The Distribution of Animals in Time (Palaeontology)
It is quite as essential to an intelligent understanding of living organisms upon the earth today, to have some vision of the long pageant of preliminary life in the past, as it is for a statesman to be well versed in the history of events leading up to the state of affairs with which his present problems are involved.
Species of animals and plants, like individuals, pass through successive stages that resemble the phases of a single life. Expanding childhood, vigorous youth, sustained maturity, and decrepit old age succeed each other only to end inevitably in death or extinction. Sometimes a species like an individual may complete its life without leaving any issue behind, but oftener, in the long course of its existence, it somehow gives rise to species different from itself, a process which has brought about the infinite diversity of living forms that connect monad with man.
Certain conservative kinds of organisms that are well adapted to their niches in nature persist, retaining their characteristics without significant evolutionary advance for unthinkably long periods of time, while other species, exhibiting a wider range of variability, live a faster, more diversified life and advance more rapidly along the transforming highway of evolution, only to meet extinction sooner. The brachiopods, Lingula and Terebratula, for example, the modern living representatives of which are hardly to be distinguished from remote fossil ancestors found buried in the most ancient sedimentary rocks, are instances of conservative species that have shown almost no progress, while trilobites, ammonites, pterosaurs, and dinosaurs are large representative groups of more ambitious animals, of astonishing diversity of form and structui al detail, which have long since paid the death penalty for their high degree of specialization.
Examples of all stages of the process of coming to an end on the part of a species may be cited. For instance, among birds the ivory-billed woodpecker and California condor are probably marked for extinction in the near future. This is not so much because they are being crowded off the earth by dominant man as because they are in the biological blind alley of overspecialization with a consequent lack of ability to adapt themselves to changing conditions, which means that they are nearing the end of their organic resources. In fact, birds taken as a group are so highly specialized that they have no future evolutionary escape, since that is possible only in generalized types having capacity for further adaptation.
There are people now living who remember the hordes of passenger pigeons that formerly darkened the skies, but the last individual of this species died in captivity only a few years ago, while the passing of the dodo, the great auk, and Steller’s sea cow are matters of recently recorded history. The hairy mammoth, New Zealand moa, sabre-toothed tiger, and woolly rhinoceros came to their end just before the beginnings of recorded human history. Back of these recent antiquities stretches a long interminable line of various species whose chapter of existence closed so long ago that our ordinary measures of time entirely fail to express the fact adequately.
There is no doubt that living species number but a small fraction as compared with vanished ones formerly peopling the globe, whose race has long since been run.
The dawn of life is unknown, for the oldest sedimentary rocks in which the first known evidences of life appear yield a wide variety of forms, such as protozoans, sponges, corals, jellyfishes, echinoderms, worms, brachiopods, mollusks, and trilobites. This means that the great Canterbury Pilgrimage of organisms had already been traveling for some time along the evolutionary road, before we catch our first glimpse of the pageant.
Fossils are nature’s hieroglyphics. They include the sum total of our actual documentary evidence of organic evolution, and besides form the alphabet in which the language of biological history is written. Sir Charles Lyell, the eminent geologist who did so much to influence young Charles Darwin at the beginning of his career, defines a fossil as “any body or traces of body, animal or vegetable, buried and preserved by natural causes.” Every fossil is either ancestral to some living thing, or is representative of an extinct line.
The science of fossils, or the ancient history of animals and plants, is called Palaeontology.
Former Ideas About Fossils
Fossil remains of animals and plants, although known for a long time, have been variously misunderstood in the past. To Aristotle and the ancients they were artificial results of spontaneous combustion, or abortive attempts of inorganic matter to take on the form of life. Empedocles, who found fossil hippopotamus bones in Sicily, thought he had discovered a battle-ground where gods and titans fought. Henrion, in 1718, regarded fossils as molds and casts left over in the creation of animals and plants. He was the cocksure writer who reported that the height of Adam was 123 feet and 9 inches, but since he carelessly neglected to specify whether or not the measurement was taken in his “stocking feet,” and as he did not make clear how he arrived at his result, his opinion is regarded with some suspicion by modern science.
As late as 1823 William Buckland of Oxford wrote learnedly of fossils under the title, On Observations on Organic Remains attesting the Action of a Universal Deluge. Lyell states that it took a hundred and fifty years of dispute and argument to persuade scholars that fossils were really remains of what were once living organisms, and a hundred and fifty years more to convince them that they were not the results of Noah’s flood.
Today a vast number of fossils have been recovered from oblivion from many parts of the world, and together they present a most illuminating and convincing mass of evidence concerning the ancient inhabitants of the earth. Even when fragmentary and imperfect, as most of them are, they furnish irrefutable proof of vanished life. The only questions that arise about fossils today concern the restoration of missing parts, the period or geological horizon when they lived, and their place in the evolutionary series. Dr. Lull of the Peabody Museum at Yale University, whose wide knowledge of fossils gives weight to his opinion, declares that “of the finally established facts which the fossils proclaim, we are as certain as we are of anything in this world.”
Conditions of Fossilization
Various factors are involved in the process of fossilization. There is no reason to believe that these factors which have been effective in the past are not at work today. The great majority of individual animals and plants do not become fossils, but return at death to their inorganic origins along the route of decay, or by being devoured by animals.
It is usually essential that hard parts like bones, teeth, shells, scales, or chitin be present, and that the conditions for natural burial and the exclusion of air be such as to aid in the preservation of these parts.
However, Dr. C. D. Walcott has published a book of unexpected facts concerning Fossil Medusae, in which are pictured a great variety of these fragile creatures which succeeded in leaving a fossil record of themselves in spite of the fact that their jellylike bodies had no hard parts, and were over 95 per cent water.
The manner of burial in fossilization may be sudden and catastrophic, as by landslide, earthquake, devastating flood, overwhelming sand storm, or by a rain of volcanic ashes such as fossilized the entire cities of Herculaneum and Pompeii, or it may be exceedingly slow, as in the formation of sedimentary rock under water, the incrustations resulting from immersion in mineral-impregnated hot springs, or by the drip of limy water which forms stalactites and stalagmites in limestone caverns.
Quicksands, swamps, and bogs may engulf animals also and thus favor fossil formation by preventing rapid decay through the exclusion of air. As a matter of fact “bog water” is said to possess antiseptic properties to a remarkable degree.
Amber, which is fossilized pitch, or the solidified juice of resinous plants, furnishes another kind of burial place. Insects crawling on the trunks of ancient conifers, that became entangled in the sticky exudations there, have succeeded far better than any ancient mummified Egyptian, dreaming of immortality, in perpetuating their mortal bodies intact in a world of universal decay.
At Rancho la Brea, near Los Angeles, California, there are famous asphalt beds in which at some time long ago a great variety of animals, horses, tapirs, llamas, elephants, mastodons, giant sloths, huge wolves, lions, and sabre-toothed tigers, were not only entrapped and killed but were also preserved as fossils.
In detritus-filled caverns where dying animals have retreated, fossils are frequently found.
Sixty miles north of the Arctic Circle at Beresovka, in Siberia, a mammoth was discovered in a pit, frozen and so perfectly preserved in ice that some of the flesh was eaten by the discoverers, many thousand years after it was accidentally placed there in cold storage. This was not an isolated case. Many other instances of frozen carcasses of mammoths have been reported in northern Siberia.
On oceanic islands, such as the Chincha Islands off the coast of Peru, where for long periods of time sea birds have resorted to nest and where there is scarcely any rainfall, the dried excreta of birds, commercially known as “guano,” is deposited, frequently to a depth of several hundred feet, forming a natural burial place for organic remains.
In the Peabody Museum at Yale University is the skeleton of an extinct species of ground sloth, JVototherium, that was recovered from a cave in New Mexico where it was buried and preserved in bat guano. The preservation was so complete that it was possible to determine by the stomach contents that it died in the spring of the year, and that the vegetation of Pleistocene times was practically like that of today.
Uses of Fossils
Fossils, as Dr. Joseph Leidy many years ago quaintly said of the Protozoa, are chiefly useful as “food for the intellect.”
Among various intellectual uses to which fossils are put, not the least is that of “faith testers,” so called by good people alarmed at the silent evidence thus presented of the great antiquity of the earth which they had been taught to believe, had been created only a few thousand years ago. To the scientist these “medallions of creation” show first of all something of the racial history of animals and plants. In the absence of direct evidence, the past history of most animals and plants must remain largely a matter of conjecture, but there are some authentic instances of modern animals whose ancestral modifications are written very legibly in the fossils that have been found. For example, the horse has a well established family tree extending backward without serious gaps for at least three million years to the little four-toed ancestor, Eohippus, of Eocene days. The actual fossil evidence for this remarkable pedigree may be seen by any visitor at the American Museum of Natural History in New York City, or at the Peabody Museum of Yale University in New Haven (Fig. 80).
Fossils are furthermore useful as indicators of past climatic conditions on the earth. The discovery of fossil palms in Wyoming, breadfruit in California; ferns in Greenland; reindeer in France; and musk oxen in Kentucky, records the indisputable fact that profound changes in climatic conditions have occurred in all of these places in the past.
Fossils also serve as measures of geologic time. Just as the date upon the corner stone indicates the year when the building was dedicated, so the presence of certain types of fossils in a particular stratum of sedimentary rock indicates the approximate time when those rocks were laid down. Or to state the value of a time measure by a further comparison, just as the character f instead of s on the page of an old book measures the limit of its publication by the year 1800, about which time the character s replaced f in general use, so the presence of a time-dating fossil on a geologic page measures the limits of its formation.
Kinds of Fossils
The following classification of different kinds of fossils is modified from that given by Professor R. M. Field in Science for June 25, 1920.
I. Those furnishing direct evidence:
1. Actual remains, such as insects in amber, and mammoths in ice;
2. Minute replacements, molecule by molecule of the original organic matter by mineral salts, resulting in petrifaction;
3. Coarse replacements, secondhand copies of originals by means of molds and casts;
4. Prints and impressions, of leaves, jellyfish, etc.
II. Those furnishing indirect evidence:
1. Coprolites, that is, solidified excreta or casts of the same;
2. Artifacts, such as ant-hills or prehistoric fashioned flints;
3. Tracks, trails, and burrows, all autographs of living animals;
4. Geologic formations, originating from organic sources, such as graphite, limestone, flint, coal, and petroleum.
Imperfections in the Record
Huxley said that the whole geologic record of fossils is “only the skimmings of the pot of life.” Although incomplete it is nevertheless the most convincing evidence of the story of the past.
The absence of suitable conditions for fossilization which surrounds the passing of the vast majority of animals and plants, as well as the inaccessibility to man of most of the fossils that actually succeed in being formed, make the task of the palaeontologist a particularly difficult one. The pages of the Great Stone Book on which the buried dead have written their own autographs cannot be freely shuffled over in order to read the story contained therein, because they are firmly stuck together. The fossil writing is, therefore, quite inaccessible except as lucky chance reveals enticing fragments of it, as when slow erosion bevels down the margin of the page exposing some few organic syllables, or when, by the puny engineering feats of man, the surface of the earth is somewhere scratched open, accidentally uncovering part of its buried treasures.
In many instances the natural sequence of rock stratification has been so confused that the student finds the pages of his book misplaced, by distortion, faulting, or folding, as in mountain formation. The more recent strata sometimes even come to lie beneath the older ones. The irregular and fragmentary character of the fossiliferous strata thus greatly increases the difficulties that confront the student who would correctly read the story of the past.
Sedimentary rocks of the earth’s crust containing fossils are not arranged in uniform continuous strata that envelop the entire globe like the layers of an onion, but form in patches of unequal thickness and extent, according to the distribution of the water areas at the time of their deposition. There is no doubt that the earlier records of life in the form of fossils have in many cases been entirely obliterated by the action of heat and pressure during the metamorphosis of rocks into gneiss, marble, and granite, while the fossils that are buried in sedimentary rocks of the ocean floor are “forever hidden from hammer and mind.”
According to the Bureau of Mines, Department of the Interior the deepest hole that man has ever made down into the undisturbed fossil-bearing epidermis of the earth is in West Virginia, where borings to the depth of 7579 feet were made in search for natural gas. The deepest mine in the world is said to be the St. John del Rev mine in Brazil, while the “Village Deep” workings of the Transvaal gold mines of South Africa take second rank with a depth of 6263 feet. In the United States the deepest mine workings are those of the “Calumet and Hecla” in Michigan which are reported to have reached 5990 feet below the surface. This is a distance of about a mile and is the nearest approach that man has ever made to the center of the earth. These extraordinary depths when compared with the total diameter of the earth, or even with the known thickness of fossiliferous rocks, are so insignificant that it is doubtful if they could be graphically represented to scale even by a shallow scratch on the surface of a four-foot globe. David Starr Jordan has truly said that the case of the palaeontologist is much like that of a traveler who, landing for five minutes on some remote corner of Australia, forthwith attempts a description of the entire continent from the observations made. The wonder is not that so little is known of the fossil record of animals and plants, but that, in the face of so many difficulties, so complete and connected a story of ancient life has been unearthed.
A Geologic Time Scale
The fragment of eternity that comes within the vision of the geologist has been divided into unequal eras of time, beginning after the earth had cooled down enough to be clothed with an atmosphere and to have its surface diversified into areas of land and water. See Table II. The succeeding eras are measured by the time taken to form stratified rocks through the erosion and disintegration of the original fire-fused rocks, and the subsequent rearrangement of their component particles as sediment under water. Such sedimentary rocks afford sanctuary to organic remains and form the happy hunting grounds of palaeontologists.
Eras from ancient to modern times are: Archaeozoic, Proterozoic, Palaeozoic, Mesozoic, and Cenozoic.
The Archaeozoic Era is characterized principally by igneous and metamorphosed rocks without proved fossils, although traces of graphite indicate that plant life, probably in the form of primitive seaweeds, must have been in existence. The fiery furnace that fashioned the archaeozoic rocks, however, was no suitable place for the preservation of whatever organic remains existed in those formative days.
The Proterozoic Era saw the slow rise of the lower plants and most of the main general types of invertebrate animals. Together with the Archaeozoic Era, according to Professor Schuchert, it constitutes over one half of the total column of known sedimentary rocks, which reaches'all together a maximum thickness of 114 miles in North America, although he qualifies this statement by saying: “In no one place, however, can be seen more than a small part of this record, for usually the local thickness is under one mile, though there are limited regions where as much as twenty miles of it are present.”
The Palaeozoic Era has been called the “Age of Fishes” because these animals became dominant during this time. The actual interval which elapsed in the Palaeozoic Era has been estimated as about 300 million years, surely sufficient time for many dynasties of plants and animals to have had their day.
Following the Palaeozoic, the Mesozoic Era witnessed the “Golden Age of Reptiles,” some 150 million years long.
Finally, the Cenozoic Era, or “Age of Mammals,” probably represents a little more than 60 million years. The geologists usually subdivide the Cenozoic into seven epochs so that beginning with the oldest, the Tertiary is made up of the Paleocene, Eocene, Oligocene, Miocene and Pliocene which are then followed by the two Ouarternary epochs, the Pleistocene and Recent. During the Pleistocene there were great climatic changes in northern Europe and North America. Possibly as many as four different times a gradual fall in the temperature of the northern United States and southern Canada resulted in a southward advance of the ice sheet to cover these areas for tens of thousands of years, only to be followed by a sufficient rise in temperature to bring about a retreat of the glaciers and return of these areas to temperate or even tropical conditions. Thus periods of glaciation alternated with interglacial periods. Similarly, areas of northern Europe were subjected alternately to long periods of glaciation and temperate climates.
The most recent episode in all this great moving spectacle of earth transformation is the story of human evolution, extending over only a few hundred thousand years at the outside, which in comparison with the stretches of time under consideration is but the thinnest surface film on the face of an abyssal ocean. Our actual fossil records of man are limited to Pleistocene and Recent times.
Pictet's Palaeontological Laws
A summary of some of the more important conclusions which follow from a study of fossils is embodied in the six “laws” adapted from Traite elementaire de palaeontologie by Jules Francois Pictet (1809-1872), as follows:
1. All stratified rocks may contain fossils, therefore, life has been present on the earth at least since the beginning of the Palaeozoic Era.
2. The oldest strata contain extinct species and largely extinct genera, while more recent strata contain forms like the living, therefore, the deeper the stratum the more divergent from those now living are the forms found therein.
3. Different fossil faunas and floras follow each other in the same sequence everywhere, the layers nearest together stratigraphically contain forms most alike, therefore, fossils show the evolution of forms from one another.
4. Constant change is the inevitable law of life. Species characteristic of one level or time are partly or completely replaced later by other species, therefore, species are not permanent or unchanging but are constantly giving way to modified forms that are presumably better qualified to occupy their place in nature.
5. Species, as well as individuals, pass regularly through a cycle, including infancy, youth, maturity, and senility, therefore, many groups of organisms (as graptolites, trilobites, and ammonites), have died out entirely and do not reappear, having completed their cycle.
6. The approximate age of any stratum may be determined by the degree of similarity of its fossils to living forms. Similar fossils in different regions are indications of geologic strata of contemporaneous formation, therefore, fossils serve to determine the age of rocks in which they are found.