Since the 1970s scientists have classified archosaurs mainly on the basis of their ankles. The earliest archosaurs had "primitive mesotarsal" ankles: the astragalus [also called the talus bone] and calcaneum were fixed to the tibia and fibula by sutures and the joint bent about the contact between these bones and the foot.
The earliest fossils of Ornithodira appear in the Carnian age of the late Triassic.... Ornithodires' "advanced mesotarsal" ankle had a very large astragalus and very small calcaneum, and could only move in one plane, like a simple hinge. This arrangement was only suitable for animals with erect limbs, but provided more stability when the animals were running. The ornithodires differed from other archosaurs in other ways: they were lightly-built and usually small, their necks were long and had an S-shaped curve, their skulls were much more lightly built, and many ornithodires were completely bipedal. The archosaurian fourth trochanter on the femur may have made it easier for ornithodires to become bipeds, because it provided more leverage for the thigh muscles. In the late Triassic the ornithodires diversified to produce pterosaurs and dinosaurs.
Tibia Fibula Astragalus Calcaneum
Primitive mesotarsal ankle.
"Advanced" mesotarsal ankle
NOTE: The advanced mesotarsal ankle shown is that of a dinosaur and not that of a pterosaur (or modern bird).
To put this in context here is a human foot (both images are of the right foot):
The bones of the tarsus with A=calcaneus, B=talus bone [astragalus], C=cuboid bone, D=navicular bone, E=lateral cuneiform, F=intermediate cuneiform and G=medial cuneiform. In dark grey the metatarsals. Left image: seen from below. Right image: seen from above.
H., Humerus; R., radius; U., ulna; r., radiale; u., ulnare; C., distal carpals united to carpo-metacarpus; CC., the whole carpal region; MC.I., metacarpal of the thumb; I., phalanx of the thumb; MC.II., second metacarpus; II., second digit; MC.III., third metacarpus; III., third digit.
F., femur; T.T., tibio-tarsus; Fi., fibula; Pt., proximal tarsals united to lower end of tibia; dt., distal tarsals united to upper end of tarso-metatarsus (T.MT.); T., entire tarsal region; MT.I., first metatarsal, free; I.-IV., toes."