New Cichlid Fossils from the Middle-Late Miocene Alkaline Lakes of Africa

Abstract

The African Cichlidae Oreochromis (Alcolapia) and Oreochromis amphimelas can survive in extremely alkaline environments and represent the only known modern alkaliphilic cichlid fish found in Africa. The presence of fossil cichlids from the Miocene of central Kenya (Tugen Hills) that are morphologically similar to Oreochromis (Alcolapia) has been noted in previous works, but the conclusions remained tentative. The purpose of this study is to examine newly discovered fossil cichlids from the Tugen Hills and to compare their osteology with that of extant Oreochromis (Alcolapia). This is performed based on a comprehensive collection of comparative material, using microscopy and computed microtomography (μCT). We provide a revised diagnosis for the genus †Rebekkachromis, and revise its systematic relationships by assigning it to the Oreochromini (rather than to the Etiini). Two new species of †Rebekkachromis are described, i.e., †R. valyricus, sp. nov., and †R. vancouveringae, sp. nov., and a morphologically diverse assemblage of cooccurring †Rebekkachromis specimens is documented. Moreover, we found that †Rebekkachromis had three sensory canal pores (instead of four) on the lower arm of the preopercle, a feature that distinguishes both the modern Oreochromis (Alcolapia) and our fossil specimens from almost all other modern African cichlid fish. Our new data indicate that alkaliphile cichlids similar to Oreochromis (Alcolapia) were present in Central Kenya about 10-13 Ma ago and that the ability of African cichlid fishes to thrive in highly alkaline waters had already developed by that time.

Major shifts in biogeographic regions of freshwater fishes as evidence of the Anthropocene epoch

“…6. In these basins, our results suggest that fossil records of Anthropocene assemblage zones may become dominated by the widely introduced species such as the European carp (C. carpio), the grass carp (Ctenopharyngodon idella), mosquito fishes (Gambusia species), tilapias (Oreochromis species), the largemouth bass (Micropterus salmoides), goldfish (C. auratus), and guppys (Poecilia reticulata), all of which are known to occur in fossil records (37)(38)(39)(40)(41)(42)(43)(44)(45)(46)(47). Most of these species were endemic to a single biogeographic region before their anthropogenic introduction; hence, the change in the fossil record will likely be noticeable.…”

“…Our results illustrate the likely changes in future paleontological records that geologists use to identify assemblage zones (i.e., the body of strata characterized by an assemblage of fossils that distinguishes it from adjacent strata (28). However, besides cranial and postcranial fossil elements that can give a relatively good taxonomic identification at the genus or species level [e.g., (35)], bones, scales, teeth, or otoliths that are the most abundant fish elements found in sediments (32,(37)(38)(39)(40)(41)(42)(43)(44)(45)(46)(47)68) can be difficult to identify (33). In this case, sedimentary ancient DNA could be a powerful proxy in the near future to track biodiversity shifts (69)(70)(71), especially for recent shifts such as those described here.…”