In the web-footed Mosasauria the tarsus, like the carpus (Figs. 146–148), progressively became more cartilaginous. In Platecarpus (Fig. 158 a) and Clidastes (Fig. 158 b) the astragalus, calcaneum, and fourth tarsale alone remain, with the divaricated fifth metatarsal, as in land lizards. In Tylosaurus, the most specialized of mosasaurs, but one, or at most two, small bones remain. Other tarsal bones remained unossified, though represented by cartilage in the adult.
Fig. 158. Limbs of aquatic reptiles: A, Platecarpus (Mosasauria), right hind leg. About one sixth natural size. B, Clidastes (Mosasauria), right hind leg and tarsus. One third natural size. C, Ophthalmosaurus (Ichthyosauria), left front paddle. One eighth natural size. D, Ichthyosaurus platydactylus (Ichthyosauria), left front paddle. One sixth natural size.
Not more than six bones of the plesiosaurs can be called tarsals, and their homologies are doubtful (Fig. 159 b, c). They have the same shapes and relations as the carpal bones and cannot be distinguished from them except by their smaller size. The three in the first row are usually called the tibiale, intermedium, and fibulare; a fourth, on the postaxial side, has sometimes been called the pisiform in both front and hind limbs, but as there never was in any terrestrial reptiles a pisiform in the tarsus, that name is of course incorrect.
