FLORAS AND FAUNAS
As the various types of sediments continue to accumulate on land and in water they produce deposits of sandstones, claystones, and limestones which in time may acquire great thickness and cover wide areas of sea floor, or continental surface. Usually there is more or less mixing of sediments resulting in sandy limestones, limy clays, and other combinations. Quite commonly, however, the types remain fairly pure but become arranged in layers which alternate from one kind of material to another. At all times the character of the deposit will depend upon the nature of the rocks which supply the materials, and any fossils that may be produced will consist of such plants and animals as live and die during the time the rock is in the making.
Some of the rock layers will be rich in plant and animal remains, others quite barren, the difference being due partly to conditions influencing the life of the region. In addition, the character and amount of rock-making materials at the time may be favorable or unfavorable to the preservation of fossils. Seas, lakes, and valleys may at any time be drained, or enlarged and deepened, by changes in the elevation of underlying rocks. The amount and variety of mineral substances dissolved in the waters of a region not only affect the character of rock deposits but also the plants and animals living in the water. Some of these chemical solutions provide cementing materials which bind together the grains of sands and mud; others have a detrimental effect upon cementing material previously deposited, and so construction and destruction go on continuously, more or less hand in hand, to produce complicated and often puzzling results.
A little more salt, or a little less of it, may change completely the variety of life inhabiting a body of water. A slight change in the depth of the water often accomplishes the same thing, for plants and animals are so delicately adjusted to their environments that conditions fatal to one race of creatures may provide the exact life requirement of another. This is a matter of practical knowledge which is being used today in the cultivation of plants and animals for market purposes. It is being demonstrated continuously, also, upon living subjects in experimental laboratories throughout the world; and, in a bigger way, the facts are observable wherever life is considered in relation to habitat. That anything so obvious should be regarded as guesswork or theorizing, or opposed to truth, when applied to former inhabitants of the earth, is somewhat surprising. And, it may be added, the cultural worth of fossil study comes to a focus on this very point, for men and women are now meddling, consciously or unconsciously, wisely or unwisely, with an all-important environment about which they have learned very little—one called, among other things, “civilization.”
For any portion of the world a complete-list of the different kinds of plant inhabitants comprises the flora of that region, and a like summary for the animal life is known as the fauna of the district. It is generally understood that different species of both plants and animals inhabit different regions of the earth, but outside of professional circles it is only beginning to be recognized that changes in floras and faunas occur from time to time, that slight differences may be noted in the course of observations extending over a period of only a few years, and that everything in a fauna or flora eventually may be displaced by new forms.
It is, however, a convenient practice to use these terms in connection with time periods, rock beds, and types of environment, as well as geographical areas. Thus we have such phrases as a “Cretaceous fauna” (attaching the name of a geologic period), a “Benton fauna” (with reference to the fossils of a rock formation), a “marine flora” (using the name of an environment), an “Arctic flora” (which applies to a definite portion of the earth surface and its plant inhabitants).
Faunas include animals which many persons do not recognize as such. Sponges, corals, insects, worms, crabs, oysters, and a host of other boneless creatures are grouped together as invertebrate animals, while another group includes the fishes, amphibians (toads, frogs, and salamanders of today), reptiles (crocodiles, lizards, snakes, and turtles being well known varieties), birds, and mammals. This second lot, provided with backbones and skeletons, comprise the great division of vertebrate animals.
Floras also include types which are commonly seen but not popularly identified as plants. The algae are perhaps best known as seaweeds, water-silk, and pond scums; fungi as toadstools and moulds. Both groups are large and of important rank in the vegetable kingdom; only the algae, however, are recognized as important fossil producers. Better known types of plants are the mosses, ferns, evergreens, grasses, and the more conspicuous flower-bearing forms, from weed size to tree size.
Many rocks owe their character to the work of large colonies of plants or animals, for the living organisms are frequently the active agency which takes dissolved mineral substance from the solvent liquid and gets it back into solid form. The liquid is, of course, the water in which the creatures live, while the mineral substance often becomes a commodity required by a plant or animal in its mode of living. Mollusks have a way of using lime in the production of shells, and many a bed of limestone consists almost entirely of this by-product of molluscan life. Tiny coral polyps build complicated and beautiful structures from the same mineral substance. Either intact or in broken condition, these structures contribute in a large way to the making of limestones. Algae, among the lowliest of plants, have done extensive work along similar lines, and numerous invertebrate animals could be named as important factors in the production of rocks. Many of the shells and other fabrications retain their peculiar patterns long after the extermination of their makers, and a highly informative part of the fossil record is provided in this manner. It is also by far the larger portion of the record, for the earlier ages of prehistoric time failed to produce a vertebrate animal of any kind, while the invertebrate record dates back to pre-Cambrian time.