Most living vertebrates, from teleosts to tetrapods, are osteichthyans (bony fishes)1, but the origin of this major group is poorly understood2. The actinopterygians (ray-finned bony fishes) are the most successful living vertebrates in terms of diversity. They appear in the fossil record in the Late Silurian but are poorly known before the Late Devonian. Here we report the discovery of the oldest and most primitive actinopterygian-like osteichthyan braincase known, from 400–million-year-old limestone in southeastern Australia. This specimen displays previously unknown primitive conditions, in particular, an opening for a cartilaginous eyestalk. It provides an important and unique counterpart to the similarly aged and recently described Psarolepis from China and Vietnam3,4. The contrasting features of these specimens, and the unusual anatomy of the new specimen in particular, provide new insights into anatomical conditions close to the evolutionary radiation of all modern osteichthyan groups.
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We thank D. Goujet and C. Poplin for helpful discussion. A.B. thanks the Australian Museum for use of facilities, and acknowledges support from an Australian Postgraduate Award and Macquarie University Postgraduate Fund. M.I.C. acknowledges support from the Biotechnology and Biological Sciences Research Council UK.
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Basden, A., Young, G., Coates, M. et al. The most primitive osteichthyan braincase?. Nature 403, 185–188 (2000). https://doi.org/10.1038/35003183
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