Fish fossils from Miocene palaeolakes in the East African Rift Valley in Kenya

Altner, Melanie (2016): Fish fossils from Miocene palaeolakes in the East African Rift Valley in Kenya. Dissertation, LMU München: Fakultät für Geowissenschaften

Abstract

Africa’s modern-day freshwater fish fauna comprises more than 3000 species, many of them endemic, and is dominated by a few teleost lineages among which the Cyprinodontiformes and the Cichlidae are especially prominent. Even though members of both groups are used as model organisms in evolutionary studies, their evolutionary history is not yet fully understood. This is in part due to their scant fossil record. The main purpose of this dissertation is to investigate the palaeodiversity of freshwater fishes on the African continent during the Miocene epoch, focusing particularly on the Lukeino and Ngorora Formations in the Central Kenya Rift. The material described in this study forms part of a collection consisting of about 650 articulated fish fossils, which were recovered from the upper Miocene Lukeino Formation and the middle–upper Miocene Ngorora Formation in the course of two field campaigns in 2013 and 2014. To provide a context for a better understanding of the new fossils of Cyprinodontiformes and Cichlidae, a comprehensive comparative dataset, including data on meristics, osteology, scale characters and otolith morphology, was assembled for extant representatives of both groups. The dataset for the Cichlidae includes all previously recognized lineages of African cichlids, and is the first of its kind designed to facilitate the phylogenetic placement of cichlid fossils. This new dataset was used to organize the taxonomic studies and to analyse the phylogenetic relationships of the fossils. The results presented here shed new light on the evolutionary history of both Cyprinodontiformes and Cichlidae, and provide new information on Miocene palaeoenvironments and hydrological networks in the Central Kenya Rift. The upper Miocene Lukeino Formation has yielded numerous well-preserved cyprinodontiform fossils of an extinct lineage of the suborder Aplocheiloidei, which represent the first fossil record of this group. On the basis of the morphological study and the phylogenetic analysis, the new taxon was assigned to the new family †Kenyaichthyidae, the new genus †Kenyaichthys and the new species †K. kipkechi sp. nov. The specimens of †K. kipkechi show wide variation in their meristic counts and morphometric traits, which is comparable to that found in recent sympatric species with variable grades of hybridization. †K. kipkechi thus presumably represents a fossil species flock in statu nascendi. The phylogenetic analysis unexpectedly places †K. kipkechi in a sister relationship to the exclusively Neotropical family Rivulidae, a probable explanation might be lack of available synapomorphies for the Rivulidae, Nothobranchiidae and Aplocheilidae. Moreover, the comparison with the recent material revealed that previously proposed apomorphic characters concerning the neural and haemal spines of the preural vertebrae in the caudal skeleton should be revised. The scarcity of other typical freshwater fishes in the Lukeino Formation and the close relationship of †K. kipkechi to the aplocheiloid families Nothobranchiidae and Rivulidae suggest a seasonally dry climate for the palaeoenvironment. Some recent members of the Nothobranchiidae and Rivulidae are known to withstand such harsh conditions due to their ability to produce desiccation-resistant eggs. The new taxon may perhaps have had a similar life cycle. In addition, deformities of the vertebral column are prominent in our sample and point to a strong influence of volcanic activity on the aquatic environment. The study of the middle–upper Miocene Ngorora fish Lagerstätte uncovered two remarkable new fossil taxa of the family Cichlidae. The first is †Protochromis pickfordi nov. gen. nov. sp. Comparative osteological and meristic studies revealed that †P. pickfordi is closely related to the present-day tribes Ectodini and Limnochromini in Lake Tanganyika. This is further supported by a Principal Coordinates Analysis based on meristic data. Due to its unique character set, which includes a tripartite lateral line and a lacrimal with six lateral line tubules, it is suggested that †P. pickfordi belongs to a precursor lineage of the ‘ancient Tanganyika mouth-brooders’. The presence of a Miocene precursor of Lake Tanganyika cichlids far outside the drainage area of the present-day Lake Tanganyika implies an ancient hydrological connection between the Central Kenya Rift and Lake Tanganyika, which supports the hypothesis that a significant portion of the genomic diversity of modern-day Lake Tanganyika cichlids is derived from riverine cichlids. The second new taxon, †Baringochromis nov. gen., is based on the analysis of 335 well-preserved fossil specimens. Its unique combination of characters, most prominently one predorsal bone, six infraorbitals including a lacrimal with four or five lateral line tubules, a partially scaled suboperculum and a low number of anal and dorsal fin spines, puts it in an intermediate position between the almost pan-African Oreochromini and the exclusively East African Haplochromini. This placement is supported by a Principal Coordinates Analysis based on meristic data. The discovery of †Baringochromis therefore suggests a scenario in which precursor lineages of the present-day Oreochromini and Haplochromini were widely distributed throughout East Africa and underwent hybridization in rivers prior to the formation of the Rift Lakes. Taken together, the data reveal that, due to their mosaic-like character sets, the new fossil cyprinodontiform and cichlid taxa cannot be confidently placed within available phylogenetic trees. This implies that the evolutionary history of these two freshwater fish groups cannot be described solely in terms of lineage splitting, and probably involved introgression and hybridization, as already suggested by studies based on molecular data for the Cichlidae. Moreover, the results highlight the value of fossil archives with high preservation quality, like the Lukeino Formation and the Ngorora fish Lagerstätte, for the understanding of past evolutionary processes.

• Dokumententyp: Dissertationen (Dissertation, LMU München)
• Keywords: fossil, teleost, Neogene, Africa, osteology
• Themengebiete: 500 Naturwissenschaften und Mathematik ; 500 Naturwissenschaften und Mathematik > 550 Geowissenschaften
• Fakultäten: Fakultät für Geowissenschaften
• Sprache der Hochschulschrift: Englisch
• Datum der mündlichen Prüfung: 27. April 2016
• 1. Bericht­erstatter:in: Reichenbacher, Bettina