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Bichirs (genus Polypterus) are primitive ray-finned fish found in Africa — and sometimes in home aquaria — that have functional lungs derived from the swim bladder and powerful pectoral fins that provide support during occasional forays ashore. This study takes Polypterus senegalus as a model for the immediate ancestors of tetrapods and quantifies the anatomical and behavioural changes that occur when these fish are ‘terrestrialized’. When raised on land, bichirs lift their heads higher off the ground, deploy their forelimbs closer to the midline and slip around much less often than those raised underwater and prompted to walk ashore for the first time. These observations, together with changes in bone structure and musculature, suggest that some of the postural changes seen in the earliest tetrapods, or rather their immediate antecedents, might have been made in response to the environment and assimilated by developmental plasticity.