Fig. 47.—Map of North America showing the area buried under ice during the Great Ice Age of the Quaternary period; the three great glacial centers; and the extent of mountain glaciers in the west. (After U. S. Geological Survey.)
Fig. 47.—Map of North America showing the area buried under ice during the Great Ice Age of the Quaternary period; the three great glacial centers; and the extent of mountain glaciers in the west. (After U. S. Geological Survey.)
The accompanying map shows the area of nearly 4,000,000 square miles of North America covered by ice at the time of maximum glaciation, and also the three great centers of accumulation and dispersal of the ice. The directions of flow from these centers have been determined by the study of the directions of many thousands of glacial scratches on rock ledges. The Labradorean (or Laurentide) glacier spread out 1,600 miles to the south to Long Island and near the mouth of the Ohio River. The vast Keewatin glacier sent a great lobe of ice nearly as far south, that is into northern Missouri. "One of the most marvelous features of the ice dispersion was the great extension of the Keewatin sheet froma low flat center westward and southward over what is now a semiarid plain, rising in the direction in which the ice moved, while the mountain glaciers on the west (Cordilleran region), where now known, pushed eastward but little beyond the foot-hills." (Chamberlin and Salisbury.)
The Labradorean and Keewatin ice sheets everywhere coalesced except in two places. One of these is an area of about 10,000 square miles mostly in southwestern Wisconsin. In spite of several ice invasions during the Ice Age, this area, hundreds of miles north of the southern limit of the ice sheets, was never ice-covered. There is a total absence of records of glaciation within this area, and so we here have an excellent sample of the kind of topography which prevailed over the northern Mississippi Valley just before the advent of the ice. A much smaller, nonglaciated area occurs in northeastern Missouri near the southern limit of ice extension.
The Cordilleran ice sheet was the smallest of the three, and it was probably not such a continuous mass of ice, the higher mountains projecting above its surface. A surprising fact is that neither this ice sheet nor any other overspread northern Alaska, which is well within the Arctic Circle, during the Ice Age. More than likely the temperature was low enough, but precipitation of snow was not sufficient to permit the building up of a great glacier.
At the same time that nearly 4,000,000 square miles of North America were ice-covered, about 600,000 square miles of northern Europe were buried under ice which spread from the one great center over Scandinavia southwest, south, and southeast over most of the British Isles, well into Germany, and well into Russia.
In both North America and Europe the high mountains, well south of the great glacier limits, especially the Sierras, Rockies, Alps, Pyrenees, and Caucasus, supported many large local glaciers in valleys which now contain none at all or only relatively small ones.
Records of glaciation, such as glacial scratches, bowlders, lakes, etc., occur high up in the White and Green Mountains, Adirondacks, Catskills, and the Berkshire Hills, thus proving that the ice must have been at least some thousands of feet thick over New England and New York. We have good reason to believe that even the highest summits, except possibly in the Catskills, from 4,000 to over 6,000 feet above sea level, were completely submerged under the ice. On top of a mountain of Archeozoic granite nearly 4,000 feet in altitude, facing the St. Lawrence Valley in northern New York, the writer has found many fragments of sandstone which were picked off by the ice in the low valley, moved southward a good many miles, and uphill several thousand feet to the top of the mountain. The reader may wonder how a great glacier at least a mile thick in northern New York could have thinned out to disappearance within the short distance to the southern border of the State, but observations on existing large glaciers show that it is quite the habit for them to thin out very rapidly near their margins, thus producing steep ice fronts.
The fact that glacial ice flows as though it were a viscous substance is well known from studies of valley glaciers in the Alps and Alaska, and the great ice sheet of Greenland. A common assumption, either that the land at one of the great centers of ice accumulation during the Ice Age must have beenmany thousands of feet higher, or that the ice must there have been immensely thick, in order to permit ice flowage so far out from the center, is not necessary. Viscous tar slowly poured upon a level surface will gradually flow out in all directions, and at no time need the tar at the center of accumulation be very much thicker than elsewhere. The movement of glacial ice from the great centers of dispersal during the Ice Age was much the same in principle, only in the case of the glaciers the accumulations of snow and ice were by no means confined to the immediate centers.
The fronts of the vast ice sheets, like those of ordinary valley glaciers, must have undergone many advances and retreats of greater or less consequence. In the northern Mississippi Valley, and also in Europe, there is positive proof for five or six important advances and retreats of the ice which gave rise to the true interglacial stages. The strongest evidence is the presence of successive layers of glacial (morainic) débris piled one upon another, a given layer often having been oxidized, eroded, and even covered with plant life before the next or overlying layer was deposited. Such is the condition of things throughout much of Iowa, where wells sunk into the glacial deposits commonly pass through layers of partly decomposed vegetable matter at depths of from 100 to 300 feet. Near Toronto, Canada, the finding of warm climate plants between two glacial deposits proves that the climate there during an interglacial stage was much like that of the southern States to-day. During the great interglacial stages the vast glaciers were notably restricted in size, and in some or possibly all, cases they may have wholly disappeared from the continent.
In former years there was a tendency to ascribe mighty erosive power to the vast slow-moving ice sheets, but to-day scarcely any geologist would hold that the ice really produced large valleys solely by ice erosion, or that mountains were notably cut down. Throughout the glaciated region, especially toward the north, the deep preglacial residual soils and rotten rocks were nearly all scoured off by the passage of the ice. That the ice, where properly shod with rock fragments, actually eroded to at least little depths into hard and fresh rocks is well known, but the evidence is clear and conclusive that the preglacial hills and mountains, and most of the valleys (including all the large ones), were rarely more than a little modified in shape and size.
One of the principal effects of the Ice Age is the widespread distribution of glacial deposits, and other deposits which were formed under water in direct association with the ice. Such materials have been described in the chapter on “Glaciers and Their Work.”
As a direct result of the Ice Age, many thousands of lakes came into existence throughout the glaciated region where few, if any, previously existed. Many of these lasted only while the ice was present because their waters were held up by walls of ice acting as dams. Thousands of others still persist, most of these having their water levels maintained by dams of glacial débris left by the ice across valleys. Good examples of lakes of both types, including a summary of the remarkable history of the Great Lakes, are considered in the chapter on “A Study of Lakes.”
Many drainage changes, gorges, and waterfalls have also directly resulted from the great Ice Age.In fact it is not too much to say that practically all true gorges and waterfalls of the glaciated region have originated as a direct result of the Ice Age. The most remarkable combination of waterfall and gorge thus produced is that of the world-famous Niagara, described in the chapter on “Stream Work.” Not only are Niagara Falls and gorge of postglacial origin but there was no Niagara River as such before the Ice Age. In New York the well-known Ausable Chasm, Trenton Falls Gorge, and Watkins Glen are all excellent examples of gorges cut since the Ice Age by streams which, because their old valleys were filled with glacial débris, have been forced to take new courses. A gorge of very special interest is that at Little Falls in central New York. This gorge, two miles long, with its precipitous walls hundreds of feet high, is the most important gateway for traffic between the Atlantic border and the Great Lakes region. The bottom of this defile contains six tracks of the New York Central and West Shore Railroads, the Barge Canal, an important highway, and the Mohawk River. Before the Ice Age there was a stream divide instead of a gorge, several hundred feet above the present river level. During a late stage of the Ice Age, when the Great Lakes drained through the Mohawk Valley, a tremendous volume of water passed over the divide and cut it down to form nearly all of the gorge except the inner or bottom trench which has since been eroded by the Mohawk River.
Fig. 48.—Sketch map of the region between Lake George and Schenectady, New York, showing how certain of the main drainage courses have been revolutionized by the great retreating ice sheet and the deposits it left. Preglacial courses shown by dotted lines only where essentially different from the present streams. (By the author, as published by New York State Museum.)
Fig. 48.—Sketch map of the region between Lake George and Schenectady, New York, showing how certain of the main drainage courses have been revolutionized by the great retreating ice sheet and the deposits it left. Preglacial courses shown by dotted lines only where essentially different from the present streams. (By the author, as published by New York State Museum.)
Only a few of the numerous stream changes directly due to the Ice Age will be briefly referred to. Certain of the principles involved are exceptionally well illustrated in the general vicinity of Saratoga Springs and Lake George, New York. During the retreat of the great glacier a lobe of ice occupied the Lake George Valley and forced the Hudson River west over a divide at Stony Creek. Then, because of heavy glacial deposits near Corinth, the Hudson could not continue south through what had been the preglacial valley of Luzerne River, but it was forced eastward over a divide in a low mountain ridge to Glens Falls. The remarkable shift of the Sacandaga River from its preglacial channel was caused by the building up of a great morainic ridge across the valley in the vicinity of Broadalbin.
The drainage of the basin of the upper Ohio River has also been revolutionized as a result of the glaciation. All the drainage of western Pennsylvania passed northward into Lake Erie just before the Ice Age instead of southwestward through the Ohio River as at present.
Rivers as large as the Mississippi and the Missouri were also more or less locally deflected from their preglacial courses. Thus the Missouri, which in preglacial time followed the James River Valley of eastern South Dakota, was forced, by a great lobe of retreating ice, to find its present course many miles farther west.
How long ago did the Ice Age end? In seeking an answer to this question we should bear in mind not only the fact that the Ice Age ended at different times, according to latitude, the more southern districts having been first freed from ice, but also the fact that approximately 4,000,000 square miles of the polar regions are now ice-covered, so that in a real sense those portions of the earth are still in an Ice Age. Some of the best estimates of the length of postglacial time for a given place arebased upon the rate of recession of Niagara Falls, the average of the estimates being about 25,000 years. The evidence for this conclusion is briefly set forth inChapter III. A careful study of the rate of recession of St. Anthony Falls, Minnesota, has led to the conclusion that the last retreat of the ice occurred there from 10,000 to 16,000 years ago. Certain clays deposited under tidewater since the last withdrawal of ice in Sweden show a remarkable succession of alternating layers thought to represent seasonal changes. By counting the layers it has been estimated that Stockholm was freed from ice only 9,000 years ago.
Although the actual duration of the Ice Age is by no means accurately known, we can be quite sure that the time represented is far longer than that of postglacial time. That it must have lasted fully 500,000 years seems certain when due consideration is given to amount of time necessary to bring about the repeated changes of climate between the glacial and interglacial stages; the amount of plant accumulation during the interglacial stages; the amount of weathering and erosion of the various layers of glacial deposits. Some estimates run as high as 1,500,000 years for the duration of the Ice Age, and an average is about 1,000,000 years, which probably indicates, at least roughly, the order of magnitude of the time involved.
When it is considered not only that the fact of the great Ice Age was not even thought of until 1837, but also that many factors enter into the general problem of the climate of geologic time, it is not surprising that the cause (or causes) of the glacial climate is still not definitely known. A few of the various hypotheses which have been advocatedto account for the glacial climate will now be very briefly referred to. One is that the increased cold (not more than 10 to 15 degrees for the yearly average) was brought about by the notably increased altitudes of late Tertiary and early Quaternary times in northern North America and Europe. In this connection it is interesting to note that the four times of real glaciation during geologic time (mid-Proterozoic, early Paleozoic, late Paleozoic, and early Cenozoic) did occur directly after great crustal disturbances and notable uplifts of land. According to this hypothesis the interglacial stages would have to be explained by a rather unreasonable assumption of repeated rising and sinking of the glaciated lands.
Another hypothesis, long held in favor, is based upon certain astronomical considerations. Thus we now have winter in the northern hemisphere when the earth is nearest the sun, but in about 10,500 years, due to wobbling of the earth on its axis, our winter will occur when the earth is farthest from the sun, thus making the winters longer and colder, and the summers shorter and hotter. After a much longer period of time the earth will be millions of miles farther from the sun in winter than in summer and this would still further accentuate the length and coldness of the winters. The interglacial stages represent the 10,500 year periods when the earth in winter (northern hemisphere) is nearest the sun. A difficulty in the way of accepting this hypothesis is that it is inconceivable that each glacial and interglacial stage lasted only 10,500 years. Another objection to the hypothesis as an explanation of Ice Ages is that it is directly opposed by the fact of widespreadglaciation at low latitudes either side of the equator during the late Paleozoic Ice Age.
Another hypothesis is based upon variations in quantity of carbonic acid gas and water vapor in the air. Increase or decrease of these constituents causes increase or decrease of temperature because they have high capacities for absorbing heat. “The great elevation of the land at the close of the Tertiary seems to afford conditions favorable both for the consumption of carbon dioxide in large quantities (by weathering of rocks) and for the reduction of the water content of the air. Depletion of these heat-absorbing elements was equivalent to the thinning of the thermal blanket which they constitute. If it was thinned, the temperature was reduced.... By variations in the consumption of carbon dioxide, especially in its absorption and escape from the ocean, the hypothesis attempts to explain the periodicity of glaciation (i.e., glacial and interglacial stages).” (Chamberlin and Salisbury.)
Still another suggested explanation is based upon variability of amount of heat radiated by the sun. Slight variations are now known to take place, and possibly in the past during certain periods of time these variations may have been sufficiently great to cause a glacial climate with interglacial stages.
Here, as in the case of so many other great natural phenomena, a single, simple explanation does not seem sufficient to account for all the features of the several well-known glacial epochs of geologic time. Two or more hypotheses, or parts of hypotheses, must more than likely be combined to explain a particular Ice Age.
CHAPTER XVII
EVOLUTION OF PLANTS
H
HAVE we any knowledge regarding the beginning of life on our planet? Our answer to this question must be decidedly in the negative. We can, however, be very positive in regard to two important matters concerning life in early geological time, namely, that plants must have existed before animals, and that the very oldest known (Archeozoic) rocks of the earth contain vestiges of organisms. We may be sure that plants preceded animals because animal life ultimately depends upon plants for its food supply or, in other words, all animals could never have been carnivorous. Now, if we can prove that organisms existed during Archeozoic time, it is evident that plants at least must have lived in that oldest known era of earth history. That living things did then exist is proved by the common occurrence of graphite, a crystallized form of carbon, in the oldest known of the Archeozoic rocks. The facts that flakes of graphite are abundantly scattered through many layers of strata of Archeozoic Age, and that adjacent layers of strata contain such varying amounts of graphite, render it practically certain that such graphite represents the carbon of organisms. Graphite existing under such conditions could not be of igneous origin. Carbonaceous or bituminous strata, so called because they contain more or less decomposed organicmatter, would, when crystallized under conditions of metamorphism, yield graphite-bearing rocks exactly like those of Archeozoic Age, and there is every reason to believe that this was their origin. But, since only graphite (carbon) of the Archeozoic organisms remains, the rest having disappeared through chemical change or decomposition, it is impossible to say whether much or all of it represents original plants or animals. In any case we can be very sure about the existence of plants (probably very simple or primitive types) in Archeozoic time, but the presence of any form of animal life has not been proved.
In the next, or Proterozoic era, some plants and animals of definite types are known to have existed and, from here on in the present chapter, it is our purpose to consider the salient points in the geological history of plants, taking up the main types in the regular order of their appearance from the remote Proterozoic days to the present. The very oldest known definitely determinable fossils of any kind are the more or less rounded masses of crudely concentric layers of carbonate of lime from one to fifteen inches in diameter found in middle Proterozoic limestone of western Ontario, Canada. Similar forms are abundant in late Proterozoic strata of Montana. They occur in large numbers as layers or reefs, in many cases repeating themselves through hundreds or even thousands of feet of strata. Careful studies have shown that these forms are the limey secretions of some of the very simplest types of plants, that is thallophytes (e.g., seaweeds), which lived in water.
Before proceeding to describe the plants of Paleozoic and later time, the reader should be impressedwith the important fact that plants of higher and higher types came into existence throughout geological time in almost exactly the botanical order of their classification, that is to say, from the very simplest types (thallophytes) of Proterozoic time there were gradually evolved, through the long geological ages, higher and higher plant forms reaching a climax in the complex and highly organized plants of the present time. This is the most significant general fact in regard to the geological history of plants. For the convenience of the reader the largest subdivisions in the classification of plants are here given.
OUTLINE CLASSIFICATION OF PLANTS
Throughout the first two periods—Cambrian and Ordovician—of the Paleozoic era, plant life appears to have made little or no progress toward higher forms. The very simple Thallophytes (e.g., seaweeds) continued to secrete concentric layers of carbonate of lime in almost exactly the same way as during the middle and late Proterozoic era. Remarkable reefs of such forms occur in the late Cambrian limestone near Saratoga Springs, New York, where one locality has been set aside as a state park. During the Ordovician there were seaweeds of the more familiar branching types without carbonate of lime supports, and these have left very perfect impressions in some of the Ordovician strata.
During the Silurian period seaweeds continued, as, in fact, they did throughout succeeding geologic time to the present. The Silurian strata seem to contain some vestiges of the first-known land plants, though the records are meager and some of the specimens are of a doubtful character. Most interesting of all is a fern or fernlike plant found in France. When we consider the profusion of land plants (all of relatively simple types) of the next or Devonian period, it seems certain that their progenitors must have existed in the Silurian, and their remains may very likely be discovered.
Beginning with the Devonian period of the Paleozoic era the records show that important advances had taken place in the evolution of the plant kingdom. Among the very simple Thallophyte plants some seaweeds of unusually large size occur in fossil form, but the important fact is that all the principal subdivisions of the typical higher non-flowering plants (Pteridophytes) as well as Pteridosperms, and even some primitive representatives of the lower order flowering plants (gymnosperms) were well represented in the Devonian. Our knowledge of land plants earlier than the Devonian amounts to almost nothing and they certainly could not have been at all prominent, but the fossil records make it very clear that many Devonian land areas were clad with rich and diversified plant life. There were even forests, probably the first on earth, but they were far different, both in general and in particular, from those of to-day because the trees were all of exceptionally low organization types. During the next two periods—Mississippian and Pennsylvanian—there was no really important progressin the evolution of plants, and since these remarkable types of land plants have left such wonderfully preserved records in strata of the Pennsylvanian or great Coal Age, we shall proceed to descriptions of the main types of that time, especially those which contributed to the formation of beds of coal.
As shown by the abundant records, the land plant life of Pennsylvanian time must have been not only prolific but exceedingly varied. Thousands of species have been unearthed from the coal-bearing formations alone, and these must represent only a fraction of all species of plants which lived during the period. Most prominent of all were the giant Lycopods constituting the lowest main subdivision of the Pteridophytes (seeabove classification). These great, non-flowering plants were at once the biggest, most common and conspicuous trees of the extensive swamp forests, and they were the greatest contributors to the formation of coal (Plate 15). Many species have been described. They commonly attained heights of 50 to 100 feet and diameters of 2 to 6 feet. In one important type the fairly numerous branches bristled with stiff, needle-shaped leaves. When the leaves dropped off the older or trunk portions, scars were left spirally arranged around the trunks of the trees. In another important type the leaf scars were vertically arranged on the lower portions of the tree trunks. The upper portions of the trunks (rarely branched) were thickly set with long, slender leaves, which in some species were two or three feet long. An interesting fact is that the inner parts of the trunks of the great Lycopods were filled with soft, pithy material. This explains why the fossil trees are nearly always flattenedout, as a result of burial within the earth. The nonbranching type of Lycopod has been totally extinct for millions of years, while the branching type is to-day represented only by small, mostly delicate, trailing plants familiarly known as “club mosses” and “ground pines.” The most conspicuous trees of the great Pennsylvanian lowlands and swamps have, indeed, left meager modern representatives, and here we have an excellent illustration of a once prominent group of plants which has dwindled away almost to extinction.
Another common type of Pennsylvanian vegetation was the so-called “horsetail” plant or giant rush. The much smaller scouring rush, represented by several species to-day, is the direct descendant of this type which, during later Paleozoic time, grew to be 50 to 90 feet high and 1 to 2 feet in diameter. The long, slender trunks filled with pith were segmented with variously shaped leaves arranged in whorls around the joints. A fine, vertical-fluted structure without leaf scars characterized the surfaces of the trunk.
Recent study has shown that many of the Pennsylvanian plants, long classed as true ferns, were really “seed ferns,” as described below. Many of the true ferns grew to be real trees up to fifty or sixty feet high, but all Paleozoic types were primitive in structure as compared with modern ferns.
Very remarkable among the later Paleozoic plants were the Pteridosperms, represented by the so-called “seed ferns.” These now wholly extinct plants seem to have formed the connecting link between the seedless, flowerless plants (Cryptogams) and the seed-bearing, flowering plants (Phanerogams), because they bore seeds but not flowers. Many of them weresmall and herbaceous, but others were tall trees, in general appearance resembling the tree ferns. “Seed ferns,” which play such an important part in the evolution of plants, are not known to have existed after Paleozoic time.
During the latter half of the Paleozoic era some very primitive types of flowering plants (Gymnosperms) existed. Most abundant of these were the so-called Cordaites, which were the tallest trees of the time, some having reached heights of over 100 feet. The upper portions only bore numerous branches supplied with many simple, parallel-veined, strap-shaped leaves up to six feet long and six inches wide. Excepting the pithy cores the trunks of these trees were of real wood covered with thick bark. Trees of this kind became extinct in the early Mesozoic era.
Very late in the Paleozoic (Permian period) two other types of the simple flowering plants (Gymnosperms) made their appearance. These were the cycads and conifers, which were the most conspicuous trees during the first two periods of the Mesozoic era. The cycads reached their culmination in the Jurassic period, but they still exist in modified form in some parts of the world. The short, stout trunk was crowned with long, stiff, palm-like leaves. In fact, the cycads are distantly related to the palms, which belong to a higher group of plants. Some specimens of cycads, especially from the Mesozoic strata of South Dakota, are so wonderfully fossilized that even the detailed structures of trunks, leaves, flowers, and seeds are so perfectly preserved that almost as much is known about these plants of millions of years ago as though they were living forms.
The conifers, with which are classed present-day pines, spruces, and many other evergreen trees, gradually took on a more modern aspect, so that late in the Mesozoic era they were much like those now living. Among the most interesting trees were the sequoias, to which the living “big trees” and red-woods of California belong. These began in relatively late Mesozoic time, reached their climax in numbers, variety of species, and widespread distribution in the early Cenozoic era; and are now almost extinct, being represented by only two species in local portions of California. Cordaites, trees which were so large and abundant in later Paleozoic time, were reduced to extinction in the early Mesozoic era.
During Mesozoic time the Thallophytes, represented by seaweeds, were common. Among the Pteridophytes the ferns and “horsetail” plants were fairly common, but the very large forms gradually gave way to much smaller ones during Mesozoic time. The giant Lycopods of later Paleozoic time dwindled almost to extinction even in early Mesozoic time, so that from that time to the present they have been very small and relatively insignificant.
Tens of millions of years of earth history had passed before the true flowering plants—the Angiosperms—appeared upon the earth. The Cretaceous period marks their advent. So far as known, these plants originated along the eastern side of North America, and very soon after their establishment they spread over the earth with amazing rapidity and dominated the vegetation as they do to-day, more than half of the existing species of plants being Angiosperms. Among the common types which have been unearthed from Cretaceous strata are palms, grasses, maples, oaks, elms, figs, magnolias, willows, beeches, chestnuts, and poplars.
Plate 13.—A Slab of Very Early Paleozoic (Cambrian) Rock, Covered with Some of the Oldest Known Definitely Determinable Animal Remains.These creatures lived in a sea which overspread the site of the Rocky Mountains of southern Canada fully 25,000,000 years ago. Most of the fossils are trilobites (including some very small ones) and other related crustacean forms (lighter portions). (After C. D. Wolcott, Smithsonian Institution, Washington, D. C.)
Plate 13.—A Slab of Very Early Paleozoic (Cambrian) Rock, Covered with Some of the Oldest Known Definitely Determinable Animal Remains.These creatures lived in a sea which overspread the site of the Rocky Mountains of southern Canada fully 25,000,000 years ago. Most of the fossils are trilobites (including some very small ones) and other related crustacean forms (lighter portions). (After C. D. Wolcott, Smithsonian Institution, Washington, D. C.)
Plate 14.—(a)Photographs of Small Slabs of Ordovician Strata Full of Fossils.These slabs are actual bits of sea bottom at least 20,000,000 years old. The left picture shows “stone-lily” stems, so-called “sea mosses,” brachiopods. Right picture shows various species of brachiopods. (Photo by the author.)
Plate 14.—(a)Photographs of Small Slabs of Ordovician Strata Full of Fossils.These slabs are actual bits of sea bottom at least 20,000,000 years old. The left picture shows “stone-lily” stems, so-called “sea mosses,” brachiopods. Right picture shows various species of brachiopods. (Photo by the author.)
Plate 14.—(b)An Outcrop of Middle Ordovician Stratified Limestone in Northern New York.This ledge is full of fossils similar to those above. The material was deposited on the floor of the Ordovician sea which overspread much of the continent. (Photo by the author.)
Plate 14.—(b)An Outcrop of Middle Ordovician Stratified Limestone in Northern New York.This ledge is full of fossils similar to those above. The material was deposited on the floor of the Ordovician sea which overspread much of the continent. (Photo by the author.)
The introduction of the higher flowering plants (Angiosperms) "was, perhaps, the most important and far-reaching event in the whole history of vegetation, not only because they almost immediately became dominant, but also because of their influence upon the animal life of the succeeding periods. Hardly had flowers appeared, before a great horde of insects, which fed upon their honey or pollen, seem to have sprung into existence. The nutritious grasses and the various nuts, seeds, and fruits afforded a better food for noncarnivores than ever before in the history of the world. It was to be expected, therefore, that some new type of animal life would be developed to take advantage of this superior food supply. As we shall see in the discussion of the Tertiary (nextchapter), the mammals, which kept a subordinate position throughout the Mesozoic, rapidly took on bulk and variety and acquired possession of the earth as soon as they became adapted to this new food, quickly supplanting the great reptiles of the Mesozoic." (Cleland.)
During the present or Cenozoic era vegetation gradually took on a more and more modern aspect until the existing species were developed. The grasses especially developed and spread rapidly, but the cereals did not evolve until late in the era. Certain single-celled plants, called diatoms, may be especially mentioned, for they must have literally swarmed in some of the Tertiary seas which spread over parts of the present lands. "The microscopic plants which form siliceous shells, called diatoms, make extensive deposits in some places. One stratum near Richmond, Virginia, is thirty feetthick and is many miles in extent; another, near Monterey, California, is fifty feet thick, and the material is as white and fine as chalk, which it resembles in appearance; another, near Bilin, in Bohemia, is fourteen feet thick.... Ehrenberg has calculated that a cubic inch of the fine, earthy rock contains about forty-one thousand millions of organisms. Such accumulations of diatoms are made both in fresh waters and salt, and in those of the ocean at all depths." (J. D. Dana.)
CHAPTER XVIII
GEOLOGICAL HISTORY OF ANIMALS (EXCLUDING VERTEBRATES)
A
ASTUDY 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.
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 organizationthan 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 recognizableanimal 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 (seeabove 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 notonly 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 whichwould 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 delicateshells 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.