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  • Ecology & evolution

Published online: 31 January 2007 | doi:10.1038/nchina.2007.070131-6

Ancient fishes: A real bonehead

Kaspar Mossman

Introduction

© (2006) Nature

Around 400 million years ago, the common ancestors of the the lobe-finned fishes and the tetrapods (that later evolved into four-limbed vertebrates) possessed a bizarre hard layer on the top of their skulls, known as cosmine. Cosmine consisted of hard enamel and flat odontodes (structures resembling teeth, with dentine and pulp) and was permeated by a network of pores. Fossils discovered in China by Min Zhu and co-workers¹ at the Chinese Academy of Sciences in Beijing revealed details of how cosmine evolved.

Four skull specimens from the Xitun formation, Yunnan, represent a new species, Meemannia eos. In the dimensions of the skull bones, Meemannia resembles ray-finned fishes (such as today's salmon), a group of animals from which tetrapods and lobe-finned fishes diverged. However, a thick layer of cosmine disqualifies them from that group. Under the microscope, the researchers saw that the cosmine consisted of four layers of enamel and odontodes.

Based on a computer analysis of 25 anatomical features, the scientists classified Meemannia as the closest known lineage to the common ancestor of the lobe-finned and ray-finned fish. In more recent fossils, the multiple cosmine layers merge into one. The function of cosmine's porous network, however, remains mysterious.

References

1. Zhu, M., Yu, X., Wang, W., Zhao, W. & Jia, L. A primitive fish provides key characters bearing on deep osteichthyan phylogeny. Nature 441, 77–80 (2006). | Article |