The most primitive osteichthyan braincase?

Most living vertebrates, from teleosts to tetrapods, are osteichthyans (bon y fishes)(1), but the origin of this major group is poorly understood(2). T he 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 repor t the discovery of the oldest and most primitive actinopterygian-like ostei chthyan braincase known, from 400-million-year-old limestone in southeaster n Australia. This specimen displays previously unknown primitive conditions , in particular, an opening for a cartilaginous eyestalk. It provides an im portant and unique counterpart to the similarly aged and recently described Psarolepis from China and Vietnam(3,4). The contrasting features of these specimens, and the unusual anatomy of the new specimen in particular, provi de new insights into anatomical conditions close to the evolutionary radiat ion of all modern osteichthyan groups.