Fossils: A Story of the Rocks and Their Record of Prehistoric Life

by Harvey C. Markman

THE PRE-CAMBRIAN COMPLEX

The rocks of Pre-Cambrian time have been buried deeply under the accumulation of younger sediments, and the resulting pressure in many places has been tremendous. In addition to the effects of pressure there also is recorded in these ancient formations the repeated movements of the materials since they were first deposited. Vertical and side adjustments of parts, with relation to other parts, have distorted the original arrangement of the rock particles to such an extent that ordinary fossils would eventually become unrecognizable. These crushing, grinding, and kneading forces working through millions of years alone would account for the absence of fossils from the older deposits. Frequently the rocks have become so changed in form that their original character can only be conjectured, and because of this change they are known as metamorphic rocks.

A few beds of Archeozoic age remain in nearly their original condition, but they are either without fossils or they have produced very questionable and unsatisfactory specimens. The existence of life during these early stages of earth history is indicated largely by chemical rather than fossil evidence. Much of the ancient limestone has been converted into marble, but it is not unreasonable to believe that plants and animals were instrumental in the production of this type of rock as they are today. Certain varieties of iron ore deposits are now being built up by the aid of plants, and similar ores in the ancient rocks may have had a like origin. The presence of great quantities of carbon, in the form of graphite, may be regarded also as a sign of life, for this substance is accumulated on a large scale by living plants, and may be retained in a solid form after the partial decay of the plant tissues.

So far as the direct evidence goes, there is no sign of any creature of large size or of such complicated structure as the common plants and animals of today. The chemistry of the mineral deposits is not entirely convincing as to the presence of life, but it is regarded as highly probable that microscopic, single-celled plants and animals, comparable to modern algae and protozoa, were in existence during Archean time. Throughout later eras there is unmistakable evidence of gradual development from simpler to more elaborate life-forms and the Archeozoic is commonly regarded as a time of preparation during which simple organisms of some kind were becoming adapted to early conditions which could not support life on a higher plane. The importance of the work done by such lowly creatures in the preparation of suitable environments for more advanced modes of living is overlooked almost entirely.

During the next era, the Proterozoic, the record of life becomes somewhat clearer. Fossils are hardly to be regarded as abundant but there were several well-defined types of animals which left shells and other parts composed of mineral matter. Among these may be mentioned the Radiolaria, Foraminifera, Bryozoa, and Sponges. Radiolaria produced delicate, often lace-like shells of many patterns adorned with the radiating filaments or spines which have suggested the name for this group. Foraminifera produced minute shells, sometimes many chambered, and often bearing a confusing resemblance to the work of snails. Common chalk is composed almost entirely of such shells and fragments of them.

Sponges and Bryozoa are animals of slightly higher organization. They are many-celled instead of one-celled and the cells have special work to perform, which is a most important step in the direction of the specialization which characterizes the structural and life pattern of later arrivals. The Bryozoa lived in moss-like colonies which have been important rock-makers; the fossil forms bear some resemblance to corals. Sponges are too well known to require description although the familiar article of commerce is merely the framework of once-living animals. They represent the earliest organization of true animal bodies even though in appearance they may have a resemblance to plants.

Actual plants of this era were of the algae class, aquatic in habit as were their animal neighbors, the first to leave a record in the form of fossils. This record, obscure and distorted, has long been a source of perplexity to investigators. Without well-defined floras and faunas to guide them, and with rocks frequently in chaotic relationships, early geologists were content to regard it all as a “Pre-Cambrian complex.” Recent studies have contributed a great deal of information not available some years ago. It is quite possible that more advanced types of life were in abundance before the close of the second era, but material on which to base sound opinion is still scarce.

Rocks of Pre-Cambrian age are plentiful in the foothills region west of Denver. The schists, gneisses, and quartzites exposed for some miles immediately beyond the red-beds are part of this great complex. The Idaho Springs formation is known to be one of the oldest in this district, although its exact age has not been determined. Other formations are recognized among the metamorphic rocks of the region but none has contributed to our knowledge of early life.