The early and middle Proterozoic strata throw no more light upon the early history of animal life than do the Archeozoic strata. The upper or later Proterozoic rocks, however, contain the oldest recognizable animal fossils. Very recently fossil remains of single-celled, shell-bearing Protozoans have been found in northern France, while the upper Proterozoic strata of the Rocky Mountains in Montana, and the Grand Canyon of Arizona have yielded worm tracks, a Molluscoid (brachiopod) and fragments of lower forms of Arthropods. This record, although very meager, clearly proves that animal life was so well advanced by late Proterozoic time, that next to the highest subkingdom was actually represented (see [above classification]), and that there must have been a long line of simpler and simpler ancestors, probably extending far back into the Archeozoic era. When we stop to consider that Archeozoic and Proterozoic time was fully as long as all succeeding geological time, it is not so surprising that fairly highly developed animals (except Vertebrates) had been evolved before the close of the Proterozoic era.
In regard to abundance of fossil animals the oldest (Cambrian) Paleozoic strata stand out in marked contrast to the Proterozoic. Many hundreds of species of animal fossils have been described from Cambrian strata, and a great many others yet remain to be discovered. Cambrian fossils are remarkably numerous, varied in species, and complex in organization ([Plate 13]). All subkingdoms of animals except the Vertebrates were represented, though usually only by the simpler types in each subkingdom. It is quite generally agreed that no less than 50 per cent of animal evolution had taken place before the beginning of the Cambrian period. The reader should, however, clearly bear in mind that tremendous advances in evolution have taken place since early Cambrian time when not only all forms from lower scale Arthropods to the highest mammals (including man) have evolved, but also when many thousands of species of lower subkingdom animals developed.
Why are the very early Paleozoic strata so rich in fossils, while the immediately preceding Proterozoic rocks show so few? The seemingly sudden appearance of so many highly developed animals in earliest Paleozoic (Cambrian) time is one of the most important considerations in the history of animal life, and it is by no means definitely understood. The following statements bear directly upon the problem: The early animal forms were probably soft or gelatinous without shells and lived mostly in the open sea where food (seaweeds, etc.) was abundant. Such animals were very unfavorable for preservation in fossil form. Then, late in Proterozoic time or very early in the Paleozoic, a severe struggle for existence set in, probably due to crowding along shores, and hard parts began to develop both for support and defensive purposes. Such hard parts or shells were commonly favorable for fossilization. This view is strongly supported by the fact that very thin shells only are known from late Proterozoic rocks, and mostly very thin shells from the earliest Cambrian, the heavier shells having been evolved later. A fact of importance to bear in mind in this connection is that just at the critical time (late Proterozoic) in shell development, the lands of the earth were undergoing widespread and deep erosion as pointed out early in the chapter on “Ancient Earth History.” The earliest Cambrian strata, therefore, nearly everywhere rest upon the deeply eroded surface of the Proterozoic rocks so that the transition strata—the very ones which would contain most fossils of the early shell development stage—are nearly everywhere missing. Finally, mention should be made of the fact, that all Archeozoic strata are profoundly altered (metamorphosed), and so are nearly all Proterozoic strata, except the later. Fossils once present in those rocks would of course have been obliterated by the process of metamorphism, but the fact remains that very considerable thicknesses of practically unaltered Proterozoic strata show few if any animal fossils.
We shall now proceed to a rather systematic consideration of the most interesting and significant types of creatures which have inhabited the earth since the beginning of Paleozoic time at least twenty-five million years ago. It is our purpose to bring out the salient features in the history of each subkingdom of animals, beginning with the lowest or simplest, and taking up in turn the higher and higher subkingdoms. By this method the reader may easily follow the main thread of organic evolution or progressive change which runs through most of the known history of animal life of our planet, and which is so important in the science of geology.
Protozoans, which include all the tiny single-celled animals, are known in fossil form even in late Proterozoic rocks and, as proved by the fossil records, they have been more or less abundant ever since, even now swarming in large portions of the surface sea waters. One of the most remarkable facts in the history of animal life is, that such exceedingly simple creatures persisted almost without change through the tens of millions of years when such profound and even revolutionary changes took place in the animal kingdom in general. The only fossil Protozoans are those which developed delicate shells either of carbonate of lime (the foraminifers) or silica. Special mention should be made of the Cretaceous period when foraminifers must have been exceedingly profuse in clear sea waters which spread over the Gulf Coastal Plain of the United States, parts of southern England, much of France, and other areas, as proved by their accumulated shells which make up formations of chalk hundreds of feet in thickness and many miles in extent.
Fig. 49.—A compound colony of fossil graptolites characteristic of late Ordovician time, fully 20,000,000 years ago. Each little prong once held a tiny individual living graptolite which was a very simple type of animal belonging to the subkingdom called “Cœlenterates.” (Modified after Ruedemann.)
The Cœlenterates, which comprise the simplest of the many-celled animals, are saclike forms with mouth openings, but with few other differentiations of parts. All are marine animals. Of these the sponges are porous, and the other types (including corals) have tentacles around their mouths. Sponges have been more or less common from early Paleozoic time to the present, and they have undergone relatively little change. “Jellyfishes,” which are in truth not fishes at all, are wholly soft or gelatinous Cœlenterates which have left some very remarkable impressions and casts in strata of very early Paleozoic age, those very ancient forms evidently having been almost exactly like those of to-day. Graptolites were slender, plumelike, delicate forms consisting of colonies of tiny individuals, in many cases in branching or radiating combinations. They existed only during the first half of the Paleozoic era. Both because they floated in the open sea, thus permitting widespread distribution, and because they underwent many distinct species changes during short geologic intervals, they are among the most useful fossils for separating the various subdivisions of strata of the earlier Paleozoic.