Before proceeding to describe the plants of Paleozoic and later time, the reader should be impressed with the important fact that plants of higher and higher types came into existence throughout geological time in almost exactly the botanical order of their classification, that is to say, from the very simplest types (thallophytes) of Proterozoic time there were gradually evolved, through the long geological ages, higher and higher plant forms reaching a climax in the complex and highly organized plants of the present time. This is the most significant general fact in regard to the geological history of plants. For the convenience of the reader the largest subdivisions in the classification of plants are here given.
OUTLINE CLASSIFICATION OF PLANTS
| I. | Cryptogams (seedless and flowerless) | 1. Thallophytes (e.g., seaweeds, mushrooms) 2. Bryophytes (e.g., mosses) 3. Pteridophytes (e.g., “club mosses,” "horsetails," ferns) | |
| II. | Pteridosperms (seed-bearing, flowerless) | { | (e.g., seed ferns—wholly extinct) |
| III. | Phanerogams (seed-bearing, flowering) | 1. Gymnosperms (e.g., cycads, conifers) 2. Angiosperms (e.g., grasses, lilies, oaks, roses) |
Throughout the first two periods—Cambrian and Ordovician—of the Paleozoic era, plant life appears to have made little or no progress toward higher forms. The very simple Thallophytes (e.g., seaweeds) continued to secrete concentric layers of carbonate of lime in almost exactly the same way as during the middle and late Proterozoic era. Remarkable reefs of such forms occur in the late Cambrian limestone near Saratoga Springs, New York, where one locality has been set aside as a state park. During the Ordovician there were seaweeds of the more familiar branching types without carbonate of lime supports, and these have left very perfect impressions in some of the Ordovician strata.
During the Silurian period seaweeds continued, as, in fact, they did throughout succeeding geologic time to the present. The Silurian strata seem to contain some vestiges of the first-known land plants, though the records are meager and some of the specimens are of a doubtful character. Most interesting of all is a fern or fernlike plant found in France. When we consider the profusion of land plants (all of relatively simple types) of the next or Devonian period, it seems certain that their progenitors must have existed in the Silurian, and their remains may very likely be discovered.
Beginning with the Devonian period of the Paleozoic era the records show that important advances had taken place in the evolution of the plant kingdom. Among the very simple Thallophyte plants some seaweeds of unusually large size occur in fossil form, but the important fact is that all the principal subdivisions of the typical higher non-flowering plants (Pteridophytes) as well as Pteridosperms, and even some primitive representatives of the lower order flowering plants (gymnosperms) were well represented in the Devonian. Our knowledge of land plants earlier than the Devonian amounts to almost nothing and they certainly could not have been at all prominent, but the fossil records make it very clear that many Devonian land areas were clad with rich and diversified plant life. There were even forests, probably the first on earth, but they were far different, both in general and in particular, from those of to-day because the trees were all of exceptionally low organization types. During the next two periods—Mississippian and Pennsylvanian—there was no really important progress in the evolution of plants, and since these remarkable types of land plants have left such wonderfully preserved records in strata of the Pennsylvanian or great Coal Age, we shall proceed to descriptions of the main types of that time, especially those which contributed to the formation of beds of coal.
As shown by the abundant records, the land plant life of Pennsylvanian time must have been not only prolific but exceedingly varied. Thousands of species have been unearthed from the coal-bearing formations alone, and these must represent only a fraction of all species of plants which lived during the period. Most prominent of all were the giant Lycopods constituting the lowest main subdivision of the Pteridophytes (see [above classification]). These great, non-flowering plants were at once the biggest, most common and conspicuous trees of the extensive swamp forests, and they were the greatest contributors to the formation of coal ([Plate 15]). Many species have been described. They commonly attained heights of 50 to 100 feet and diameters of 2 to 6 feet. In one important type the fairly numerous branches bristled with stiff, needle-shaped leaves. When the leaves dropped off the older or trunk portions, scars were left spirally arranged around the trunks of the trees. In another important type the leaf scars were vertically arranged on the lower portions of the tree trunks. The upper portions of the trunks (rarely branched) were thickly set with long, slender leaves, which in some species were two or three feet long. An interesting fact is that the inner parts of the trunks of the great Lycopods were filled with soft, pithy material. This explains why the fossil trees are nearly always flattened out, as a result of burial within the earth. The nonbranching type of Lycopod has been totally extinct for millions of years, while the branching type is to-day represented only by small, mostly delicate, trailing plants familiarly known as “club mosses” and “ground pines.” The most conspicuous trees of the great Pennsylvanian lowlands and swamps have, indeed, left meager modern representatives, and here we have an excellent illustration of a once prominent group of plants which has dwindled away almost to extinction.
Another common type of Pennsylvanian vegetation was the so-called “horsetail” plant or giant rush. The much smaller scouring rush, represented by several species to-day, is the direct descendant of this type which, during later Paleozoic time, grew to be 50 to 90 feet high and 1 to 2 feet in diameter. The long, slender trunks filled with pith were segmented with variously shaped leaves arranged in whorls around the joints. A fine, vertical-fluted structure without leaf scars characterized the surfaces of the trunk.
Recent study has shown that many of the Pennsylvanian plants, long classed as true ferns, were really “seed ferns,” as described below. Many of the true ferns grew to be real trees up to fifty or sixty feet high, but all Paleozoic types were primitive in structure as compared with modern ferns.