The palaeoecology of the primitive, limbed snake Pachyrhachis is reevaluated. Previously considered to have been preserved in a shallow bay with a nearby freshwater source, it is here demonstrated to have inhabited an inter-reef basin. Evidence for this comes from the position of the nearest palaeoshoreline (450 km away), the finely laminated carbonate mudstones, and the absence of bioturbation, alluvium, and unequivocally terrestrial or freshwater taxa. Marine adaptations of Pachyrhachis include the laterally compressed body and heavily ossified (pachyostotic) vertebrae and ribs. Hydrodynamic considerations indicate that it was a slow swimmer, and thus an ambush rather than pursuit predator. Morphology of the anterior vertebrae suggests that the strike muscles were well-developed. The narrow head and neck might have been adaptations to reduce water resistance during rapid lunges, and/or to forage in burrows or tight crevices.