PERMIAN HARDSHIPS
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.
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.