So That's a Fossil!

So That's a Fossil!

Millions of years before man made his appearance on the earth, countless species of animals and plants flourished and dominated the land, the sea, and even the air. A record of such prehistoric life has been preserved in the rocks that make up the crust of the earth. Scientists refer to the remains of these ancient inhabitants as fossils. But what things qualify as fossils?

The word fossil is derived from the Latin word meaning "to dig up." Early usage of the word in modern science included anything dug out of the earth, whether organic or inorganic. This usage undoubtedly led to much confusion, since "anything dug from the earth" could include a mineral or a rock or interred remains of practically anything within the category of fossil. As the sciences of geology and paleontology developed, the term fossil became restricted to include only those things that were remains or traces of organisms that lived during the geologic past.

Several years ago, for show—and—tell a geology student brought to class the mummified remains of a cat and rat that, he assured his fellow students, he had found the previous summer in the ruins of a Scottish castle. There was definite evidence to show that the animals had died in combat with each other. In the dry atmosphere of these ancient ruins the two animals had managed to avoid decomposition, and both were mummified in their entirety. Despite the remarkable state of preservation and the information that could be obtained from these specimens, they could not be classified as fossils, simply because they were not old enough.

In 1900 a hunter following a wounded deer in eastern Siberia happened upon the carcass of an elephant. The huge beast had been partially exposed by a recent cave—in of a river bank. The hunter's curiosity prevailing over fear and superstition, he approached the remains and fell to examining them. When he was satisfied with his investigation, he resumed his search for the wounded deer, but not before breaking off a tusk, which he later sold to a cossack. The buyer, upon his return to St. Petersburg, relayed the information that another frozen mammoth had been discovered.

The Imperial Academy of Science organized an expedition as soon as it was possible. The site of the discovery was at least 3,000 miles away, so it was more than a year later that the expedition arrived at the scene. By this time the exposed portions of the mammoth had undergone ravages of two summer suns, and wolves had gnawed on its flesh. When the excavation finally began, the scientists were pleased to find that much of the carcass was still intact. Evidently the beast had fallen into a crevasse, breaking its hip. In a frenzied attempt to climb out it had pulled an avalanche of snow down on itself and was quickly frozen.

In its chest was a mass of clotted blood, and in the stomach was much undigested food. Between its clenched teeth there was food not yet swallowed, leaving no doubt as to the suddenness of its death. Frozen bacteria were later found in the mammoth's blood, suggesting that it must have had an infection at the time it was killed. The death of this unfortunate animal occurred some 45,000 years ago. The entire carcass of this mammoth, as well as the bacteria in its blood, can be considered fossils. Why? Because they lived in the geologic past, well over 10,000 years ago.

Several hundred million years ago an animal crawled out on the soft mud near the town formerly known as Mauch Chunk, Pennsylvania. It may have been merely enjoying the afternoon sun, or perhaps it was on the never—ending quest for food. Whatever its reasons, it left behind perfect impressions of its feet in the soft mud, which was baked and hardened by the hot afternoon sun. The timeless processes of erosion and deposition covered and buried the footprints with sediments. Further geologic activity caused the mud to become sedimentary rock, and the footprints were preserved for eons to come. Subsequent erosion in recent times stripped the overlying rocks and exposed several of the footprints to view. Although no single fragment of the animal is present, these footprints, as evidence of its having been there once ages ago, are therefore fossils.

In the summer of 1955, while prospecting for uranium in the vicinity of Price, Utah, a geologist and his party discovered the bones of a dinosaur. The remains were uncovered in sandstone from the Jurassic Period, indicating that this animal had lived over one hundred million years ago. It was identified as a swamp—dwelling, herbivorous dinosaur, the Stegosaurus. The bones were quite radioactive, a condition not uncommon in areas of uranium deposits. How it met its death was not evident, but the fact remains that the body was quickly covered by sediments and preserved. In time erosion removed the overburden and revealed this denizen of ancient days. The bones of this Stegosaurus are fossils since the animal lived in the extreme geologic past.

About fifteen miles east of the town of Wheeling, West Virginia, near streams along the old U.S. 40, is a well—known fossil plant quarry. The plants are nearly 300 million years old and are unquestionably fossils. Associated with the plants are petrified ripple marks showing evidence of a stream contemporary with the plants. Anyone who has glanced into a shallow streambed has noticed that the flowing water can produce ripples on the soft bottom muds. Most of the time these marks are readily destroyed by changing stream velocities. On occasion, however, the ripple marks are covered by subsequent deposition of sediments and are preserved. Ripple marks have been preserved in every geologic era; some to them are billions of years old. These ripple marks, which were preserved along with the plant life found in the quarry, are also 300 million years old. The plants are of course fossils, but the ripple marks, although they were formed in the same remote geologic era, could not be classed as fossils because they are not organic in origin. Mud cracks and raindrop imprints are also common evidence of events in the very distant past but, like the ripple marks, are not fossils.

Some years ago a geology professor welcomed a weatherbeaten wanderer with an Australian accent into his office. The guest was looking for a professional opinion of the contents of the box he was carrying. He opened six paper—wrapped packages, each containing a rock slab in which the remains of a perchlike fish were embedded. Although the professor had seen many fossil fish before, these caught his attention because the organic matter in each of the fish had been replaced with precious opal. The eyes were the most breathtaking — opalized gemstones the size of quarters, reflecting rainbow colors in the sunlight. The Australian wouldn't reveal the location of his find, nor has he yet responded to the professor's impassioned entreaties to take him along the next time he goes fishing. But somewhere in Australia . . .

Some forty million years ago, in the Baltic peninsula of Europe, there existed a species of tree that almost constantly exuded resin. Insect life must have been abundant, because the sap frequently descended on some unwary insect clinging to the tree or flying directly into the gooey material. The resin hardened around the encased insects and eventually developed into amber. Both the amber and its insect prisoner are considered fossils because both were organic and lived in the geologic past.

Despite the many ways in which fossils can be preserved, they must meet the requirements of being signs or remains of organic life from the geologic past. This category can include the merest fragments of bones and teeth or entire skeletons, shells, leaves or trees, an imprint, a cast, or mold that once encased ancient living matter.

Recent references to fossils have included "fossil fuels," those polluters of the biosphere that result from combustion of such materials as coal, oil, and natural gas. The term is completely valid, for every coal seam represents the remains of a Carboniferous forest, and every pool of oil began as millions of microscopic animals from a prehistoric sea. Even the white cliffs on both sides of the Strait of Dover are composed of remains of Cretaceous (Lat. creta, "chalk") sea animals whose shells are visible under microscopic examination.

Only an infinitesimal percentage of life in the past has been preserved in the fossil record. Incomplete as it may be, it has provided indisputable evidence of the progression of life on our planet for nearly four billion years.

From the book: 
Our Fascinating Earth