Dinosaurs | Page 2

in the icy covering of Greenland and Antarctica.
The Age of Mammals. Before this period was a very much longer one--at least thirty times as long--during which modern quadrupeds were slowly evolving from small and primitive ancestors into their present variety of form and size. This is the Tertiary Period or Age of Mammals. Through this long period we can trace step by step the successive stages through which the ancestors of horses, camels, elephants, rhinoceroses, etc., were gradually converted into their present form in adaptation to their various habits and environment. And with them were slowly evolved various kinds of quadrupeds whose descendants do not now exist, the Titanotheres, Elotheres, Oreodonts, etc., extinct races which have not survived to our time. Man, as such, had not yet come into existence, nor are we able to trace any direct and complete line of ancestry among the fossil species known to us; but his collateral ancestors were represented by the fossil species of monkeys and lemurs of the Tertiary period.
[Illustration: Fig. 1.--The Later Ages of Geologic Time.]
The Age of Reptiles. Preceding the Age of Mammals lies a long vista of geologic periods of which the later ones are marked by the dominance of Reptiles, and are grouped together as the Age of Reptiles or Mesozoic Era. This was the reign of the Dinosaurs, and in it we are introduced to a world of life so different from that of today that we might well imagine ourselves upon another planet.
None of the ordinary quadrupeds with which we are familiar then existed, nor any related to nor resembling them. But in their place were reptiles large and small, carnivorous and herbivorous, walking, swimming and even flying.
Crocodiles, Turtles and Sea Reptiles. The Crocodiles and Turtles of the swamps were not so very different from their modern descendants; there were also sea-crocodiles, sea-turtles, huge marine lizards (Mosasaurs) with flippers instead of feet; and another group of great marine reptiles (Plesiosaurs) somewhat like sea-turtles but with long neck and toothed jaws and without any carapace. These various kinds of sea-reptiles took the place of the great sea mammals of modern times (which were evolved during the Age of Mammals); of whales and dolphins, seals and walruses, and manatees.
Pterodactyls. The flying Reptiles or Pterosaurians, partly took the place of birds, and most of them were of small size. Strange bat-winged creatures, the wing membrane stretched on the enormously elongated fourth finger, they are of all extinct reptiles the least understood, the most difficult to reconstruct and visualize as they were in life.
Dinosaurs. The land reptiles were chiefly Dinosaurs, a group which flourished throughout the Age of Reptiles and became extinct at its close. "Dinosaur" is a general term which covers as wide a variety in size and appearance as "Quadruped" among modern animals. And the Dinosaurs in the Age of Reptiles occupied about the same place in nature as the larger quadrupeds do today. They have been called the Giant Reptiles, for those we know most about were gigantic in size, but there were also numerous smaller kinds, the smallest no larger than a cat. All of them had short, compact bodies, long tails, and long legs for a reptile, and instead of crawling, they walked or ran, sometimes upon all fours, more generally upon the hind limbs, like ostriches, the long tail balancing the weight of the body. Some modern lizards run this way on occasion, especially if they are in a hurry. But the bodies of lizards are too long and their limbs too small and slender for this to be the usual mode of progress, as it seems to have been among the Dinosaurs.
ANIMALS OF THE AGE OF REPTILES. LAND REPTILES. DINOSAURS corresponding to the larger quadrupeds or land mammals of today. CROCODILES, LIZARDS AND TURTLES still surviving. SEA REPTILES. PLESIOSAURS } corresponding to whales, dolphins, seals, ICHTHYOSAURS } etc., or sea-mammals of today. MOSASAURS } FLYING REPTILES OR PTEROSAURS. BIRDS WITH TEETH (scarce and little known). PRIMITIVE MAMMALS of minute size (scarce and little known). FISHES and INVERTEBRATES many of them of extinct races, all more or less different from modern kinds.
Fishes, large and small, were common in the seas and rivers of the Age of Reptiles but all of them were more or less different from modern kinds, and many belonged to ancient races now rare or extinct.
The lower animals or Invertebrates were also different from those of today, although some would not be very noticeably so at first glance. Among molluscs, the Ammonites, related to the modern Pearly Nautilus, are an example of a race very numerous and varied during all the periods of the Reptilian Era, but disappearing at its close, leaving only a few collateral descendants in the squids, cuttlefish and nautili of the modern seas. The Brachiopods were another