James Maguire, age 8, of Sarasota, Florida, for his question:
How are fossils made?
Making a fossil is somewhat like cooking a dinner. The meat and vegetables once belonged to living plants and animals. The oven is the ground and the ingredients are buried between heavy layers of rock. Slowly, very slowly, the earth's dry and gentle warmth changes them into durable fossils.
The earth has several recipes for making fossils. But one thing is always so. Every recipe takes many millions of years. The slow slow job must begin with material created by living plants or animals. It may be an elegant shell that a small animal built for himself as he grew ages ago in some ancient sea. It may be a twig or a giant fern that grew millions of years ago in a strange, swampy forest. It may be a tooth that belonged to a long gone tiger. It may be a gob of gummy resin that oozed from a wound in the trunk of an ancient pine tree. It may be the bulky body of a departed dinosaur. In any case, we know that fossil making ingredients belonged to plants and animals that lived on earth in the dim distant past. We know they lived long ago because it takes ages and ages to finish the fossil making recipe.
As a rule, the remains of departed plants and animals fall to the ground and decay. The chewy parts are eaten by hungry animals and the tougher parts are broken up and chewed by bugs and tiny bacteria that live in the soil. The rotting materials soon become powdery dust. But sometimes this decaying work is impossible. The job needs the help of special decay bacteria. These midgets are too small for your eyes to see but things must be just right for them to survive. They cannot live without oxygen and a little moisture. Decay bacteria are scarce in the dry desert sands and also ,in swamps where no oxygen is mixed with the stale and stagnant waters.
When departed plants and animals leave their remains fn these regions, they can not rot and decay into dust. The remains are soon covered with mud or with blowing sands. As the years go by, they are buried under deeper and deeper layers of dust and silt. The layers on top get heavier and the ground below is gently warmed. Some of the light chemicals in the fossil materials seep upward and escape into the air. The rest are squeezed dry and pressed hard. After hundreds of millions of years. The buried remains of ancient swamps and forests become coal and coal is a fossil material. After countless ages, all that remains of a long departed dinosaur are his fossilized bones. And that gob of old resin becomes a hard lump of glassy, honey¬ colored amber.
The rocky crust of the earth is forever changing and so is the climate. The dry deserts of Arizona once had swampy forests. The climate changed and the fallen trees were buried under heavy layers of sandy rock. Underground water dissolved away their woody cells and replaced them with perfect copies made of stony chemicals. In time, the remains of these trees became a forest of petrified wood. Petrified means "turned to stone" and petrified wood is a fossil material.
Experts rate all sorts of other things as fossils. Sometimes a lumbering dino¬saur left his footprints on a bank of moist mud, and right away the mud dried hard and solid. Dust and debris covered it over without disturbing the footprints. After perhaps 100 million years, someone dug down and unearthed the layer with the ancient footprints. They are rated as fossils. Birds have very light and fragile bodies and only a very few of their bones became fossils. But often a fallen bird left his imprint in the ground, feathers and all. This imprint is rated as a fossil. So is the imprint of a ferny frond pressed between layers of coal.