DINOSAURIA. A group of reptilian ani mals which flourished during the Mesozoic Era, or the Age of Reptiles. They were in many ways the ruling or dominant forms of that time and as such filled the various roles of ter restrial animals as do the warm-blooded mam mals of to-day. Dinosaurs were air-breathing animals and, in common with other reptiles, such as the modern crocodiles, to which they were somewhat distantly related, possessed a more or less scaly or armored skin and were prob ably cold-blooded or poikilothermous, that is, possessing no mechanism for the maintenance of bodily heat, but having an internal tempera ture which varied with that of the outside air. It is assumed that they were egg-laying, but this can neither be proved nor disproved until either eggs or unborn young are found.
In their anatomy the dinosaurs show certain features which link them with the crocodiles on the one hand and with the birds on the other, and while due in part to community of habit, such as bipedal running on the part of dino saurs and birds and the consequent modification of the hind limbs, these similarities of structure also imply genetic or blood relationships.
In size the dinosaurs ranged from that of a house cat to a length and bulk exceeded only by the greater of the modern whales — upward of 90 feet long and 40 tons in weight. In habits they were as varied as in size, for some were light of foot and bipedal while others were quadrupeds of unwieldy bulk, some armored, others armorless, some endowed with horns or with talons and terrible teeth, yet others whose only apparent means of defense lay in their huge size and inaccessible habitat. They were carnivorous and herbivorous, some of the latter having very defective dentition which made mastication of the food impossible, while others were provided with a dental battery of marvelous detail and perfection.
Their habitat was in all probability the land, at any rate at the beginning of their racial career; indeed, the initial stimulus to their evolution may well have been aridity of climate, which is generally an incentive to the develop ment of traveling powers and hence may have brought about the cursorial adaptation which so distinguishes the earlier forms. Later the
dinosaurs, at any rate such as are known to us, were inhabitants of low-land lying along' the shores of seas and oceans and some were actually partly, if not wholly, water-living, al though none show an extreme of adaptation to aquatic life and none were salt-waler inhabit ants, the occasional inclusion of their remains in marine strata being the result of accident.
There is reason to believe that the dinosaurs were diphyletic, that is, consisted of two races which ran separate though in many ways parallel evolutionary courses, the one group, the carnivores (Theropoda) and their plant feeding derivatives (Sauropoda) being nearer the crocodiles, and the true herbivores (Pre dentata) being nearer the birds. It is prob able, however, that each phylum was derived from the same ancestral stock, but that the divergence began at once, possibly in Permian time.
The duration of dinosaurian existence was immensely long, for their remains are first found in rocks of Middle Triassic time, but these are already in a state of development that implies a antecedent evolution. And they continue, despite the extinction of certain lines, until the very close of the Mesozoic, a lapse of time measured by millions of years. Their fos sils appear first in Germany, but this does not necessarily imply a Germanic origin; on the contrary, the belief has been expressed that one must go farther west to a continental mass which once linked Europe and Nor* America across what is now the North Atlantic to find their ancestral home. Thence they spread the world over, their remains being found in Europe, North and South America, Africa, Madagascar and Australia; but, with the ex ception of India, they are as yet unknown from the great continent of Asia. Whether this is significant or merely because they have not thus far been discovered, is not known.
The classification of the dinosaurs may be given as follows: