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 co-occurring †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.

摘要

非洲丽鱼科罗非鱼（Alcolapia）和橙胸口孵非鲫可以生存在极端碱性环境，代表唯一已知的现代嗜碱性丽鱼在非洲。在先前的工作中已经注意到，肯尼亚中部中新世（图根山）的化石丽鱼科鱼的形态与奥利奥克雷非斯（Alcolapia）的形态相似，但结论仍是暂定的。这项研究的目的是检查新发现的来自图根山的化石丽鱼科鱼，并将它们的骨学与现存的Oreochromis（Alcolapia）。这是在使用显微镜和计算机显微断层照相术（μCT）的比较材料的全面收集的基础上进行的。我们为† Rebekkachromis属提供了经过修订的诊断，并通过将其分配给Oreochromini（而不是Etiini）来修改其系统关系。描述了† Rebekkachromis的两个新种，即† R. valyricus，sp。nov。和† R. vancouveringae，sp。11月，并记录了共同出现的† Rebekkachromis标本的形态多样的组合。此外，我们发现† Rebekkachromis在前oper的下臂上有三个感觉管孔（而不是四个），这是现代人的特征。Oreochromis（Alcolapia）和几乎所有其他现代非洲丽鱼科鱼类的化石标本。我们的新数据表明，大约10至13 Ma以前，肯尼亚中部就存在类似于Oreochromis（Alcolapia）的嗜碱丽鱼科鱼，到那时非洲丽鱼科鱼在高碱性水中的繁殖能力已经得到了发展。