During the Paleozoic era land plants appeared, and before its close the continent was densely clothed with forests consisting of flowerless plants such as ferns and club-mosses, together with a less abundance of trees related to the existing conifers.
Great additions to the world's knowledge of the varied and beautiful floras of the swamps in which the coal-beds of Pennsylvania, Ohio, Nova Scotia, etc., were accumulated have been made by H. D. Rogers, J. S. Newberry, Leo Lesquereux, J. W. Dawson, I. C. White, David White, and others.
The Mesozoic era is characterized among other events by the first appearance and rapid development of flowering plants, the cycads being especially numerous, and of our ordinary broad-leaved trees, such as the oak, willow, sassafras, etc., and by the coming in of palms; and in the animal kingdom by the culmination of reptilian life and the advent of birds and mammals.
The American Mesozoic rocks have yielded a rich store of fossil plants, as is well known from the painstaking studies of J. S. Newberry, Leo Lesquereux, W. M. Fontaine, L. F. Ward, F. H. Knowlton, and others. These same students of the progress of plant life on the continent have also made extensive and critical studies of the Cenozoic floras.
The relics of reptilian life brought to light from the Mesozoic rocks of New Jersey, Kansas, Wyoming, etc., by Joseph Leidy, O. C. Marsh, E. D. Cope, and others, have astonished the world, even though marvellous results in a similar direction had previously been made known in Europe. The reptilian age was marked in America by the presence of such huge reptiles, and by the strange development and adaptations in various directions that they surpass the wildest dreams of fable. Lizard-like reptiles walked the earth that were 40 to 60 feet in length and stood 10 to 14 feet high where the massive hind limbs joined the body. Their thigh-bones in certain instances measured over 6 feet in length. Some of these monsters, it is estimated, weighed at least 10 tons. These, the hugest of all land animals, were vegetable feeders. Others, of less size, although still gigantic and more active, were carnivorous. Some of the old lizard-like forms which left their footprints in great abundance in the sands now hardened into sandstone in the Connecticut Valley and New Jersey walked on their hind feet, after the manner of birds, and left three-toed footmarks, some of them 20 inches in length, which are strikingly bird-like in appearance. Other great reptiles, whale-like in appearance, inhabited the ocean. Yet more marvellous forms were provided with wings, resembling those of bats, and in the case of the great Pteranodons found in the rocks of Kansas had a "stretch of wing" of fully 20 feet. But the strange menagerie that has been resurrected contains such a marvellous array of grotesque shapes that not even a catalogue of the genera can be presented here.
While the Mesozoic era was emphatically the age of reptiles, the coming of a more highly developed fauna was foreshadowed. Bird life was represented, and the skeletons of reptilian birds, or birds with teeth like those of reptiles, have been discovered in the Mesozoic rocks of Kansas. Important additions to our knowledge of these strange creatures, which furnish much instructive data in reference to the development of the higher from the lower forms of life, have been made by O. C. Marsh. The humble
beginning of mammalian life is shown by insectivorous marsupials, the jaws of which were discovered in the Newark system (Lower Mesozoic) of North Carolina.
The Cenozoic era is the age of mammals, so called because during that time brute mammals succeeded reptiles as the rulers of the earth. From the rocks deposited in North America during this era, principally the sediments of fresh-water lakes and the gravel-beds laid down by streams in the Pacific mountain region, a great number of skeletons of truly remarkable mammals, differing widely from anything now living, have been discovered by Joseph Leidy, O. C. Marsh, E. D. Cope, H. F. Osborn, and others. The profound interest attached to this fauna, and the bearings it has on the study of the geographical distribution of animals, climatic changes, etc., is indicated by the fact that it includes forms related to the rhinoceros, elephant, camel, etc., which are not represented among the animals now living on the continent, although having relatives in other and principally tropical countries.
During the Psychozoic era mind gained ascendency over brute force, and man became the leader. The mammals continued to dominate the earth throughout the Pleistocene period and were then probably more numerous and of even larger size than during the preceding Tertiary period. During the Pleistocene great climatic changes occurred, and large glaciers existed in several regions which now enjoy a temperate climate and are densely populated.