Foot posture in a primitive pterosaur

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The nature of the hindlimb posture and gait of pterosaurs has been controversial, partly because most of the pterosaur skeletons that have been found were flattened in thin-bedded rocks, therefore obscuring three- dimensional anatomy. A major controversy concerns the extent to which pterosaurs move on the ground; they have been variously interpreted as ranging from sprawling, quadrupedal walkers to erect, bird-like bipedal cursors. Study of pelvis and femur material from the derived group Pterodactyloidea has resolved which movements are possible at the hip, but the lack of three-dimensional, articulated pterosaur feet has prevented examination of all of the movements that are possible within the foot. We have found a large, uncrushed, partial skeleton of a new species of the basal pterosaur Dimorphodon in thick-bedded deposits of Tamaulipas, Mexico; this material includes such a three-dimensional foot. The nature of this skeleton contradicts an important part of the cursorial interpretation, that is, that only the toes contacted the ground during terrestrial locomotion. The flattened metatarsal-phalangeal joint at the base of the first four toes of this specimen would not allow such a digitigrade posture without separating most of the joints. A flat-footed stance is consistent with presumed footprints of pterosaurs that show impressions of the entire sole of the foot.

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