DINOSAURS—ANCESTORS OF BIRDS
No class of the extinct reptiles is so familiar, by name at least, as is that of the dinosaurs, mainly because of the enormous size of some of them, and the fact that their prodigious skeletons are exhibited complete in many museums. No other land animals ever approached some of them in bulk. A great number of species have been exhumed, yet as a group these reptiles are only imperfectly known, for the fossils are not scattered throughout the whole extent of Mesozoic deposits, but only in two limited periods of that era separated by two or three millions of years. 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 on the hind limbs, like ostriches. They ranged in size from that of a cat to the prodigious bulk of the diplodocus or the brontosaurus, seventy feet long and perhaps twenty feet high at the hips, while an East African species appears to have been even far bigger. Some were herbivorous, and dwellers mainly in marshes and swamps; others ranged the uplands, armored for defense against huge predatory kinds, and still others had horny beaks like birds. It is believed, in fact, that our birds are descended from the same stock as these creatures, through an early offshoot.
| MONOCLONIUS RESTORATION (DECKERT) IN TYPICAL LANDSCAPE |
| (American Museum of Natural History) |
CROCODILES AND ALLIGATORS
These repulsive and ferocious reptiles (Crocodilia) are the bulkiest of the whole class, and most resemble the ancient aquatic dinosaurs, with which they are undoubtedly allied, although their precise derivation is undetermined. Their general shape is in conformity with the reptilian model, rather than indicative of any close relationship to lizards; indeed their closest living relatives are the tuatara and the chelonians. They have four legs of nearly equal size in modern examples, but in some of the older extinct forms of the Lias and Jurassic strata the hind legs were much longer than the fore pair, and broadly webbed, while other features indicate a purely marine life; but it appears plain that the crocodilians originated as land dwellers whose descendants, early in the history of the group, took to an amphibious method of life.
The eyes, nostrils, and external ears are situated on the upper surface of the head, so that breathing, seeing, and hearing are unimpaired in the water, the upper part of the head being usually raised above the surface when swimming. The nostrils and ears have valves which are shut when the animal is under water.
Crocodiles and alligators are mainly carnivorous, feeding on mammals and waterfowl, for which they lie in wait close to the edge of the water, sweeping them in by a blow of their tail, but the gavials feed almost exclusively on fish. All are oviparous, laying oval, hard-shelled eggs.
The order is represented by a single family, Crocodilidæ, including six genera scattered through the tropical and subtropical parts of the globe in the strange fashion that characterizes many of the ancient groups of which present species are mere relics. Thus the American genus of alligators has also a species in China; and the Old World crocodiles are represented by a single, narrowly restricted species in the region of the Gulf of Mexico. Our common alligator (A. mississippiensis) inhabits the low coastal rivers and swamps from North Carolina to the Rio Grande, and remains abundant in the steaming bayous along the lower Mississippi, and in the swamps of Louisiana, but in Florida has been killed off, not only by the demand for its hide (as leather), but in a wanton way by tourists and sportsmen, until now its numbers are greatly reduced. It would have become nearly extinct were not its prolificness great, each female depositing thirty-five to forty eggs in the layers of the cone-shaped heap of a nest she makes on the bank.