The crocodiles are followed in the classification of the reptiles by several extinct groups, known only as fossils of the Mesozoic Age. The first of these is the subclass Plesiosauria, containing a series of predatory creatures characterized by very long necks, short tails, and feet that in the older forms indicate a terrestrial existence, but later exhibit a progressive change to paddles, showing that finally the plesiosaurs were wholly aquatic. This was accompanied by a steady increase in size, until finally a length of at least forty-five feet was reached—chiefly by extension of the neck—in the elasmosaurus of the Cretaceous rocks of Kansas.