The posterior epipodials in aquatic reptiles (Figs. 159, 158) are almost indistinguishable from the anterior ones, except that they are somewhat, or much, smaller. As in the front leg there may be accessory epipodials in both the plesiosaurs and ichthyosaurs. This shortening of the epipodials, so characteristic of aquatic animals, is seen to a moderate extent in the earliest known reptile, Eosauravus (Fig. 151 b) from the middle Pennsylvanian, as also in the Proganosauria (Fig. 153 a) and Choristodera. It is much more pronounced in the Mosasauria (Figs. 146–148, 158 a, b), Aigialosauria, Thalattosauria, and Thalattosuchia (Fig. 150). The elongation of the tibia and fibula, so characteristic of the cursorial or leaping forms, reached the maximum in the Pterosauria (Fig. 155 c).
The tibia and fibula of some temnospondylous amphibians are quite indistinguishable from those of many Cotylosauria.
Mesopodials
Numerous modifications have occurred in the structure of the carpus and tarsus of reptiles in adaptation to diverse habits of life. The carpus (Fig. 134) or wrist of the earliest known reptiles is composed of eleven freely articulated bones, none small: four in the first row, called respectively, from the preaxial to the postaxial side, the radiale, intermedium, ulnare, and pisiform, corresponding quite to the scaphoid, lunar, pyramidal, and pisiform of the human wrist; two in the second row, the radial or first, and the ulnar or second, centrale; and five in the third row, the carpalia, the first four corresponding to the trapezium, trapezoid, magnum, and unciform of the human wrist. Watson has recognized a small third centrale in the curious genus Broomia (Fig. 137 e) from South Africa, unknown as an ossified element in other reptiles, though perhaps represented by a cartilage in the young of the modern Sphenodon.
Carpus
The carpus is known in but two temnospondylous amphibians, Eryops (Fig. 136) and Trematops. In both, the preserved bones are the same in number as in the early reptiles and some modern ones. The radius of Eryops, however, articulates with three bones, the supposed radiale, intermedium, and ulnare, while the pisiform is large, and an articular surface on the postaxial distal margin of the ulna
