Dialipina

Abstract: Ray-finned bony fish (ranging from cod to guppies) and lobe-finned bony fish (lungfish, coelacanths and the ancestors of the land vertebrates) were clearly once close relatives, but a large morphological gulf still separates fossils of both groups dating from times when they looked much more similar than they do now. A fossil of a primitive lobe-finned fish that combines many features found in both groups has now been discovered in the Yumman fossil beds in China, and this new species offers a bridgehead between the two distinct lines of fishy descent. A primitive fossil species of lobe-finned fish combines many histological features found in both ray-finned bony fishes and lobe-finned bony fishes. Osteichthyans, or bony vertebrates, include actinopterygians (teleosts and their relatives) and sarcopterygians (coelacanths, lungfishes and tetrapods). Despite features found in basal actinopterygians (for example, Dialipina and Ligulalepis)1,2,3 and basal sarcopterygians (for example, Psarolepis and Achoania)4,5, the morphological gap between the two lineages remains wide and how sarcopterygians developed a dermal surface covering known as cosmine (composed of a pore–canal network and a single layer of odontodes and enamel) is still poorly known6,7,8,9,10. Here we describe a primitive fossil fish, Meemannia eos gen. et sp. nov., that possesses an actinopterygian-like skull roof and a cosmine-like dermal surface combining a pore–canal network (found in various fossil sarcopterygians) with superimposed layers of odontodes and enamel (previously known in actinopterygians and some acanthodians11,12,13). This 405-million-year-old fish from the Lower Devonian of Yunnan (China) demonstrates that cosmine in many fossil sarcopterygians arose step by step through the acquisition of a pore–canal network followed by the subsequently developed ability to resorb previous generations of odontodes and enamel. Meemannia provides key characters for studying deep osteichthyan phylogeny and indicates a possible morphotype for the common ancestor of actinopterygians and sarcopterygians.