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.