Very remarkable among the later Paleozoic plants were the Pteridosperms, represented by the so-called “seed ferns.” These now wholly extinct plants seem to have formed the connecting link between the seedless, flowerless plants (Cryptogams) and the seed-bearing, flowering plants (Phanerogams), because they bore seeds but not flowers. Many of them were small and herbaceous, but others were tall trees, in general appearance resembling the tree ferns. “Seed ferns,” which play such an important part in the evolution of plants, are not known to have existed after Paleozoic time.
During the latter half of the Paleozoic era some very primitive types of flowering plants (Gymnosperms) existed. Most abundant of these were the so-called Cordaites, which were the tallest trees of the time, some having reached heights of over 100 feet. The upper portions only bore numerous branches supplied with many simple, parallel-veined, strap-shaped leaves up to six feet long and six inches wide. Excepting the pithy cores the trunks of these trees were of real wood covered with thick bark. Trees of this kind became extinct in the early Mesozoic era.
Very late in the Paleozoic (Permian period) two other types of the simple flowering plants (Gymnosperms) made their appearance. These were the cycads and conifers, which were the most conspicuous trees during the first two periods of the Mesozoic era. The cycads reached their culmination in the Jurassic period, but they still exist in modified form in some parts of the world. The short, stout trunk was crowned with long, stiff, palm-like leaves. In fact, the cycads are distantly related to the palms, which belong to a higher group of plants. Some specimens of cycads, especially from the Mesozoic strata of South Dakota, are so wonderfully fossilized that even the detailed structures of trunks, leaves, flowers, and seeds are so perfectly preserved that almost as much is known about these plants of millions of years ago as though they were living forms.
The conifers, with which are classed present-day pines, spruces, and many other evergreen trees, gradually took on a more modern aspect, so that late in the Mesozoic era they were much like those now living. Among the most interesting trees were the sequoias, to which the living “big trees” and red-woods of California belong. These began in relatively late Mesozoic time, reached their climax in numbers, variety of species, and widespread distribution in the early Cenozoic era; and are now almost extinct, being represented by only two species in local portions of California. Cordaites, trees which were so large and abundant in later Paleozoic time, were reduced to extinction in the early Mesozoic era.
During Mesozoic time the Thallophytes, represented by seaweeds, were common. Among the Pteridophytes the ferns and “horsetail” plants were fairly common, but the very large forms gradually gave way to much smaller ones during Mesozoic time. The giant Lycopods of later Paleozoic time dwindled almost to extinction even in early Mesozoic time, so that from that time to the present they have been very small and relatively insignificant.
Tens of millions of years of earth history had passed before the true flowering plants—the Angiosperms—appeared upon the earth. The Cretaceous period marks their advent. So far as known, these plants originated along the eastern side of North America, and very soon after their establishment they spread over the earth with amazing rapidity and dominated the vegetation as they do to-day, more than half of the existing species of plants being Angiosperms. Among the common types which have been unearthed from Cretaceous strata are palms, grasses, maples, oaks, elms, figs, magnolias, willows, beeches, chestnuts, and poplars.
Plate 13.—A Slab of Very Early Paleozoic (Cambrian) Rock, Covered with Some of the Oldest Known Definitely Determinable Animal Remains. These creatures lived in a sea which overspread the site of the Rocky Mountains of southern Canada fully 25,000,000 years ago. Most of the fossils are trilobites (including some very small ones) and other related crustacean forms (lighter portions). (After C. D. Wolcott, Smithsonian Institution, Washington, D. C.)