NATURE AND USES OF FOSSILS.

Scope of Geology.—

The science of GEOLOGY, of which PALAEONTOLOGY or the study of fossils, forms a part, is concerned with the nature and structure of the earth, the physical forces that have shaped it, and the organic agencies that have helped to build it.

Nature of Fossils.—

The remains of animals and plants that formerly existed in the different periods of the history of the earth are spoken of as fossils. They are found, more or less plentifully, in such common rocks as clays, shales, sandstones, and limestones, all of which are comprised in the great series of Sedimentary Rocks (Fig. 1).

According to the surroundings of the organisms, whether they existed on land, in rivers, lakes, estuaries, or the sea, they are spoken of as belonging to terrestrial, fluviatile, lacustrine, estuarine, or marine deposits.

The name fossil, from the Latin ‘fodere’ to dig,—‘fossilis,’ dug out,—is applied to the remains of any animals or plants which have been buried either in sediments laid down in water, in materials gathered together by the wind on land as sand-dunes, in beds of volcanic ash, or in cave earths. But not only remains of organisms are thus called fossils, for the name is also applied to structures only indirectly connected with once living objects, such as rain-prints, ripple-marks, sun-cracks, and tracks or impressions of worms and insects (Fig. 2).

Preservation of Fossils.—

In ordinary terms, fossils are the durable parts of animals and plants which have resisted complete decay by being covered over with the deposits above-named. It is due, then, to the fact that they have been kept from the action of the air, with its destructive bacteria, that we are able to still find these relics of life in the past.

Petrifaction of Fossils.—

When organisms are covered by a tenacious mud, they sometimes undergo no further change. Very often, however, moisture containing mineral matter such as carbonate of lime or silica, percolates through the stratum which contains the fossils, and then they not only have their pores filled with the mineral, but their actual substance may also undergo a molecular change, whereby the original composition of the shell or the hard part is entirely altered. This tends almost invariably to harden the fossils still further, which change of condition is called petrifaction, or the making into stone.

Structure Preserved.—

Petrifaction does not necessarily destroy the structure of a fossil. For example, a piece of wood, which originally consisted of carbon, hydrogen, and nitrogen, may be entirely replaced by flint or silica: and yet the original structure of the wood may be so perfectly preserved that when a thin slice of the petrifaction is examined under a high power of the microscope, the tissues with their component cells are seen and easily recognised (Fig. 3).

Early Observers.—

Remains of animals buried in the rocks were known from the earliest times, and frequent references to these were made by the ancient Greek and Roman philosophers.

Xenophanes.—

Xenophanes, who lived B.C. 535, wrote of shells, fishes and seals which had become dried in mud, and were found inland and on the tops of the highest mountains. The presence of these buried shells and bones was ascribed by the ancients to a plastic force latent in the earth itself, while in some cases they were regarded as freaks of nature.

Leonardo da Vinci.—

In the sixteenth and seventeenth centuries Italian observers came to the fore in clearly demonstrating the true nature of fossils. This was no doubt due in part to the fact that the Italian coast affords a rich field of observation in this particular branch of science. The celebrated painter Leonardo da Vinci (early part of the sixteenth century), who carried out some engineering works in connection with canals in the north of Italy, showed that the mud brought down by rivers had penetrated into the interior of shells at a time when they were still at the bottom of the sea near the coast.

Steno.—

In 1669, Steno, a Danish physician residing in Italy, wrote a work on organic petrifactions which are found enclosed in solid rocks, and showed by his dissection of a shark which had been recently captured and by a comparison of its teeth with those found fossil in the cliffs, that they were identical. The same author also pointed out the resemblance between the shells discovered in the Italian strata and those living on the adjacent shores. It was not until the close of the eighteenth century, however, that the study of fossil remains received a decided impetus. It is curious to note that many of these later authors maintained the occurrence of a universal flood to account for the presence of fossil shells and bones on the dry land.

Fossils an Index to Age.—

A large part of the credit of showing how fossils are restricted to certain strata, and help to fix the succession and age of the beds, is due to the English geologist and surveyor, William Smith (Fig. 4). “The Father of English Geology,” as he has been called, published two works[1] in the early part of last century, in which he expressed his view of the value of fossils to the geologist and surveyor, and showed that there was a regular law of superposition of one bed upon another, and that strata could be identified at distant localities by their included fossils. Upon this foundation the work of later geologists has been firmly established; and students of strata and of fossils work hand in hand.

Stratigraphy.—

That branch of geology which discusses the nature and relations of the various sediments of the earth’s crust, and the form in which they were laid down, is called Stratigraphy. From it we learn that in bygone times many of those places that are now occupied by dry land have been, often more than once, covered by the sea; and thus Tennyson’s lines are forcibly brought to mind—

“There where the long street roars hath been
The stillness of the central sea.”

Elevated Sea-beds.—

A striking illustration in proof of this emergence of the land from the sea is the occurrence of marine shells similar to those now found living in the sea, in sea-cliffs sometimes many hundreds of feet above sea-level. When these upraised beds consist of shingle or sand with shore-loving shells, as limpets and mussels, they are spoken of as Raised Beaches. Elevated beaches are often found maintaining the same level along coast-lines for many miles, like those recorded by Darwin at Chili and Peru, or in the south of England (Fig. 5). They also occur intermittently along the Victorian coast, especially around the indents, where they have survived the wear and tear of tides along the coast line (Fig. 6). They are also a common feature, as a capping, on many coral islands which have undergone elevation.

Sea-beds far from the Present Coast.—

Marine beds of deeper water origin may be found not only close to the coast-line, but frequently on the tops of inland hills some miles from the sea-coast. Their included sea-shells and other organic remains are often found covered by fine sediment forming extensive beds; and they may frequently occur in the position in which they lived and died (Fig. 7). Although it is well known that sea-birds carry shell-fish for some distance inland, yet this would not account for more than a few isolated examples.

Raised Beaches as Distinct from Middens.—

Again, it may be argued that the primitive inhabitants of countries bordering the coast were in the habit of piling up the empty shells of the edible molluscs used by them for food: but these “kitchen middens” are easily distinguished from fossil deposits like shelly beaches, by the absence of stratified layers; and, further, by the shells being confined to edible species, as the Cockle (Cardium), the Blood-cockle (Arca), the Mussel (Mytilus), and the Oyster (Ostrea) (Fig. 8).

Submerged Forests.—

Evidence of change in the coast-line is shown by the occurrence of submerged forest-land, known as “fossil forests,” which consist of the stumps of trees still embedded in the black, loamy soil. Such forests, when of comparatively recent age, are found near the existing coast-line, and may sometimes extend for a considerable distance out to sea (Fig. 9).

From the foregoing we learn that:—

1.—Fossils afford data of the various Changes that have taken place in past times in the Relative Positions of Land and Water.

Changes of Climate in the Past.—

At the present day we find special groups of animals (fauna), and plants (flora), restricted to tropical climates; and others, conversely, to the arctic regions. Cycads and tree-ferns, for example, seem to flourish best in warm or sub-tropical countries: yet in past times they were abundant in northern Europe in what are now temperate and arctic regions, as in Yorkshire, Spitzbergen, and Northern Siberia, where indeed at one time they formed the principal flora.

The rein-deer and musk-sheep, now to be found only in the arctic regions, once lived in the South of England, France and Germany. The dwarf willow (Salix polaris) and an arctic moss (Hypnum turgescens), now restricted to the same cold region, occur fossil in the South of England.

In Southern Australia and in New Zealand, the marine shells which lived during the earlier and middle Tertiary times belong to genera and species which are indicative of a warmer climate than that now prevailing; this ancient fauna being like that met with in dredging around the northern coasts of Australia (Fig. 10).

From the above evidence we may say that:—

2.—Fossils teach us that in Former Times the Climate of certain parts of the earth’s surface was Different from that now existing.

Fossils as Guides to Age of Strata.—

In passing from fossil deposits of fairly recent origin to those of older date, we find the proportion of living species gradually diminish, being replaced by forms now extinct. After this the genera themselves are replaced by more ancient types, and if we penetrate still deeper into the series of geological strata, even families and orders of animals and plants give place to others entirely unknown at the present day.

From this we conclude that:—

3.—Fossil Types, or Guide Fossils, are of great value in indicating the Relative Age of Geological Formations.

Gradual Evolution of Life-forms from Lower to Higher Types.—

As a general rule the various types of animals and plants become simpler in organisation as we descend the geological scale. For example, in the oldest rocks the animals are confined to the groups of Foraminifera, Sponges, Corals, Graptolites, Shell-fish and Trilobites, all back-boneless animals: whilst it was not until the Devonian period that the primitive fishes appeared as a well-defined group; and in the next formation, the Carboniferous Series, the first traces of the Batrachians (Frog-like animals) and Reptiles are found. Birds do not appear, so far as their remains are known, until near the close of the Jurassic; whilst Mammals are sparsely represented by Monotremes and Marsupials in the Triassic and Jurassic, becoming more abundant in Cainozoic times, and by the Eutheria (Higher Mammals) from the commencement of the Eocene period.

It is clear from the above and other facts in the geological distribution of animal types that:—

4.—The Geological Record supports in the main the Doctrine of Evolution from Simpler to more Complex types; and fossils throw much light upon the Ancestry of Animals and Plants now found Living.