Читать книгу All About Strange Beasts of the Past - Roy Chapman Andrews - Страница 4

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The word “fossil” comes from the Latin word fossilis, which means “dug up.” Therefore, a fossil must have been buried at some time. It is the remains of a plant or animal that once lived upon the earth or in the water. Usually fossils are bones, but they can be rock impressions of plant or insects or shells or even footprints. Some fossils are millions of years old. Others may be only a few thousand years old.

Because of their great age fossils help us understand the story of past life. Some of them give us information about the world before man came upon the earth and made written records. From this information we can understand the present and predict some things about the future.

If a bone is not buried, it will be destroyed sooner or later. It may be destroyed by weather; for sun, rain, frost, snow and wind soon make powder out of bones. Also flesh-eating birds and animals, such as dogs, wolves, cats, hyenas, eagles and vultures, tear the carcass apart and carry off the bones. Even if there were no flesh eaters, little animals like rats and mice would gnaw the bones and destroy them. So a bone has little chance of surviving unless it is covered quickly by some sort of sediment.

Almost all fossils are found in what we call sedimentary rocks. The commonest kinds of sedimentary rocks are shales, which are hardened mud or clay, and sandstones, which are hardened sand. Limestone, another sedimentary rock, may be formed by lime dissolved in water or from the limy parts of animals or plants.

On dry land there is little chance for a bone to become a fossil. It might be covered quickly by blown sediment in a sand or dust storm, but that doesn’t often happen. The most usual way is for an animal to sink in quicksand, bogs or mud holes. That is what happened in the tar pits of Rancho La Brea.

Very often animals die on the banks of a stream or lake. A rainstorm or flood washes their bodies into the water. If there is no current, they may sink quietly into the mud. More probably, the carcass will drift downstream until it comes to rest in a backwater or eddy. There it drops to the bottom, the flesh slips away, and the bones lie naked. Then fine mud settles over the bones like dew on grass.

Illustration Caption: Frequently animal bones will come to rest in a backwater or eddy of a lake or stream where they will be fossilized.

If a river carrying much sediment in its current pours into the backwater, the bones will be covered quickly. But sometimes months or years will pass before they are covered. Occasionally a skeleton is broken up by swift water, and all the bones are separated. They may be scattered far and wide over a large area. As they are rolled over and over on the bottom, only the hardest parts like the teeth and big bones remain.

Every bone is composed of hard and soft matter. When an animal dies, the soft matter usually decays, and the hard matter crumbles to dust. But if the bone is covered, a very slow change takes place. The soft matter disappears, leaving cavities and canals. These are filled by whatever mineral is in the earth where they are buried. If it is limestone, the bone cavities fill up with limestone and thus turn the bone to a fossil. Or it may be sandstone or even iron that transforms the original bones to fossil bones. I once found a dinosaur skeleton that had become iron. In that case the skeleton had been in water containing a great deal of iron. The iron had completely replaced the animal matter in the bones. In the La Brea tar pits the bone cavities were filled with tar or asphalt.

In the case of fossilized wood, the change is greater than in bone. The original vegetable matter is replaced by mineral substances. But even though the change is complete, the cellular form of the wood can still be seen under the microscope. In Arizona there is a place called the Petrified Forest. Here hundreds of fossilized trees lie on the ground. They have the shape of the original trees, but are now turned to stone.

Frequently we find stones that show the natural mold of a hard body, such as a sea shell. After the original shell was enclosed in mud or sand, it dissolved. Then the space was filled up with lime or silica which hardened into a fossil shaped like the original shell.

Illustration Caption: Sometimes the natural molds of sea shells are left in rock.

Fossil leaves are really imprints in stone. Suppose a leaf falls on the still surface of a pond and slowly sinks to the bottom where mud covers it. The vegetable matter rots away leaving a perfect impression of the leaf. Then the minerals of the water fill the space made by the hard fibers of the leaf. These minerals turn to stone and we have a fossil imprint of the leaf. So it happens with plants of many kinds and also with insects. Their soft little bodies are cast in extremely fine mud which turns to stone. But the actual insect has disappeared.

By studying fossilized wood, plants, insects and animals, a scientist can tell about the climate millions of years ago. That is because the kinds of animals of a region indicate whether the country was plains or forest, and if there was much water, as in lakes or rivers. If the fossilized wood is that of conifers, such as pines or spruce, he can be sure that the climate was cold or temperate. If palms are found, he knows it must have been warm or tropical. So, with evidence from fossilized plants, animals, trees, and insects, the scientist can give a good picture of what the world was like at that time and place.

Fossils were once living things just as much as you and I are now, or as are your dog or cat and the plants around you. What makes one really understand this fact, is a fossil footprint. These footprints are “motions turned to stone.” They show where animals ran or jumped or sat.

Illustration Caption: Fossil footprints show where a dinosaur walked.

Even the paths to and from their feeding places are evident.

In the American Museum of Natural History in New York, a great dinosaur skeleton is on display. Behind it there is a whole series of dinosaur tracks. The deep impressions of the feet were made in mud that hardened into stone. These fossil footprints were taken up in blocks and laid in the museum as though the reptile had just walked on that path. They give a startling feeling of aliveness to the huge dinosaur skeleton.

Thousands of dinosaur tracks can be seen in the Connecticut River Valley. These are at most two hundred million years old.

But plants and animals are not the only things that have left a fossil record. In one place even the weather was fossilized. That sounds impossible, but it is true. On a certain day, millions of years ago, it rained. Not a long rain, but a short, hard shower. Big drops fell on soft mud and made little holes. The mud baked in the sun. Finally the mud became solid stone which showed the holes made by the driving rain. So the story of a storm was written for those who can read the record of the rocks.

Usually fossilization is a very slow process. Sometimes it takes millions of years. Still, if conditions are just right, it could happen in a few thousand or possibly hundreds of years.

Illustration Caption: Heavy raindrops made little holes in soft mud which later became solid rock.

When it comes to hunting fossils, people often ask, "How do you know where to dig?" The answer is, you don’t dig—or at least not often. The fossil hunter must first find sedimentary rocks. Then the rocks must be bare. Deserts and dry areas are best because there is little surface grass or vegetation to cover the rock. Also dry country is often cut into ravines and gullies by flash floods or winds and frost and rain. These cuts are helpful because they show a cross section of the earth. When you walk along the sides of a gully or ravine, you may see fossil bones which are partly exposed. These can be carefully uncovered with whisk brooms and small instruments.

The study of fossils has been carried on for only about one hundred and fifty years. At first most fossil bones were found by accident. But about the year 1800, a great French naturalist, Georges Cuvier, became seriously interested in studying fossils. He collected many bones and wrote scientific papers about them. It may be said that he was the founder of the science of paleontology, the study of fossils. Literally paleontology means “the science of ancient living things.”

Not until the end of the War Between the States was there a real program of fossil collecting in America. At that time the United States government sent out exploring parties into the West. One thing they were to find out was what minerals existed in this little-known region. Nearly every exploring party contained at least one geologist to study the soil, rocks and minerals. During their work, the geologists discovered many fossil bones.

These were of special interest to two scientists: Professor Cope of Philadelphia and Professor Marsh of Yale University. They were both rich men and friends. Each sent out fossil collectors at his own expense between 1870 and 1895. Each studied and named many new animals. But soon personal rivalry developed. The two brilliant scientists seemed to think there weren’t enough fossils for everyone. They became bitter enemies.

Nevertheless, their work greatly helped the study of paleontology all over the world. Scientists realized that fossils existed on every continent. If they found similar fossils in different parts of the world, they would know that plant and animal life had been similar. The record of the fossils would give a record of life in the distant past. So museums began to send out expeditions to collect fossils in almost every part of the earth.

Central Asia was one of the last areas to be explored. No one knew whether it contained fossil bones. But scientists did know that many ancient animals of Europe and North America were closely related. Central Asia lies right between the two continents. Perhaps some of the animals of Europe and North America had originated in Central Asia. From this great middle area they may have spread to the other continents.

Some years ago I made up my mind to explore Central Asia, hunting for fossils and other natural-history specimens. The Gobi Desert was the place to begin work. For two thousand miles it stretches east and west right through the center of Mongolia. It is one of the driest and most fearful deserts of the world.

At that time no one had traveled in the Gobi Desert except by camel. But those creatures travel very slowly, averaging only ten miles a day on a long march. I decided to use automobiles instead. Everyone thought the expedition would be a failure and that none of us would return. But the cars were very successful, and we drove many thousands of miles in the desert. There were forty men, eight motor cars, and 150 camels to carry gasoline and supplies. The expedition was made up of some of the best scientists in the world. We discovered great fields of fossils including bones of many unknown animals. As we expected, some of the animals were closely related to those of either Europe or North America or of both continents.

In the book entitled All About Dinosaurs, I have told of the dinosaur bones we discovered. Those great creatures lived in the Age of Reptiles, long before mammals dominated the earth.

Illustration Caption: Motor cars were used for the first time in exploring the Gobi Desert.

We also discovered the fossil bones of many strange beasts that followed the reptiles. In this book I want to tell of these creatures, for they will help to give a picture of the world in the Age of Mammals that followed the Age of Reptiles.

Illustration Caption: Modern Mammals

All the creatures on this page are classified as mammals. Unlike reptiles, all true mammals have hair, give birth to their young alive, and have warm blood. It is easy to see that mammals are of many kinds, sizes and shapes.