A team of Ludwig-Maximilians-Universitaet (LMU) in Munich has identified the first fossil specimens¹ of a major group of killifishes that is widely distributed in freshwater habitats today. The 6-million-year-old material sheds new light on the evolution of the bony fishes. Killifish are true survivors². These colorful little fish are perfectly³ adapted to the demands of their ephemeral habitats. They spend their short lives in temporary freshwater pools that form during the rainy season, and owe their long-term survival to the fact that their eggs are resistant⁴ to desiccation. Although they are a species-rich group, and are widely distributed in the tropics and subtropics, their fossil record is sparse⁵. But now LMU palaeontologists Professor Bettina Reichenbacher and Melanie Altner have identified the first fossil representatives of one of the two extant suborders of killifish. "The specimens are exceptionally well preserved, date from about 6 million years ago, and were discovered in Kenya by French palaeoanthropologists," says Reichenbacher. "Our studies have now shown that they are members of a previously⁶ unknown genus that is now extinct, which we have named Kenyaichthys -- the fish from Kenya."

 

A cache that includes 77 complete specimens

 

The fossils originate from a site located in the Tugen Hills, which lie in the Eastern arm of the East African Rift⁷ Valley. During the Late Miocene -- about six million years ago -- the site formed part of a lake, and the newly described specimens, each 2 to 4 cm long, were preserved in the sediment⁸ beds that accumulated on the lake bottom. "The sample comprises a total of 169 individuals, and 77 of these are complete," says Altner. The anatomical details discernible in the impressions left in the sediments⁹ enabled the two researchers to conclusively¹⁰ identify all of these individuals as killifishes. "Analysis of the structures of the tailfin, the pelvic fins¹¹ and the bones in the skull¹², in particular, yielded crucial information that convinced us that this material constituted the first fossils attributable to the killifish Suborder Aplocheiloidei. This group also encompasses¹³ modern African killifishes, such as Pachypanchax from Madagascar, the striped panchaxes of Southeast Asia and the rivulids of South America," Altner explains.

 

In addition to the fossil aplocheiloids, only a few other freshwater forms were found at the site. Reichenbacher and Altner assume that the prevailing¹⁴ environmental conditions were too extreme for less specialized¹⁵ species. During the Late Miocene, the climate got drier and extensive areas of savannah developed. "We believe that, like modern killifish species, Kenyaichthys was well equipped to survive long periods of drought, and could cope better with such conditions than other species of fish," says Reichenbacher.

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