FOOT POSTURE IN A PRIMITIVE PTEROSAUR

The nature of the hindlimb posture and gait of pterosaurs has been con troversial(1-16), partly because most of the pterosaur skeletons that have been found were flattened in thin-bedded rocks, therefore obscuri ng three-dimensional anatomy. A major controversy concerns the extent to which pterosaurs move on the ground; they have been variously inter preted as ranging from sprawling, quadrupedal walkers to erect, bird-l ike bipedal cursors(1). Study of pelvis and femur material from the de rived group Pterodactyloidea(11-13) has resolved which movements are p ossible at the hip, but the lack of three-dimensional, articulated pte rosaur feet has prevented examination of all of the movements that are possible within the foot. We have found a large, uncrushed, partial s keleton of a new species of the basal pterosaur Dimorphodon in thick-b edded deposits of Tamaulipas, Mexico; this material includes such a th ree-dimensional foot, The nature of this skeleton contradicts an impor tant part of the cursorial interpretation, that is, that only the toes contacted the ground during terrestrial locomotion(2-7). The flattene d metatarsal-phalangeal joint at the base of the first four toes of th is specimen would not allow such a digitigrade posture without separat ing most of the joints. A hat-footed stance is consistent with presume d footprints of pterosaurs(8-10) that show impressions of the entire s ole of the foot.