The turtle-group (Chelonia) makes
its appearance in the Triassic and thrives in the Jurassic. Its
members are extinct and primitive forms of the thick-shelled
reptiles, but true turtles, both of marine and fresh water,
abound before the close of the Mesozoic. The sea-turtles attain
an enormous size. Archelon, one of the primitive types, measured
about twelve feet across the shell. Another was thirteen feet
long and fifteen feet from one outstretched flipper to the other.
In the Chalk period they form more than a third of the reptile
remains in some regions. They are extremely interesting in that
they show, to some extent, the evolution of their characteristic
shell. In some of the larger specimens the ribs have not yet
entirely coalesced.
The Crocodilians also appear in the later Triassic, abound in the
Jurassic, and give way before the later types, the true
Crocodiles, in the Cretaceous. They were marine animals with
naked skin, a head and neck something like that of the
Ichthyosaur, and paddles like those of the Plesiosaur. Their back
limbs, however, were not much changed after their adaptation to
life in the sea, and it is concluded that they visited the land
to lay their eggs.
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