The great tribe of true bony fishes, such as the cod and perch, which includes more than ninety percent of the fishes living today, was not yet in existence. About one-third of the many kinds of fishes then living were related to the sharks, a group which is relatively insignificant in recent years. Nearly one-fourth of the total belonged to a tribe of enamel-scaled fishes, now represented only by a few sturgeon and gar-pike.
Lung fishes have never been a large group but it is noteworthy that they have had existence since Middle Devonian time. Living members of the race, inhabitants of Africa and South America, make a practice of burrowing into the mud of stream channels during dry seasons and are provided with lungs which enable them to breathe air in the manner of higher vertebrates. They survive the complete drying-up of the streams and live for months without water. Other forms, with less development of lungs, frequent stagnant pools and come to the surface occasionally for a breath of air. All are provided with gills also, which enables them to obtain their oxygen as other fishes do. They are believed to be a connecting link between the fishes and the early amphibians. More accurately, perhaps, they should be regarded as holding an intermediate position without being directly ancestral to any higher type of vertebrate animal.
Still dominant among the invertebrates were the brachiopods, on the whole averaging a little larger in size, and otherwise indicating congenial times for that type of organism. They reached the peak of their development during this period. Trilobites were declining although a few new and strangely ornamented varieties made a brief appearance. Crinoids apparently found living conditions less favorable during Devonian time, but in a later era they again became prominent. Corals were favored only at times and in certain localities. Along with the crinoids they appear to have suffered from the presence of an unusual amount of mud in the waters of their customary habitats. Both had a preference for clear water as indicated by the absence of fossils from limestones containing more than a very small percentage of muddy sediments. Crustaceans, similar to the sea-scorpions and better known as eurypterids, became prominent among fresh-water animals. Some were unusually large for creatures of this class, lengths of several feet being recorded from fragments. Gastropod mollusks came into prominence in localities where living conditions were favorable. Bivalves continued to thrive but the cephalopods had a rather meager development considering the heights they were to achieve in subsequent periods.
In western North America the large expanse of territory known as the Great Plains was evidently well above sea level during this entire period, for no beds of this age are found in eastern Colorado. West of the Front Range, however, there was some deposition of marine sediments during late Devonian time. Formations of this age are exposed near Salida and Glenwood Springs, on the White River Plateau, and in the San Juan region.
The Carboniferous period gets its name from the vast deposits of coal which were developed during that time in many parts of the northern hemisphere. Depressed land surfaces bordering the continents, and extending well into the interior of present boundaries, supported dense growths of vegetation and provided the swampy conditions most favorable to coal production. Varieties of plants which are now of small size and lowly position in the botanical world acquired the proportions of large trees.
CARBONIFEROUS FORESTS
Best-known fossils of the period are carbonized portions of the larger trees, and impressions left in the muds and sands of ancient bogs. Forest trees of several kinds reached the height of a hundred feet, with a trunk diameter of two to six feet. This size often is exceeded in modern forests, but by trees of an entirely different type. Considering the amount of development among the plants of earlier periods, Carboniferous forests provide an outstanding spectacle of advancing life.
Quite common among the larger trees were two varieties of club-mosses, also known as scale trees. They were cone-bearing evergreens with only slight resemblance to modern conifers. Instead of seeds they produced spores, a method of reproduction which is practiced among ferns. The trunks were marked from bottom to top with uniform patterns of cushions and scars indicating the points at which leaves were attached during the earlier stages of growth. In the Lepidodendrons the rows of scale-like cushions wind spirally upward while among the Sigillaria there is a vertical arrangement of leaf-scars which resemble the imprints of a seal, these impressions being in straight and parallel rows on a surface which may be either ribbed or smooth. The leaves of scale trees were stiff and slender, and arranged in grass-like tufts at the top.
Calamites, related to our horsetail rushes, were somewhat smaller than the scale trees. Their trunks consisted of a thin, woody cylinder with a pithy interior, and were marked at intervals by nodes which gave them the “jointed” appearance of a bamboo stem. Leaves were arranged in circles around the nodes of main stem or branches. Spore-bearing cones appeared at the tips of the stems.
