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.
[Illustration: Prehistoric Plants
Some of the larger and better known plants of past ages are shown as
reconstructed by artists. Finer details of the reconstructions often
have to be neglected because of uncertainties due to the scattered
and fragmentary character of the fossil record.]
LEAF IMPRESSIONS
Carboniferous Ferns
Strap-leaf Conifer (Cordaites)
MODERNIZED TYPES
Sequoia Cone and foliage
Miocene Fossils (Florissant Shales)
Maple
Willow
Eocene palm (Denver Beds)
HORSE TAIL RUSHES
Restoration (Calamites)
Fossils Leaves and Stem
CYCADS
Restoration
Fossil Trunk
CLUB MOSSES
Restoration (Sigillaria)
Fossils Trunk Impressions
SCALE TREES
Restoration (Lepidodendron)
Fossil Leaf scars
Also included among the larger trees were the Cordaites or large-leaved
evergreens, tall and slender, seed-bearing but not true conifers as yet.
Leaves were strap-shaped or grass-like, the larger ones having a length
of six feet and a width of six inches. Trunks were woody, resembling
pine, but with a central pith. The flowers were small and resembled
catkins in form.
Ferns and fernlike plants were so numerous that the period has been
known as an age of ferns. Earlier knowledge of these forests was based
on fossils of a fragmentary character from which an accurate association
of parts could rarely be obtained. A general relationship with the ferns
was apparent, but careful study of additional material has given us a
rather different view of Carboniferous plant life and we note a highly
diversified array of forms with many suggestions of modern tendencies.
The flora as a whole may be regarded as highly specialized for the
conditions which prevailed at the time and were not to continue through
subsequent periods. Warm temperatures and abundant moisture were
essential especially to spore-bearing types, and the cold, arid
conditions of the next period put an end to many of the groups, or
greatly reduced their prominence.
This could be regarded equally well as an age of insects, for some of
these invertebrates acquired the greatest size they have ever had,
particularly the dragonflies with a wing-spread of more than two feet in
one of the largest fossils so far discovered. Cockroaches numbering
upwards of five hundred species have been named. Though large they are
hardly to be regarded as giants, lengths of three or four inches being
about the limit.
Some of the insect types of today quite evidently existed among the
inhabitants of Carboniferous forests, but it is apparent that there were
also some antiquated forms which may have descended from the trilobites.
Although some authorities regard this as the period in which insects
originated, there are others who maintain that definite beginnings are
not established so readily on present evidence. Spiders are believed to
have made their appearance at this time.
Four-footed vertebrates resembling salamanders were prominent animals of
the Carboniferous swamps. At first adapted to a life in water and later
to land conditions, they are known as amphibians, the name being based
on the ability to live in two different kinds of environment. Common
living representatives of this group are the toads and frogs, but these
tailless forms are not known among fossils of the Paleozoic era and are
almost unknown throughout the Mesozoic. The Age of Amphibians, as we
apply that phrase, was definitely not an age of toads and frogs.
These primitive land animals were of different types, ranging from much
smaller sizes up to the length of a crocodile. Most of them had short
legs, and feet which were suitable for locomotion upon land, but many of
the creatures probably spent most of their lives in the water. Tails
were usually high and flattened as if for swimming, sometimes long, at
other times greatly shortened in proportion to the body. Heads were
generally large, jaws long, and mouths wide.
Before the close of the period true reptiles appear, but this race of
animals is destined to make a more spectacular advance than the
amphibians and will be discussed in connection with Mesozoic life. The
amphibians, however, are regarded as being the ancestors of the reptiles
as well as the higher quadrupeds which follow them. Although living
reptiles are readily distinguished from living amphibians there is a
different situation with regard to these primitive forms, for among the
fossils it becomes increasingly difficult to separate the two groups as
new material is investigated.
Invertebrates had their ups and downs during the period. Trilobites
became scarce, and brachiopods for a time were the most abundant of the
shelled animals but later declined rapidly. In favorable localities the
crinoids established a wonderful record for new species before the
period had advanced very far. Hundreds of species of Carboniferous
invertebrates are known, and in many of the rocks of the period they are
the only fossils to be found, for the vertebrates were still unable to
venture far from the swampy districts, and much inland territory was too
well drained to support either the floras or faunas then existing.
In the Colorado area there are both marine and continental formations
but the great coal-making forests and their inhabitants were limited to
other localities. As a consequence this region is not famous for
Carboniferous fossils.