The Devonian is one of the most outstanding of all periods from the
viewpoint of life development. Dominance of the fishes is its greatest
achievement, the invertebrates remaining about as they were and the
higher vertebrates barely in evidence, but life on a large scale was no
longer confined to the seas. Fresh-water fishes became prominent and
land plants well established. The first forests appeared, with fern-like
plants predominating although woody trees of several types and
considerable size were included. It is quite possible that extensive
land areas had been well supplied with vegetation during earlier times,
but the delicate tissues of plants are far less likely to be preserved
than the limy parts of animals. The fossil record, therefore, cannot be
expected to reveal more than a suggestion of the progress made at this
level of living. The story of plant life becomes much clearer in the
next period when conditions were more favorable for the production and
preservation of plant fossils.
Land animals of the time are almost unknown. A few snails and scorpions
have been found, and some footprints made by early amphibians. Insects
probably were in existence although the evidence is not quite clear on
this point. The increasing number of fresh-water fishes, however, may be
regarded as a sure indication that inland conditions were becoming more
favorable for plant and animal inhabitants of all kinds.
The extent of development among the fishes cannot be accurately
indicated by naming a few types, for it is mainly in the number of
species and genera within the larger groups that progress is seen. In
general it may be stated that the fishes of the period had not yet
acquired the bony skeleton and typical form of familiar modern species.
Skeletons were of cartilage, partly hardened in some instances by lime.
Armor plates were customary with certain races but were not present
among all fishes. Neither were these armored forms exceptionally large,
as compared with living sharks. Although occasional giants appeared, the
majority were small. Many were sluggish creatures with poorly-developed
jaws, living as scavengers on sea and stream bottoms. Tail fins were
usually unbalanced as in the sharks, or pointed and rounded rather than
evenly forked.
[Illustration: Modernized Types of Fishes from Eocene Shales of
Southwestern Wyoming]
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.