94 WATER REPTILES OF THE PAST AND PRESENT 



That many of them were rovers is quite certain. With the skeleton 

 of a large plesiosaur found some years ago in western Kansas, there 

 were many sihceous pebbles which could have come only from the 

 shores of the old Cretaceous seas about the Black Hills, hundreds 

 of miles distant. Some of the pebbles are red quartzite, quite 

 identical with that of the bowlders brought to Kansas milUons of 

 years later by the glacial drift from outcroppings near the northern 

 line of Iowa. The bones of plesiosaurs are often found in deposits 

 believed to be of deep-water origin. But they are also found in 

 Kansas associated with the remains of small turtles, flying reptiles, 

 and birds which could only have lived near the shores. Indeed, 

 their remains have often been found with those, of strictly fresh- 

 water animals which had been brought down by the floods to the 

 seas. Their wide but rather sparse distribution in all kinds of 

 marine sediments would rather indicate that they were at home far 

 out in the tempestuous ocean or near the shores in protected bays, 

 though probably they preferred the shallow-water Mttoral regions. 

 One conclusion is quite justified : they were not gregarious, as were 

 the ichthyosaurs. 



It is not certain that the plesiosaurs were viviparous, though 

 there are good reasons for the behef that they were. Remains of 

 two embryos were found years ago in England associated in such 

 a way that it is reasonable to suppose they were unhatched young, 

 though embryos have never yet been found associated with skele- 

 tons of adults, as have those of ichthyosaurs in numerous instances. 

 Bones of young, often quite young, plesiosaurs, are frequently 

 found in shallow-water deposits, and if the young were actually 

 born ahve they must have swum freely in the open waters while 

 yet of very tender age. Rather singularly, however, the remains 

 of these young plesiosaurs always occur as isolated bones. 



In geological range the plesiosaurs were very persistent, extend- 

 ing 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 begin- 

 ning 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 



