, 2013 and 2014 in the Tugen Hills area, Kenya, which is located within the eastern branch of the East African Rift System. The area was selected because it is located close to the hotspot of modern cichlid biodiversity, and is known for the excellent preservation of its fossils. The area is rich in fine-grained, finely stratified lake sediments, representing ancient lake basins dating to the middle to upper Miocene. These basins were filled with sediments resulting from the tectonic activity of the eastern branch of the East African Rift System, with the entire lacustrine succession spanning several hundreds of meters. At least 10 new fossil-bearing localities were discovered in the area. They furnished numerous fish fossils, notably cichlids, whose preservation quality ranges from good to exceptional. As a result, sediments from this area are now characterized as a conservation and concentration Lagerstätte (Rasmussen et al. 2017).
Here we focus on the cichlid fossils from two of the new localities in the Tugen Hills and compare them with previously published fossils from Africa, as well as modern African cichlids. The first new site, Yatianin, is dated to about 12.1 million years. The material comprises complete and well-preserved skeletons of more than 20 individual cichlids. The other locality, Terenin, is dated to about 13 million years. More than 10 individuals were collected from this locality. This material was even better preserved than those at Yatianin, but most of the skeletal elements were disassociated from each other.
The fossils from the new sites were prepared under a stereoscopic microscope using fine carbide needles (0.17–0.7 mm in diameter). Unlike the case for many fish fossils, the contrast between bones and surrounding sediment was markedly high when viewed under X-rays, permitting the application of microCT technology to the best preserved specimens. MicroCT was carried out on a phoenix nanotom m at the facilities of the Bavarian Natural History Collections in Munich. The data were analysed and imaged with the software Amira.
On the basis of the microCT data, several delicate structures, some never before observed in fossil cichlids, were revealed in great detail (Figure 1). The study of these characters enabled us to establish a number of similarities and differences between the cichlids so far recovered from the Tugen Hills. Many individuals possess tricuspid teeth in the outer and inner rows of their oral dentition. Others exhibit shouldered or conical unicuspid dentition, or a mixture of tricuspid and unicuspid oral teeth. Some specimens possess one and others two supraneural bones. The hypural plates 1-4 of the caudal fin show different combinations of fusion between them. However, there are also characters that are shared by all individuals from Yatianin and Terenin: the urohyal is slender and lacks an anterodorsal projection, the pharyngeal dentition is unicuspid, the scales are cycloid and the preopercle bears six lateral-line foramina, three on the upper and three on the lower branch.
Based on these differences among the best preserved specimens, at least three species can be recognized in the sample from Yatianin. They were compared with other modern and extinct cichlids and found to be unique, thus constituting new species. Interestingly, the above-mentioned similarities that are shared by the cichlids from Yatianin and Terenin seem to be shared as well with cichlids from two previously described localities which are relatively close in time and in space to Yatianin: †Rebekkachromis (about 11 million years, see Kevrekidis et al. 2019) and †“Sarotherodon” martyni (about 12 million years, see Van Couvering 1982).
Based on the presence of tricuspid teeth in the inner rows of their dentition, the cichlids from the Tugen Hills are hypothesized to belong to the haplotilapiines, the most diverse clade of African cichlids. Nevertheless, the systematic placement of this group beyond this level remains uncertain, as no known modern tribe of African cichlids exhibits the combination of features described above.
In conclusion, it seems that during the middle to upper Miocene, the lakes in the Tugen Hills abounded in representatives of a previously unknown group of cichlids that possibly diversified in this area. These observations raise a number of questions related to the evolution of this group, such as their systematic relationships with the modern haplotilapiines, their geographic distribution, as well as the timing, the places and causes of their emergence and extinction. Further efforts to investigate the simultaneous presence of other cichlid groups in the same area (Altner et al. 2017) may elucidate whether the lakes of the Tugen Hills became a "melting pot" of different lineages. As more material from these expeditions is currently under study, it is conceivable that at least some of these questions might be answered in the foreseeable future.
Legend:
Figure 1. The specimen OCO 2014-11-13 from the locality of Yatianin. (A) Photograph of the right side, seen medially and (B) microCT visualization of the left side, seen laterally.
References
Altner, M., Schliewen, U. K., Penk, S. B. R., and Reichenbacher, B. (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: e1297819. doi:10.1080/02724634.2017.1297819.
Kevrekidis, C., Valtl, M., Penk, S. B. R., Altner, M., and Reichenbacher, B. (2019). 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. doi:10.1007/s10750-018-3715-8.
Rasmussen, C., Reichenbacher, B., Lenz, O., Altner, M., Penk, S. B. R., Prieto, J., and Brüsch, D. (2017). Middle–late Miocene palaeoenvironments, palynological data and a fossil fish Lagerstätte from the Central Kenya Rift (East Africa). Geological Magazine 154, 24–56. doi:10.1017/s0016756815000849.
Van Couvering, J. A. H. (1982). Fossil cichlid fish of Africa. London: Special Papers in Paleontology 29: 1–103.
Keywords:
microCT,
3D paleontology,
Pseudocrenilabrinae,
tricuspid teeth,
Radiation,
East African Rift System,
African Great Lakes
Conference:
XVI European Congress of Ichthyology, Lausanne, Switzerland, 2 Sep - 6 Sep, 2019.
Presentation Type:
Oral
Topic:
MORPHOLOGY, ONTOGENY AND PALAEONTOLOGY
Citation:
Kevrekidis
C,
Ruthensteiner
B and
Reichenbacher
B
(2019). Evidence for the existence and diversification of a new group of cichlids in the Miocene lakes of the Tugen Hills, Kenya.
Front. Mar. Sci.
Conference Abstract:
XVI European Congress of Ichthyology.
doi: 10.3389/conf.fmars.2019.07.00074
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Received:
29 May 2019;
Published Online:
14 Aug 2019.
*
Correspondence:
Mx. Charalampos Kevrekidis, Department of Earth and Environmental Sciences, Faculty of Geosciences, Ludwig Maximilian University of Munich, München, Germany, xaralamboskevrekidis@yahoo.gr
Mx. Bettina Reichenbacher, Department of Earth and Environmental Sciences, Faculty of Geosciences, Ludwig Maximilian University of Munich, München, Germany, b.reichenbacher@lrz.uni-muenchen.de