CHAPTER XVIII

GEOLOGICAL HISTORY OF ANIMALS (EXCLUDING VERTEBRATES)

A study of the animals of the past is not only of great interest in itself, but also it furnishes a mainstay of the great doctrine of organic evolution. At the very outset of our discussion the reader should have already in mind at least the main subdivisions of the animal kingdom in order to reasonably well understand where the important animal types of the different geological ages fit in, and how those types bear upon the doctrine of evolution. The accompanying, very brief, general classification includes the usually recognized subkingdoms with special reference to representatives of those which are of most geological and evolutionary significance. Reading downward in this classification, the degree of complexity of organization steadily increases from single-celled animals to man himself.

I. Protozoans, e.g. foraminifers (with lime carbonate shells)

{Sponges II. Coelenterates, e.g. {So-called "jellyfishes," graptolites {Corals

{So-called "sea lilies" III. Echinoderms, e.g. {So-called "starfishes" {So-called "sea urchins"

IV. Worms,

V. Molluscoids, e.g. {So-called "sea mosses" {Brachiopods

{Clams, oysters { {Snails {Pearly nautilus, VI. Mollusks, e.g. { { ammonites { {Cephalopods, e.g. {So-called { { "cuttle fishes"

{Trilobites VII. Arthropods, e.g. {Crabs, lobsters {So-called "sea scorpions" {Insects

{Ostracoderms {Fishes VIII. Vertebrates, e.g. {Amphibians {Reptiles {Birds {Mammals (including man)

Before entering into a brief but rather systematic discussion of some of the most important types of animals which lived during geological time, it may be well for the reader to have in mind some of the most important conclusions which have been reached as a result of the study of the fossil animal records. These conclusions may be summarized as follows:

1. Animal life existed many millions of years ago.

2. Not only the animals of to-day, but also those of any given geological period, directly descended from those of preceding geological periods.

3. Animal life has undergone continuous change since its introduction upon the earth, so that each group of strata, representing a particular geological age, contains a characteristic assemblage of fossil animals.

4. Many of the changes in the history of animals have been progressive or evolutionary, so that strata of early geological time contain distinctly more primitive or lower order forms than the strata of late geological time. But, while the line of evolution has been maintained without a break, culminating in man, there have been many offshoots of a retrogressive nature.

5. Even as far along in geological time as the early Paleozoic era, the highest subkingdom--vertebrates--had no representative whatever. In other words, all the important subdivisions of animal life from a little below fishes to man have been evolved since about the close of the Ordovician period.

6. Any species of animal which ever became extinct has never been known to reappear, and literally tens of thousands of species are known to have become extinct.

7. No species like those now living are found in the more ancient strata, such being confined to the strata of relatively recent geological dates.

8. While more and more highly organized animals have continuously been evolved, many of the earlier and simpler types have persisted, a remarkable case in point being the single-celled animals called foraminifers which may be traced, without very notable change, through the tens of millions of years of geological time from the late Proterozoic era to the present day.

9. Many species have been able to maintain themselves practically without change through long stretches of geological time, while others have had only very brief existence.

When did animal life begin on the earth, and what were the first forms like? We can only partially answer the first question by saying that animals have existed for tens of millions of years, certainly as early at least as Proterozoic time. Up to the present time we are utterly in the dark as to what the earliest animal forms looked like, but we have positive knowledge that the oldest forms found as fossils in the rocks represent creatures which were far more primitive and lower in organization than many animals of to-day, and that since those oldest known forms lived, the animal kingdom has undergone various profound alterations. In view of the above statements, and also the fact that the oldest known plant forms were extremely simple or single-celled, it is more than likely that the first animal life of the earth was single-celled. In harmony with this view is the fact that fossil single-celled animals are found in the very oldest (Proterozoic) rocks which contain any definitely determinable fossil animals.

Do the most ancient known rocks show that animal life existed during Archeozoic time? In the preceding chapter we pointed out the fact that the carbon (in the form of graphite), so commonly present in those most ancient known strata, proves the existence of life of some kind during Archeozoic time. But because nothing like definitely determinable fossil forms have thus far been discovered in those rocks, we cannot be sure whether the carbon represents plant or animal life or both, though certainly plants of very low order at least must have existed. Because of the intense alteration (metamorphism) of those very old strata, all definite forms have long since been obliterated as such. We may, however, in the light of the vast evolution which took place through succeeding geological time, be very sure that any animals which may have existed during Archeozoic time were in general much simpler forms than those of even early Paleozoic time.

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.

The Coelenterates, 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 Coelenterates 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.

Corals comprise another important branch of the Coelenterates. During the Cambrian period there were corallike sponges and possibly simple corals, but from the early Ordovician to the present true corals have been common, especially in the clearer, warmer seas. Their carbonate of lime skeletons have accumulated to help build up great limestone formations representing almost every geologic age from early Paleozoic time to the present. Paleozoic corals were in general notably different from those of later time. There were three main types including the compound "honeycomb" and "chain" types, and the solitary or compound "cup" type. They all had four, or multiples of four, radiating partitions; were rarely branched; and were generally large, some individual cup corals ranging in length from half an inch to a foot or more. Modern corals (beginning with the Mesozoic) have six or eight partitions; are nearly all profusely branched; and are mostly tiny individuals.

Echinoderms are all marine animals, including the so-called "starfishes," which are not really fishes. They have body cavity, with digestive canal, low order nervous system, and a water circulatory system. Most of them have radially segmented shells or skeletons. The oldest fossil forms are found in Cambrian strata, these being very simple or primitive types, with a bladderlike head set on the end of a segmented stem, both head and stem having been supported by carbonate of lime. Such forms lived only to middle Paleozoic time. Ordovician strata contain representatives of all the main types of Echinoderms in well-fossilized forms.

A stemmed Echinoderm of special interest, first known from the Ordovician, has persisted to the present day. It is the so-called "sea lily" or "stone lily," consisting of a complex, headlike portion attached to the sea bottom by a long segmented stem, the whole being supported by lime carbonate. They were very numerous during the Silurian, but they seem to have culminated in variety of species and numbers of individuals during the Mississippian period when they were exceedingly profuse. Hundreds of species of "stone lilies" are known from Mississippian strata alone, and in certain localities, as at Crawfordsville, Ind., and Burlington, Ia., the "stone lily" remains are so numerous that when living they must have literally forested parts of the sea bottom. From Mississippian time to middle Mesozoic time they occupied a relatively subordinate position when they again developed in great profusion. The Mesozoic forms were distinctly more like those of to-day, and it scarcely seems credible that any creature could have contained such a multiplicity of hard parts, more than 600,000 segments having been counted in a single fossil from Jurassic strata. The "sea lilies" of to-day are relatively unimportant.

The familiar five-pointed "starfishes," so common along our seacoasts, are first known from the Ordovician, and they persisted through the many millions of years to the present time with remarkably little change. The so-called "sea urchins" live in rounded, segmented lime-carbonate shells bristling with movable spines. "Sea urchins" are first known from the Ordovician, but they did not become abundant and diversified until Mesozoic time, when many of them took on a very modern aspect.

Worms are known to have existed ever since late Proterozoic time, as proved by the occurrence of tracks, borings and more rarely delicate impressions on rock surfaces. Because of their softness they have rarely been well fossilized and are, therefore, of no great evolutionary or geological importance.

The subkingdom Molluscoids has been richly represented by both the so-called "sea mosses" and brachiopods. The "sea mosses" form colonies of tiny mosslike tufts, resembling corals outwardly, though they are much more highly organized. They have been common from Ordovician time to the present, their carbonate of lime skeletons often having contributed to the building of limestone formations. Brachiopods always have two external shells or valves, in most cases working on a hinge, and also a pair of long, spiral-fringed arms associated with the soft part of the animal inside the shells. They differ from the other type of bivalve (e.g., clam, oyster) in that they are symmetrical with reference to a plane passed through the middle of the shells at right angles to the hinge line. They have rarely grown to be more than a few inches long. A few scant brachiopod remains are known from the late Proterozoic, but throughout known geologic time they reached their greatest development in the Paleozoic era, more especially in the Devonian period. Combining number of species and number of individuals, the brachiopods probably hold the record of all important groups of fossil animals, more than 7,000 species being known. Many layers of rock are filled with their shells (Plate 14). Since the close of the Paleozoic they have fallen off notably, and are now represented by relatively few small forms. From the standpoint of evolution it is interesting to note that in very early Paleozoic time the brachiopods were mostly small, of relatively simple organization, and their thin shells were not joined by hinges. Later they became larger and more complex and their thicker shells worked on hinges. Nearly all the Paleozoic forms had long, straight hinge lines, which made it difficult for their enemies to open them. Along with the change to narrower, curved hinge lines came the decline of the tribe. They have been of great value to the geologist in subdividing the geological column of strata into its many formations.

The Mollusks, which are more highly organized than the Molluscoids, have more or less distinctly developed heads and locomotive organs. Many thousands of species are now extinct, the classes of most geological importance being represented by clams, snails, and the pearly nautilus. Most of them have shells and gills for breathing. The members of the simplest group, well represented by the clam tribe, possess two similar shells working on hinges, so that in this regard they are much like brachiopods, but, unlike the latter, they are not symmetrical with reference to a plane at right angles to the hinge line. Cambrian strata contain the oldest known of the fossil forms where they are small, relatively thin-shelled, and rare. In marked contrast to the brachiopods these bivalves have rather steadily increased in numbers of species and individuals to the present time, now being represented by thousands of forms. During the Mesozoic era they greatly out-numbered the brachiopod bivalves and took on a more distinctly modern aspect, when the oyster tribe and closely related types were prominently developed. Culmination in size and thickness of shell seem to have been reached in early Cenozoic time, strata of that age in certain places, for example in Georgia and southern California, being filled with oyster shells 10 to 20 inches long and 4 to 6 inches thick! In addition to their gigantic size and thickness, many of the shells were fluted or ribbed, and so they represented an extreme type of defensive armor among the lower animals.

Snails have existed from the earliest Paleozoic era to the present time, and the outstanding fact of interest concerning them is that they furnish one of the finest illustrations of an important class of animals which has undergone practically no conspicuous change or evolution during all those millions of years of time.

We shall now turn our attention to the highest order of Mollusks--the cephalopods. These creatures, whose heads are armed with powerful tentacles and supplied with complex eyes, propel themselves by forcible ejection of water. One general type--the chambered cephalopod--has a shell divided into compartments (e.g., modern pearly nautilus) which are successively built up and abandoned by the animal as it grows larger. These chamber-shelled cephalopods constitute one of the most remarkable and instructive illustrations of evolutionary change within any important subgroup of invertebrate animals, ranging from early Paleozoic to the present. Both because of the abundance of fossil forms in rocks of all these periods of geological times, and because certain of the evolutionary changes are so clearly expressed in the well preserved shell portions, they are specially adapted for study. In the late Cambrian only straight and slightly curved forms with smooth, nearly straight chamber partitions existed. Notable advance took place during the next (Ordovician) period when there were straight, curved, open-coiled, and even close-coiled forms. All had simple partitions, and the straighter forms predominated. "The size attained by the Ordovician cephalopods was probably never surpassed by representatives of the class. Some of the (straight) shells were twelve to fifteen feet in length, and a foot in diameter. From this great size they ranged down to or below the size of a pipe stem." (Chamberlin and Salisbury.) They were more than likely the undisputed masters of the Ordovician seas. Silurian time marked no important change in their structures, but the coiled forms predominated for the first time. During the second half of the Paleozoic era all preceding types with simple partitions persisted, but in some forms the simple partitions gradually became angled and finally rather complexly curved. During the Mesozoic era the partition lines of the close-coiled forms evolved until a most remarkable degree of complexity was attained, comparable, indeed, to the sutures of the human skull plates. These remarkable forms called ammonites, of which more than 2,000 species are known, began with the Mesozoic, reached their climax, and passed out of existence toward the close of the same era. Certain strata of Jurassic age are literally filled with ammonites, some shells being several feet in diameter. Various eccentric changes took place in the ammonites shortly before their extinction. Some shells became uncoiled and even straight, thus outwardly at least showing reversion to the original early Paleozoic ancestors, but with retention of the complex partitions. Others assumed spiral shapes and still others became curved or coiled at each end. While these extraordinary evolutionary changes were going on among the chambers of cephalopods during Mesozoic time, some of the ancient close-coiled forms with very simple partitions managed to persist. In fact this simple type, almost exactly like its early Paleozoic ancestor, has been the only one out of this whole remarkable class of animals to persist to the present time, being now barely represented by the well-known pearly nautilus of the Indian Ocean.

During the Mesozoic era the highest type of cephalopod, represented by modern squids and so-called "cuttlefishes," branched off and developed in great profusion. These had slender internal shells, but no external chambered shells. An inky black liquid secreted in a bag was forced out to cloud the water when the animal was escaping its enemy, thus antedating by millions of years the principle of smoke screen so effectively used by ships during the World War. Some Jurassic species got to be over two feet long, and a few specimens of that age have been found in such perfect state of preservation that drawings of the fossils have actually been made with the ink (after moistening) taken from their own ink bags.

Before concluding this chapter we shall take up the salient points in the geological history of Arthropods which constitute the highest subkingdom of all animals except the Vertebrates. They are now very abundant and varied, familiar examples being crabs and insects. A few scant remains of simpler forms are known from the Proterozoic, but since very early Paleozoic time they have been very common and have undergone great evolutionary changes. A few striking examples only will be dwelt upon. Among the most common and interesting of all Paleozoic animals were the trilobites, distantly related to modern lobsters and crabs.

Some of these grew to be two feet long, but usually they were only one or two inches long. First known from the earliest Paleozoic, they reached their culmination relatively early in the era and then dwindled away to utter extinction before its close. "They were characteristic of the Paleozoic era, beginning in great variety in the Lower Cambrian and dominating the seas of the Cambrian (300 species) and Ordovician (950 species). In the Silurian, though they were still common, the trilobites were nevertheless on the decline (485 species), and this ebbing of their vital force is seemingly shown in many picturesque forms replete with protuberances, spines, and exaggeration of parts. As a rule, in evolution, one finds that when an organic stock is losing its vital force there arises in it an exaggeration of parts, as if heroic efforts were being made to maintain the race. Spinosity in animals is often the prophecy of tribal death. In the Devonian, the variety and number of the trilobites were greatly reduced (105 species), at a time when the ancient types of fishes, which undoubtedly fed on these crustaceans (trilobites), began to be common in the seas. In the later Paleozoic seas, the trilobites were relics, or animals surviving from a time better suited to their needs, and one by one they vanished, until a little before the close of the Paleozoic era none were left." (Schuchert.)

An extraordinary type of Arthropod which ranged throughout Paleozoic time and became extinct at its close was the so-called "sea scorpion," closely related to the modern scorpion. Their five or six pairs of appendages all came out from the head portion, one pair in some cases having been developed as powerful pincers. Their culmination in size was reached during the Devonian when some forms grew to the astonishing length of over eight feet! Such gigantic creatures must have been tyrants of the seas until they were subdued by the oncoming powerful fishes. True scorpions are known from rocks as old as the Silurian. Lobsters and crabs made their appearance during the Mesozoic era.

Since insects constitute the highest subdivision of Arthropods, they include the very highest forms of animal life except the Vertebrates. The oldest known fossil insects are from Pennsylvanian strata, more than 1,000 species having been described from rocks of that age. They were all simple or primitive types like cockroaches and dragon flies, and were remarkable for size. Giant cockroaches got to be four inches long. One form of dragon fly, with a spread of wing of over two feet, was probably the largest insect which ever lived (Plate 15). Development of insect life was especially favored during the great Coal Age because of the prolific vegetation, but more than likely insects originated somewhat earlier. Early in the Mesozoic era a great progressive change began to come over insect life and higher forms gradually evolved until by the close of the era many of the highest types like flies, ants, and bees were common. As might be expected, the highest insects did not develop until after the appearance of the true flowering plants in later Mesozoic time, butterflies apparently not having evolved until early in Cenozoic time. Many of the thousands of known species of fossil insects are from strata of Tertiary age during which time they may have been even more numerous than to-day, although there are about 400,000 species now living. An almost incredible case is a Tertiary stratum only a few feet thick in Switzerland from which nearly 1,000 species of insects have been unearthed. Another famous locality is Florissant, Colorado, where during early Tertiary time there was a small lake into which showers of fine volcanic dust fell and entombed vast numbers of insects, more than 2,000 species having been unearthed. Still another extraordinary occurrence is along the shores of the southern Baltic Sea where more than 2,000 species of insects have been found in a fossil resin called amber. The insects were caught in the still soft sticky resin while it was exuding from the trees, and thus we have the insects, fully two or three million years old, literally embalmed and marvelously preserved, often in beautifully transparent amber.