Rapid braincase evolution between Panderichthys and the earliest tetrapods

THE panderichthyids (or elpistostegids) are the most tetrapod-like fishes that still retain paired fins rather than limbs, During the transition from fish to tetrapod, the braincase, previously subdivided by a joint, was remodelled into a solid structure(4). Here we present the complete braincase of the fish Panderichthys rhombolepis, a Middle Devonian(1) member of the tetrapod stemgroup(2,3). Panderichthys has an externally tetrapod-like skull(2), but we show that the braincase retained the intracranial joint, conforming wholly to the generalized pattern of lobe-finned fish, and sharing no obvious derived features with tetrapods, This places the braincase transformation between Panderichthys and the earliest tetrapods exemplified by Acanthostega(4). The timing of the braincase transformation closely matches that of the limbs. There are also striking similarities with the braincase transformation in the lungfish lineage. Both phenomena may reflect developmental linkages and canalization.