Tom Lyon, age 13, of Muncie, Indiana, for his question:

How many different fossils have been found?

Nobody seems to have taken a precise census of the fossil species so far found and identified. Several estimates have been made, all of them ranging into hundreds of thousands. These figures are merely educated guesses. So are those based on estimates of how many past species left no identifiable remains. Some experts suspect that only one in a thousand species was preserved as a recognizable fossil.

As one's interest in fossils grows deeper, the number of identified species becomes less important. Counting them seems pretty dull when, for example, one studies the fossil of a cockroach ancestor who lived 320 million years ago in the Carboniferous Period of fabulous coal forests. One's curiosity is prodded by questions about how he lived, his environment and what other plants and animals shared his long gone environment. One's imagination stretches to comprehend the vast and varied time span between then and now.

However, counting the species of a particular era has a function. It helps a paleontology student to piece together the existing environment     how, for example, that cockroach found food, his neighbors, competitors and likely enemies. But, this function is limited because we have only a very small section of the total picture. Most species of the era were too fragile to leave identifiable remains and so far we have found only a few of those that did.

The really fascinating aspects of a fossil are when he lived and his possible relationship to living species. In the past generation, paleontologists have developed skillful techniques for dating a fossil specimen. Other data from his era help to reconstruct his environment. But tracing his family tree both backwards and forwards is an enormous and engrossing project, fraught with many problems.

For example, the ancestry. of our noble horse has been traced back at least 50 million years. His earliest ancestor was a toe footed forest dweller no bigger than a smallish dog. Through the ages, his world changed and he changed with the times. His descendants grew larger and gradually developed longer, stronger legs, different teeth and hoofed feet. If the original dawn horse were still living, he would be classed as a very different species from a modern race horse.

The big classification problem lies in his gradual change. Horse fossils of 36 million years ago reveal enough new features to class him as a different species from his original ancestor. Some paleontologists define four different species on the horse family tree, others suspect there may be more. This is the major reason why counting the number of different fossils is difficult    and perhaps downright impossible.

The horse left a fairly complete record of his fossilized ancestors. Even so, we cannot say whether it includes four, six or more extinct species. Most other fossil animals are much harder to count because records of their age old family trees are very scanty. Fossilized plants present even more of a problem because most of their remains were mashed beyond recognition.