With the exception of a single vertebra of doubtful affinities and the cast of a turtle-shell no vertebrate fossils have ever been discovered in the extensive sandstones of Dakota age, the lowermost of the Upper Cretaceous. From the next horizon above the Dakota, the Benton Cretaceous, chiefly marine limestones, at least three genera of plesiosaurs are known from Kansas, Texas, and Arkansas, with two or three more from the limestone shales of Wyoming. A few specimens of armored dinosaurs, two genera of ancient crocodiles, nearly the last of their kind, some marine turtles, and a few vertebrae of ichthyosaurs, the last of the order known anywhere in the world, are also known from the Benton Cretaceous of Wyoming.
Continuous with the Benton limestones above in Kansas are the famous beds of Niobrara chalk; perhaps no fossil deposits in the world are more famous. Exposures covering hundreds of square miles in western Kansas, almost pure chalk, have furnished fossil-hunters during the past forty years literally thousands of specimens of mosasaurs, hundreds of pterodactyls, and scores of plesiosaurs and marine turtles, in addition to the famous birds with teeth and countless fishes of diverse kinds. Two or three specimens of spoon-billed dinosaurs have been found in these deposits, but no other reptiles of any kinds. Beds of like age in Colorado and New Mexico have furnished a few specimens of mosasaurs.
From the marine beds of Fort Pierre age, next above the Niobrara in the west, have come some excellent specimens of two genera of mosasaurs, three or four forms of plesiosaurs, a few pterodactyls, the largest of all marine turtles, and still fewer specimens of dinosaurs, in Kansas, South Dakota, Wyoming, and Montana. From deposits of approximately like age in Mississippi, Alabama, and New Jersey, many incomplete specimens were found years ago of mosasaurs, plesiosaurs, and turtles, the last of the amphicoelian crocodiles, the first of the procoelian crocodiles, and the famous specimen of Hadrosaurus which served for the Hawkins restoration, the first attempt of its kind.
From the uppermost Cretaceous beds of America, the Lance, Judith River, or Belly River beds as they are variously called, have come the remains of a marvelous reptilian fauna. These beds may be grouped together though not all contemporaneous, and there is dispute about their age, some excellent paleontologists insisting that the uppermost are really of Eocene age. From Colorado east of Denver, from eastern Wyoming, from Montana, and especially from the vicinity of Edmonton in Canada, as also occasionally in western Texas and New Mexico, have come many marvelous specimens of dinosaurs, huge bipedal carnivorous dinosaurs, great spoon-billed aquatic dinosaurs, armored stegosaurian dinosaurs, and many kinds of the great horned dinosaurs, the Ceratopsia, so far known only from these beds. Here at the very close of the Age of Reptiles, at the close of the Age of Dinosaurs, are found the ultimate specializations of all the chief groups of dinosaurs except the long-necked quadrupedal dinosaurs which gave up the ghost in Lower Cretaceous times. Many were provided with horns and spines, some indeed seemed to have bristled with spines throughout, a sure sign that they were approaching the end of their career. The modern type of crocodiles had usurped the ancient forms of the early Cretaceous, and reached the largest size of their race perhaps, though but few specimens are known. Here also in these beds we find the first representatives of lizards and snakes in America, though snakes have been described from earlier strata, perhaps, in Brazil. Those archaic, old-fashioned rhynchocephalians described on a later page as the Choristodera appeared also for the first time in these beds, and persisted for a little while in the Eocene, in Europe and America. And with all these there has very recently been described the last of the plesiosaurs, whose race went out with the dinosaurs at the very close of the Mesozoic. It is needless to say that the turtles also occur, for, as a general rule, wherever vertebrate fossils are found, in rocks of the land or the sea, marine or fresh-water, there will be some bones of turtles among them.
With the beginning of the Cenozoic the record of the reptiles becomes relatively scanty in America. In the warm waters of the old Eocene lakes and rivers of Wyoming lived countless crocodiles, true crocodiles of modern aspect and of large size. But, as the climate of North America grew progressively colder, the crocodiles retreated to the south, till, in the Oligocene, the scanty remains of the last crocodiles are found in the American Tertiary. On the other hand, as the open lands appeared toward the close of the Eocene, and in the Oligocene and Miocene, the land tortoises throve and grew greatly in size. In the Bad Lands of South Dakota one may see their remains in almost incredible numbers. And in equally great numbers are these land tortoises, in shape much like the common box tortoise of today, but vastly larger, found in the rocks of the late Miocene or early Pliocene age in western Kansas. And these are the last records of the big tortoises in North America; their descendants are perhaps yet living in the Galapagos Islands.
The history of the lizards and snakes, the only other reptiles found in the Cenozoic rocks of America, is very brief. A few specimens from the Lower Eocene of Wyoming; a few skinks and amphisbaenas from the Oligocene Bad Lands of South Dakota, and some bones of a python-like snake in the early Eocene of Wyoming are about all that we know of the Squamata in the Tertiary. Doubtless snakes and lizards were just as abundant then as now, though but few were preserved, for they are and always have been distinctly terrestrial animals, that only by accident fell into places where they could be fossilized.
The author has collected reptile bones from nearly all of the horizons here mentioned and believes that the list is complete.
CHAPTER V
ADAPTATION OF LAND REPTILES
TO LIFE IN THE WATER
In the never-ceasing struggle for existence all forms of life upon the earth, whether consciously or unconsciously, are continuously striving for improvement; striving to flee from adverse environments, or to adapt themselves better to those which must be endured; to escape their enemies, or to find means whereby they may withstand them; to find more or better food, or to prevent others from despoiling them of what they have. There is always more or less of unrest, more or less of discontent, if such terms may be used of the lower organisms. It sometimes happens with groups of organisms that by reason of unusual or extraordinary traits they become so perfectly adapted to their environments, to their surroundings, or so easily adaptable to changes in their environments, that they remain for long ages securely protected and little changed. But, as with man himself, improvement is usually the result of adversity—adversity which stimulates but does not destroy. And the word improvement, translated into biological language, means simply specialization, that specialization which adapts the organism better to its mode of life, which fits it the better to excel its less ambitious or less capable competitors. No animals or plants are perfect; if they were, there would be no advancement, no struggle. If all physical conditions stood still, or remained uniform, perhaps life would stand still, but conditions never have and never will stand still, and life must change to meet changed conditions.
Thus it is that which makes life easier, which lessens the dangers of destruction, which insures the continued prosperity of the race, is seized upon and utilized by all plants and animals, so far as possible. As said long ago by Tennyson,[2] the first law of life is not the preservation of self, but the prosperity of the race. Whatever the causes may be whereby the offspring are better adapted to conquer in the struggle for existence, whatever may be the laws governing changes and specialization, whether heredity, Mendelism, mutation, natural selection, or Lamarckism, we call the process evolution.