Page:The Osteology of the Reptiles.pdf/219

Rh toes have become shortened and the phalanges on the postaxial side reduced, the dinosaurs have rather long digits, but they had become distinctively digitigrade, shortening the portion resting upon the ground, like the reduction of the digital formula in the plantigrade reptiles. In all such reptiles with the more mammal-like mode of locomotion, the foot is more mesaxial or preaxial, as in the mammals, where the fourth is very seldom the strongest toe.

The chief joint between the foot and legs in mammals is between the end of the tibia and the first row of the tarsals. In reptiles it is intratarsal, that is, between the first and second rows of the tarsus. In those reptiles which walked more or less upon the toes, digitigrade, there was a progressively closer articulation between the tibia and the astragalus, giving a firmer and closer ankle which otherwise would have been subject to injury with the heel elevated far above the ground. In the bipedal Theropoda (Fig. 156 ), the astragalus, while perhaps never fully fused with the tibia, acquired a long ascending process which fitted closely into a groove in front of the distal end of the tibia. In the still more elongated feet of the pterodactyls (Fig. 155 ) the astragalus became indistinguishably fused with the tibia, as in birds, and the joint, while actually, as formerly, intratarsal, was functionally between the leg and tarsus as in mammals.

Short toes and reduction of phalanges, then, mean a more mammal-like mode of locomotion and posture of the feet. In the turtles this has been produced by the exigency of the immovable shell, and by the greater or lesser twisting of the epipodials upon the propodials.