In geological range the plesiosaurs were very persistent, extending through nearly all the Mesozoic. They began their career as fully evolved plesiosaurs, so far as we now know, near the close of the Triassic period, and reached their culmination in the Upper Cretaceous, but survived to the close of that period. In the beginning of their career they were associated with the marine crocodiles and the ichthyosaurs, but outlived them to find companions and probably enemies in the huge and voracious mosasaurs of the later Cretaceous times. At no time do they appear to have been especially numerous, nor does it seem probable that they were ever a dominant type of marine vertebrate life, though their remains occur everywhere that marine deposits of the Jura and Cretaceous are known. Indeed, it may be said with almost certainty that rocks of these ages and of that character everywhere in the world contain fossil plesiosaurs. Their bones have been made known from Europe, Asia, Africa, Australia, and North and South America. From North America thirty or more species have been described from New Jersey, Alabama, Mississippi, Texas, Arkansas, Kansas, Nebraska, Colorado, New Mexico, Wyoming, North and South Dakota, California, etc. The cause of their final extinction no one knows, nor can we conjecture much about it with assurance. That climatic conditions became unfavorable for them is highly improbable, considering their cosmopolitan habits; they were not discriminating in their environments. After successfully withstanding their fiercest foes, the ichthyosaurs, crocodiles, and mosasaurs, and large carnivorous fishes, it does not seem probable that they would succumb to lesser enemies, though it may be that they were finally attacked successfully, not in the fulness of their strength as adults, but while young, by more insidious enemies. More probably after their long life of millions of years they had grown old, as everything grows old, and had become so fixed and unplastic in their structure and habits that even slight causes were at last their undoing. When we shall have bridged over that still imperfectly known transition period between the great Age of Reptiles and the greater Age of Mammals we shall have learned more definitely some of the causes of the extraordinary revolution in vertebrate life that then occurred. The plesiosaurs went out with nearly all of their kind, the mosasaurs, the pterodactyls, the dinosaurs; and, so far as we now know, their places in the sea, land, and air were not immediately taken by any other creatures. [Illustration: FIG. 44.—_Nothosaurus_; restoration after E. Fraas; landscape by Dorothy Williston.] NOTHOSAURIA