Читать книгу All About Dinosaurs - Roy Chapman Andrews - Страница 5

Table of Contents

I have been talking about fossils without explaining what they are, or how they are formed. The name 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 an animal that once lived upon the earth.

Fossils are being made today just as they were a million years ago. A bone your dog has buried in the ground may some day become a fossil if it remains long enough.

When an animal dies, there is a great chance that its skeleton will be pulled apart by other animals and destroyed. But that doesn't always happen. Sometimes wind may cover it quickly with sand, or rain may wash mud over it. This heaps up higher and higher. Water, which contains minerals in solution, drips through it. Then a very slow change takes place. Particle by particle the animal matter in the bone is replaced by mineral matter. So it is petrified or "turned to stone."

This change may be rapid or very slow. It depends upon conditions. A bone may be completely petrified in a few hundred years if the water is heavy with minerals. Or it may lie for thousands of years without being entirely mineralized. Then it is called a "sub-fossil." Most often bones are changed to sandstone or limestone. But sometimes they are formed by unusual minerals such as iron or even opal.

In the Gobi Desert we discovered the skeleton of a dinosaur in the side of a cliff. It was in an iron deposit, and the bones had been completely changed to iron.

Quite often an animal may die on the bank of a stream or in the water. The body floats along until it comes to rest on a sand bank or in a backwater. There it sinks to the bottom. The flesh decays, and the bones are gently covered with very fine mud. It fills every groove and pore and preserves the mark of each ridge or furrow. After a long while the sediment is pressed together into rock. The stream dries up or changes its course. The bones are left enclosed in stone, perhaps to be found by some fossil hunter millions of years later.

Some parts of the skeleton are more frequently preserved than others. Because they are hard and solid, skulls and teeth are often found when nothing else is present. Leg and arm bones and ribs are easily broken. Very rarely the flesh, skin or tendons are entirely or partly preserved. In such cases, the soft parts were protected in some way from decay, and mineralization was very rapid.

Quite often fossils are formed as molds, or casts, like the dinosaur tracks from Connecticut that I described. In the Gobi Desert we found impressions of plants and insects in what are called "paper-shales." These were made from extremely fine sediment, deposited in horizontal layers, which separate into sheets as thin as paper. In one is the perfect imprint of a mosquito. In another the imprint of a butterfly's wing is so beautifully preserved that you can see the most delicate veins under a magnifying glass.

These paper-shales were formed in sheltered pools of quiet water. Leaves or insects which died upon the surface sank to the bottom and were gently covered with a blanket of the finest mud. As the animal matter decayed, their tiny bodies left a perfect impression in the mud. It was exactly like the mold that one makes in plaster.

Mary Anning was hunting for fossil sea shells when she discovered the first marine reptile in England. These were actually the casts of shells. The shells were pressed down into the mud. After a while all the animal matter was dissolved by the water. But a hole, or mold, was left where the shell had been. Later this mold was filled with sediment which hardened into a cast of the shell. We have found casts of the birds' eggs. Often the kernels of nuts are fossilized. Many of the small animals, like crabs, that lived in the sea or swamps have been preserved.

Most of you have seen fossilized wood. In Arizona there is a place called the "petrified forest" where hundreds of fallen trees lie on the ground. Some are great trunks thirty or forty feet long. Chips and chunks are scattered about. The vegetable matter in the wood has been replaced by minerals. So completely was it done that, with a microscope, you can see the cellular structure of the wood.

The impressions of leaves, of seeds, and of wood tell an important story. From them it is easy to decide what the vegetation was like when those plants and trees lived. We can even know the climate. If the trees were of desert type, there must have been little rainfall. If they were those of the tropics, like palms, certainly the weather was warm with much rain. So you see how the history of past life on the earth has been written in the rocks. You can read it easily, once you have learned the language.

Of course fossils can only be preserved in sedimentary rocks like sandstone, limestone, slate and shale. These rocks are made up of small particles of sediment pressed together. You couldn't find fossils in granite or volcanic rocks which have been formed by heat and change. So when we go fossil hunting, we first have to know that we are on sedimentary rocks.

Next, the surface must be cut up into ravines and gullies and canyons. Thus we have a cross section of the land. Usually it is desert or dry country with little grass, trees, or other vegetation to protect the soil from the wind and weather. Rain and "flash floods" cut deep gullies in the surface. These expose the bones that may lie buried underneath.

A cake is a good example of what I mean. If it is covered with frosting, you don't know what kind of a cake it is. It may be a plain cake or a layer cake, or it may contain raisins or nuts. But if you slice through it, then you can tell. If nuts or raisins are there, you probably will see some of them in the cross section from top to bottom.

That is what ravines and gullies and canyons do for the fossil hunter. They give him a cross section of the land. He walks along the sides of the ravines or in a dry river bed. Some bones will probably show if they have been buried there, just as the nuts and raisins show in the cake.

It is quite useless to dig for fossils unless there is some indication that bones are there. You just can't go out and say: "Here I will dig for a fossil. I hope I'll find one." No, you must look for bits of bone, for discolorations in the rock, for something that will show that a fossil might be there. When we were in the Gobi Desert, we traveled for hundreds of miles across sedimentary rocks. We knew fossils might lie underneath the surface, but there was no place where we had a cross section of the land. Finally we found it, and then we discovered the first fossils ever known in Mongolia.