Harvey C. Markman

“ The study of biology, which is the science of life, has provided an instructive viewpoint that enables us to see ourselves against the vast background built up by investigations into the nature of the earth and its ancient inhabitants. This science deals with living creatures as organisms—plants and animals so organized as to be capable of existence only in an environment which provides exact life requirements. ”

“ Man’s efforts to bring about an adjustment between himself and his civilization have centered largely on the method of forcing himself into the mold that happens to be present, one pattern today, another tomorrow. No creature of the past has had to adapt itself to anything so radically new or so thoroughly revolutionary. The vital problem now is whether this man-made environment will prove helpful or disastrous. ”

“ The development of life is perhaps the one outstanding feature of the time divisions, but for the most part the changes in floras and faunas have been gradual rather than abrupt, and this makes it very difficult to draw sharp lines or to visualize beginnings and endings of the various stages of development. Occasionally there is good excuse for drawing a line, where the record is broken and resumed again after a long lapse of time. The principal cause of such breaks is the elevation of great land masses, which brings on an interval of erosion and surface destruction for the areas uplifted. ”

“ The striking feature of life development in the Cenozoic era is the great progress and expansion over the earth of the mammalian races. The division of the era into periods, however, was based largely on a study of fossil mollusks. In the Paris basin of France, it was noticed by the geologists of a century ago that the youngest of the sedimentary beds contained the greatest number of recent or still living species. Successively downward into the older beds the percentage of recent species decreased until there were practically no living species represented in the oldest rocks of the series. ”

“ Explanation of this is that plant and animal tissues quickly decay under ordinary conditions when life ceases. Unless protected from destructive agencies which are especially active at the surface of the ground, even the heavier bones of animals and the large trunks of fallen trees will soon crumble into shapeless masses. The usual method employed by nature to prepare a fossil specimen is so closely related to the natural process of rock making that a little knowledge of that subject will be necessary in order to know what fossils are and how they are preserved for so long a time. ”

“ PLANT LIFE AND CLIMATE
Some idea of the Mesozoic climate is obtained from the character and distribution of the plant life. Triassic floras are not large and there is very little fossil evidence for the earlier half of the period. It is quite possible that arid or desert conditions prevailed for a time in much of North America, as at the close of the Paleozoic era. Plant life was at first not abundant, and conditions were unfavorable for the production of fossils. In Upper Triassic rocks of Virginia, however, there are signs of swampy conditions, with rushes and ferns predominating. ”

“ A formation may be regarded as an extensive rock mass, variable, in thickness and other proportions, as well as in composition, but representing a period of time during which there was no great change in the character of plant and animal life, and no serious interruption in the depositing of the rock-making materials. Occasionally the lower and upper limits of a formation are well defined and readily located. ”

“ The past history of horses is well known from an abundance of fossil material, ranging in age from the Eocene down to the present. Modern horses have only one toe in each foot, but there are remnants of two additional toes which may be seen only in the bony structure underlying the skin. Most of their ancestral relatives were three-toed as far back as the Oligocene period. During Eocene time, however, there was a stage which may be regarded as four-toed although it was evidently a temporary condition, linking known horses with more remote forms having five toes. ”

“ Many rocks owe their character to the work of large colonies of plants or animals, for the living organisms are frequently the active agency which takes dissolved mineral substance from the solvent liquid and gets it back into solid form. The liquid is, of course, the water in which the creatures live, while the mineral substance often becomes a commodity required by a plant or animal in its mode of living. Mollusks have a way of using lime in the production of shells, and many a bed of limestone consists almost entirely of this by-product of molluscan life. ”

“ Zoologically, man is merely one of the creatures that arrived in the course of time, along with other mammals. Just when he arrived and how he looked at the time of his coming cannot be determined from a study of fossils. Perhaps it is of no importance. There is nothing to indicate his existence before the Cenozoic, no completely satisfactory proof of existence before the Pleistocene period. As with other inhabitants of the earth, it is probable that he became prominent only after a great deal of competition with other creatures which kept his ancestors submerged for thousands of years. ”

“ Invertebrate animals appear to have made much progress, but plants were either scarce or too small and delicate to be productive of fossils. It is probable, however, that seaweeds and other algae were flourishing along with the invertebrates, because animal life is directly or indirectly dependent on the existence of plants. The latter sustain themselves by taking carbon and nitrogen from air, water, and soil, but animals must obtain their requirements by eating plants or eating each other. They cannot obtain what they need from the inorganic world without this help from the vegetable kingdom. ”

“ Much of this requires more than the few score years of a human lifetime for its accomplishment, but many of us have seen muds become so solidified, by merely drying, that they could hardly be distinguished from prehistoric shales. It is to be noted also that some ancient fossils come from sandstones which are scarcely more rock-like than the loose sands of an ocean beach. Thus we learn that firm consolidation of rock-making materials is not always a sign of great antiquity, and that hardness of rock is not always essential to the preservation of imbedded plants and animals. ”

“ In many cases, so-called fossil shells are not shells at all; instead, they are merely a stony filling which was once surrounded by 11 shell substance. In other instances the original shell remains as it was during the life of its former occupant, preservation of the shell being due largely to the substitution of a mineral filler for the soft animal tissues once present.
Limestone comes into existence through a more elaborate process than that which produces sandstone and shales. It is one of the three types of common rocks, known collectively as the sedimentaries, in which fossils are found. ”

“ Since a formation may contain a variety of beds, including sandstones, shales, limestones, and all sorts of mixtures, there is sometimes need of subdividing it; but formations are the smallest units commonly shown on geological maps. They are actual rocks which fit into a historical scheme of things and may be regarded aptly as the pages of a book which nature has done in stone. ”

“ Over-specialization in the production of tusks appears to have been the principal factor in the downfall of the mammoths. The large size of the animals and the difficulties of finding sufficient food to sustain life must have been a serious handicap at times, but their ability and inclination to travel over long distances enabled some of them to find tolerable living conditions until the end of the Glacial Period. They are now extinct and the nearest living relatives are the elephants, somewhat reduced in size of tusks and body but otherwise very similar. ”

“ It is generally understood that different species of both plants and animals inhabit different regions of the earth, but outside of professional circles it is only beginning to be recognized that changes in floras and faunas occur from time to time, that slight differences may be noted in the course of observations extending over a period of only a few years, and that everything in a fauna or flora eventually may be displaced by new forms. ”

“ One of the first estimates provided a total of only twenty-five millions of years and included a great stretch of time during which the earth, according to prevailing theory, was more sun-like than rock-like, a time when planets were being born and the earth could not have been in its present physical condition, which is the chief concern of the geologist. Since those earlier conditions could not have supported life as we know it, our knowledge of cosmic history renders small service in the study of fossils. ”

“ It should be understood first that a fossil is some record, commonly preserved in rock, of a kind of plant or animal which no longer exists 6 as a living type. This, at least, is the ordinary sense of the word and more elaborate definitions are of small service to anyone. It may be necessary to add, however, that all things which have lived at any time are regarded as either plants or animals. ”

“ A slight change in the depth of the water often accomplishes the same thing, for plants and animals are so delicately adjusted to their environments that conditions fatal to one race of creatures may provide the exact life requirement of another. This is a matter of practical knowledge which is being used today in the cultivation of plants and animals for market purposes. It is being demonstrated continuously, also, upon living subjects in experimental laboratories throughout the world; and, in a bigger way, the facts are observable wherever life is considered in relation to habitat. ”

“ The monotremes or egg-laying types of mammals such as the duck-bill and spiny anteaters which now inhabit Australia are almost unknown as fossils. Marsupials, the next higher living group, which includes the opossum and kangaroo, appeared at the end of Cretaceous time along with the placentals or higher mammals which dominate the history of the Cenozoic era. Nevertheless, there are a few teeth and jaws from rocks of Triassic and Jurassic age to indicate that small mammals, from the size of mice to slightly larger than rats, existed throughout most of the Age of Reptiles. ”

“ Animals that perish in bogs or quicksands are soon covered over; in many localities wind-blown dust and sand do the work; and flooded river valleys provide an abundance of mud for the necessary burial of others. Even underground, the decay of soft tissues is too rapid to permit of replacement by mineral substance in a manner that would reproduce form and texture. Skin and flesh are almost invariably lost, although in a few instances the thick scaly hides of dinosaurs are known to have produced natural molds and casts by the method explained in connection with footprints and other impressions. ”

“ Coal beds often produce fossils of an unusual sort. In the formation of coal, plant material gradually loses some of its more perishable substances but retains carbon which has better lasting qualities and slowly accumulates to produce the seams and beds that are mined. In the early stages of the process the original vegetation undergoes little change in appearance but eventually practically all of its character is lost. Many fossil leaves are found as thin layers of carbon, bedded in the clays which are commonly associated with coal deposits. ”

“ A few beds of Archeozoic age remain in nearly their original condition, but they are either without fossils or they have produced very questionable and unsatisfactory specimens. The existence of life during these early stages of earth history is indicated largely by chemical rather than fossil evidence. Much of the ancient limestone has been converted into marble, but it is not unreasonable to believe that plants and animals were instrumental in the production of this type of rock as they are today. ”

“ Some of the rock layers will be rich in plant and animal remains, others quite barren, the difference being due partly to conditions influencing the life of the region. In addition, the character and amount of rock-making materials at the time may be favorable or unfavorable to the preservation of fossils. Seas, lakes, and valleys may at any time be drained, or enlarged and deepened, by changes in the elevation of underlying rocks. ”

“ Four-footed vertebrates resembling salamanders were prominent animals of the Carboniferous swamps. At first adapted to a life in water and later to land conditions, they are known as amphibians, the name being based on the ability to live in two different kinds of environment. Common living representatives of this group are the toads and 43 frogs, but these tailless forms are not known among fossils of the Paleozoic era and are almost unknown throughout the Mesozoic. The Age of Amphibians, as we apply that phrase, was definitely not an age of toads and frogs. ”

“ During the Pliocene period, in the genus Pliohippus and also in Hipparion, the feet were far advanced in structure, with most of the species single-toed, the side digits having reached the splint stage. Pleistocene horses of the genus Equus, like living species of that genus, were strictly one-toed animals, ranging over grassy areas and highly specialized for a life in that kind of environment.
Specialization is to be noted partly in the foot and leg structures where the modifications have contributed to greater speed and travelling ability. ”

“ The story of plant life becomes much clearer in the next period when conditions were more favorable for the production and preservation of plant fossils.
Land animals of the time are almost unknown. A few snails and scorpions have been found, and some footprints made by early amphibians. Insects probably were in existence although the evidence is not quite clear on this point. The increasing number of fresh-water fishes, however, may be regarded as a sure indication that inland conditions were becoming more favorable for plant and animal inhabitants of all kinds. ”

“ Flexibility in other directions is sacrificed for greater strength, and the foot incidentally becomes less suited for other purposes.
This is what is meant by “specialization”—a departure from “generalization.” The study of fossils provides numerous illustrations of specialized development which contributes greatly to an interest in prehistoric life. Any specialized structure or habit which increases fitness for a particular way of living is also known as an “adaptation.” ”

“ Isolated patches of fossil-bearing sediments frequently record the migration of animals in unmistakable terms, but the details of the wanderings and the conditions encountered in the newly established habitats are often left in doubt. To correlate the facts revealed at one locality with findings at other places and, if possible, to date all prehistoric events with a greater degree of accuracy are among the major tasks of current investigations. ”