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The importance of articulated skeletons in the identification of extinct taxa: new fossils of cichlids from the Miocene of Kenya and clupeids from the Miocene of Greece (Teleostei)
The importance of articulated skeletons in the identification of extinct taxa: new fossils of cichlids from the Miocene of Kenya and clupeids from the Miocene of Greece (Teleostei)
Fishes are important components of aquatic faunas, but our knowledge on the fossil record of some taxa, relative to their present diversity, remains poor. This can be due to a rarity of such fossils, as is the case for the family Cichlidae (cichlids). Another impediment is the rarity of well-preserved skeletons of fossil fishes. This becomes even more problematic for taxa whose modern representatives are challenging to distinguish based on osteological data, as is the case for the cichlids and also for the representatives of the family Clupeidae (clupeids: herrings, shads and allies). Our limited understanding of the past diversity of these taxa hinders efforts to address questions regarding their evolution. In this study, new material of well-preserved and articulated skeletons of fishes belonging to the aforementioned families is presented. There are few areas in Africa with sediments that could hold fossils of freshwater fishes. Recently, however, the Ngorora Formation at the Tugen Hills area in Kenya has been recognized as a conservation and concentration Lagerstätte, furnishing complete skeletons of cichlids dating to the middle-late Miocene. For some of these fossils, the use of μCT technology allowed the imaging of anatomical details which would otherwise be hard or impossible to observe. Material from three new localities is presented: from the sites Rebekka (ca. 11 Ma), Yatianin (ca. 11 Ma) and Terenin (ca. 13 Ma). Based on this material, four new species of fossil cichlids, placed in one new genus, †Rebekkachromis, are described. In the course of this project, new data on the osteology and dentition of modern cichlids were collected and/or compiled and presented, concerning 1) the size of the oral teeth relative to their position on the jaws, 2) the number of sensory canal pores on the preopercle, 3) the number of lateral-line tubules on the lacrimal, 4) the number of supraneural bones and 5) the fusion pattern of hypural plates. The results of these investigations corroborate the use of the above-mentioned characters for taxonomic purposes, as the intraspecific variation is very low. At the same time, they allowed the examination of the systematic relationships of the fossil fishes from the Tugen Hills. The results regarding the placement of oral teeth in the mouth helped establish that †Rebekkachromis is a haplotilapiine genus. The most notable and unexpected result from these investigations, however, was that the species of the modern subgenus of alkaliphile cichlids Oreochromis (Alcolapia) have three sensory canal pores on the lower branch of their preopercle, as does †Rebekkachromis, unlike any other haplotilapiine known to date. This new evidence, together with other meristic and morphological characters (e.g. the minute scales on the nape, throat and belly), indicates a strong resemblance of †Rebekkachromis to Oreochromis (Alcolapia). This observation provided further support to the idea that the middle-upper Miocene sediments of Tugen Hills hold not only the earliest haplotilapiine, but more precisely the earliest oreochromine cichlids. Because of the high concentration of fishes in the Tugen Hills, we can make inferences not only about individual species, but also about the communities of the fishes that lived in those lakes. For instance, the consistent absence of accompanying fauna and flora in the examined sites, together with the geological context of the Tugen Hills, indicate that †Rebekkachromis was living in alkaline lakes. The modern alkaliphile cichlids of the subgenus Oreochromis (Alcolapia) live further south, close to the border of Kenya with Tanzania. As tectonism and volcanism proceeded with a north-to-south direction along the East African Rift System, it makes sense that alkaliphile cichlids developed in the same direction. Furthermore, the genus †Rebekkachromis seems to have included a diverse morphogroup of related cichlid species. The occurrence of different species, and/or intermediate forms between those species, living in the same paleolakes may indicate the presence of species flocks. The paleolakes in the Tugen Hills seem to have favored the development of species flocks, as attested by the presence of other possible species flocks in the upper Miocene of the same area. The last section of this Thesis concerns fishes from a different continent and family, but within a similar timeframe. Well-preserved skeletons of clupeids were recovered from a new terrestrial locality, dating to the late Miocene of the Serres Basin in Northern Greece. These fossils represent a new species that cannot be attributed to any of the modern clupeid genera, but which has the least differences with Hilsa, a genus known from the Indo-West Pacific. The new fossil species was tentatively attributed to the fossil genus †Pseudohilsa, known from the middle Miocene of Azerbaijan. †Pseudohilsa, together with some other fossils attributed to “Hilsa” from the middle Pliocene of Abkhazia in the Black Sea region, might indicate that fishes similar to the modern-day genus Hilsa were once living in the Eastern Paratethys realm and its successors. The new fossil species from Greece are therefore the first Hilsa-lookalikes known from the Mediterranean Basin. The results of this Thesis highlight the importance of localities which furnish well-preserved and articulated skeletons, which are necessary to investigate the systematics of fossil fishes. The new osteological data on modern cichlids presented in this study demonstrated the potential of morphological characters to inform us about the relationships of fossil fishes. This work enriches our knowledge of the past diversity of African cichlids and clupeids from the Balkans, with the description of new taxa and inferences about their environment and distribution.
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Kevrekidis, Charalampos
2021
English
Universitätsbibliothek der Ludwig-Maximilians-Universität München
Kevrekidis, Charalampos (2021): The importance of articulated skeletons in the identification of extinct taxa: new fossils of cichlids from the Miocene of Kenya and clupeids from the Miocene of Greece (Teleostei). Dissertation, LMU München: Faculty of Geosciences
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Abstract

Fishes are important components of aquatic faunas, but our knowledge on the fossil record of some taxa, relative to their present diversity, remains poor. This can be due to a rarity of such fossils, as is the case for the family Cichlidae (cichlids). Another impediment is the rarity of well-preserved skeletons of fossil fishes. This becomes even more problematic for taxa whose modern representatives are challenging to distinguish based on osteological data, as is the case for the cichlids and also for the representatives of the family Clupeidae (clupeids: herrings, shads and allies). Our limited understanding of the past diversity of these taxa hinders efforts to address questions regarding their evolution. In this study, new material of well-preserved and articulated skeletons of fishes belonging to the aforementioned families is presented. There are few areas in Africa with sediments that could hold fossils of freshwater fishes. Recently, however, the Ngorora Formation at the Tugen Hills area in Kenya has been recognized as a conservation and concentration Lagerstätte, furnishing complete skeletons of cichlids dating to the middle-late Miocene. For some of these fossils, the use of μCT technology allowed the imaging of anatomical details which would otherwise be hard or impossible to observe. Material from three new localities is presented: from the sites Rebekka (ca. 11 Ma), Yatianin (ca. 11 Ma) and Terenin (ca. 13 Ma). Based on this material, four new species of fossil cichlids, placed in one new genus, †Rebekkachromis, are described. In the course of this project, new data on the osteology and dentition of modern cichlids were collected and/or compiled and presented, concerning 1) the size of the oral teeth relative to their position on the jaws, 2) the number of sensory canal pores on the preopercle, 3) the number of lateral-line tubules on the lacrimal, 4) the number of supraneural bones and 5) the fusion pattern of hypural plates. The results of these investigations corroborate the use of the above-mentioned characters for taxonomic purposes, as the intraspecific variation is very low. At the same time, they allowed the examination of the systematic relationships of the fossil fishes from the Tugen Hills. The results regarding the placement of oral teeth in the mouth helped establish that †Rebekkachromis is a haplotilapiine genus. The most notable and unexpected result from these investigations, however, was that the species of the modern subgenus of alkaliphile cichlids Oreochromis (Alcolapia) have three sensory canal pores on the lower branch of their preopercle, as does †Rebekkachromis, unlike any other haplotilapiine known to date. This new evidence, together with other meristic and morphological characters (e.g. the minute scales on the nape, throat and belly), indicates a strong resemblance of †Rebekkachromis to Oreochromis (Alcolapia). This observation provided further support to the idea that the middle-upper Miocene sediments of Tugen Hills hold not only the earliest haplotilapiine, but more precisely the earliest oreochromine cichlids. Because of the high concentration of fishes in the Tugen Hills, we can make inferences not only about individual species, but also about the communities of the fishes that lived in those lakes. For instance, the consistent absence of accompanying fauna and flora in the examined sites, together with the geological context of the Tugen Hills, indicate that †Rebekkachromis was living in alkaline lakes. The modern alkaliphile cichlids of the subgenus Oreochromis (Alcolapia) live further south, close to the border of Kenya with Tanzania. As tectonism and volcanism proceeded with a north-to-south direction along the East African Rift System, it makes sense that alkaliphile cichlids developed in the same direction. Furthermore, the genus †Rebekkachromis seems to have included a diverse morphogroup of related cichlid species. The occurrence of different species, and/or intermediate forms between those species, living in the same paleolakes may indicate the presence of species flocks. The paleolakes in the Tugen Hills seem to have favored the development of species flocks, as attested by the presence of other possible species flocks in the upper Miocene of the same area. The last section of this Thesis concerns fishes from a different continent and family, but within a similar timeframe. Well-preserved skeletons of clupeids were recovered from a new terrestrial locality, dating to the late Miocene of the Serres Basin in Northern Greece. These fossils represent a new species that cannot be attributed to any of the modern clupeid genera, but which has the least differences with Hilsa, a genus known from the Indo-West Pacific. The new fossil species was tentatively attributed to the fossil genus †Pseudohilsa, known from the middle Miocene of Azerbaijan. †Pseudohilsa, together with some other fossils attributed to “Hilsa” from the middle Pliocene of Abkhazia in the Black Sea region, might indicate that fishes similar to the modern-day genus Hilsa were once living in the Eastern Paratethys realm and its successors. The new fossil species from Greece are therefore the first Hilsa-lookalikes known from the Mediterranean Basin. The results of this Thesis highlight the importance of localities which furnish well-preserved and articulated skeletons, which are necessary to investigate the systematics of fossil fishes. The new osteological data on modern cichlids presented in this study demonstrated the potential of morphological characters to inform us about the relationships of fossil fishes. This work enriches our knowledge of the past diversity of African cichlids and clupeids from the Balkans, with the description of new taxa and inferences about their environment and distribution.