For a time there was no great change in North America following the
opening of the Permian. Then began a series of mountain-making movements
and continental uplifts which drained the swamps, lakes, and inland
seas. With the passing of the vegetation which had established itself in
and around these areas much of the animal life followed. It is probable
that a considerable proportion of the marine life survived, much more
than is indicated by the fossil record, but the receding seas carried
the survivors into territory which is now inaccessible to fossil
hunters.
After Middle Permian time the climate everywhere seems to have been cold
and dry. By the end of the period there had been accomplished more
geographical change throughout the world than at any time since the
beginning of the Paleozoic era. Traces of the crustal movements which
produced new mountain ranges can be followed in Europe, Asia, and North
America. The Appalachian region was raised to a great height, possibly
in excess of three miles. A major disturbance of this character is known
among geologists as a revolution, and to this particular one the name
“Appalachian Revolution” has been given.
The elevation of continents necessarily changes their coast lines. This,
in turn, influences ocean currents which have an important bearing on
climatic conditions. In addition to this, the elevation of mountain
systems not only rearranges the distribution of hot and cold winds over
the land areas but it may produce barriers to the migrations of floras
and faunas, confining them to areas in which it is no longer possible to
live. When the effect upon plants and animals is considered, it is
easier to understand why a line is drawn across the geological time
chart at such a point and an era of prehistory is regarded as closed.
During the Permian period there was recorded in the rocks more
widespread glacial action than ever before or since. With less inland
water to provide the necessary evaporation there was a marked decrease
in rainfall, and arid or semi-arid conditions replaced the hospitable
climate that had been such an important factor in the prolific life of
the Carboniferous. The struggle for existence became intense, but
hardier types of plants and animals, with greater ability to adapt
themselves to adverse conditions, established themselves here and there,
as ancestral forms became extinct. Most of the large spore-bearing trees
died out and seed-producing varieties began to acquire prominence, among
them the coniferous evergreens. Ferns, however, proved their
adaptability by producing some new forms which became prominent in
Permian floras.
The prehistoric amphibians have been divided into three orders, one of
which includes all the larger forms. This group, known as the
labyrinthodonts, continued on through Permian time but began to show
backward tendencies, with dwindling limbs and a return to life in the
water. Among the larger land varieties are typical fossils ranging from
about fifteen inches to five feet in length. In outward appearance they
differed from Carboniferous amphibians. One of the other orders,
including a great diversity of smaller forms, became extinct during this
period, leaving no known descendants. The third order is regarded as the
oldest, and probably the ancestral group from which the modern newts and
salamanders originated.
The most successful of Permian land animals were the peculiar reptiles
that learned to live in drier regions. Like the horned toad and Gila
monster of our arid southwestern United States, the larger Permian
reptiles were four-footed animals. In size and shape they were not
greatly different from amphibians then living. An exception to this
rule, among some of them, is the development of long, bony spines above
the vertebrae of the back. A fairly common fossil of this type, found in
Texas and known as _Dimetrodon_, had a total length of six feet, about
half of this being in the tail. The tips of the spines adorning the back
reached a height of three feet or more and there was probably a covering
of skin over these bones, which would produce a sail-like structure or
“fin” of large size. Its use has not been explained but it provides an
easy name for these odd creatures—the “fin-back lizards.”
Rock deposits produced in arid regions usually have characters which are
not difficult to recognize. Gritty texture, irregular bedding, red
color, and gypsum are common features. Formations of Permian age are to
be found in Colorado but better fossil deposits have been discovered in
Kansas and Texas.
[Illustration: Marine Reptiles: Plesiosaur (Lower Skeleton) and
Mosasaur
The Mesozoic Era produced many types of reptiles besides the
dinosaurs. Two of the marine forms are shown in this illustration,
both from Cretaceous beds of the western United States. Plesiosaurs
were the giants of the seas in their time, lengths of forty to fifty
feet being not uncommon. A long flat tail provided locomotion for
the mosasaurs whereas the plesiosaurs resorted to the peculiar limb
structures known as flippers or paddles.]