Gaps in the fossil record: a case study

Creationists often point out that there are gaps in the fossil record, as if that somehow invalidates it. The Morrison Formation illustrates the impossibility of an absolutely complete record - and the coherent, detailed picture it nevertheless gives of a time and place.

Creationists often point out that the fossil record has gaps, and they seem to think that those parts we do have are somehow invalidated because we don't have the entire record. It's as if the books in the public library were somehow made worthless because the library doesn't have every book ever published. The creationists' dream world is a sort of paleontological Forest Lawn Cemetery where all animals (and people) are carefully laid out and preserved. That this idealized concept does not conform to the jumbled, chaotic, infinitely complicated real world is in no way a failure of the real world. It is a failure of the concept.

Of course there are gaps in the fossil record, and always will be. For a fossil to be found, a complicated series of steps must occur in sequence. The first is that the animal (or plant) must be buried quickly. Animals that die on the plains or in the mountains are soon found by scavengers, such as hyenas or ceratosaurs, and soon reduced to bone chips. Most animals that are fossilized are caught in a flash flood, or die in or near a river and are buried in a sand bar or an overbank flood, or are caught in a sandstorm. If the current in the river is fairly strong, even those few animals that die in the water are soon torn apart and their bones scattered over acres of river bottom. It is estimated that perhaps one animal in a thousand is fossilized, likely a generous estimate.

The second condition necessary for an animal to be fossilized is that it must be buried in a depositional area: that is, more and more layers of mud or gravel must be laid down over it. If the area is subject to erosion - and nearly all land surfaces are - the fossil will soon be washed out and destroyed.

The third step is that this depositional area must at some time become an erosional area, so that wind and water wear it down and uncover the buried remains.

The fourth step necessary for the recovery of a fossil is that when the fossil is uncovered, someone knowledgeable has to walk along that ridge, or study the face of that cliff, and locate the fossil and recover it. The time frame for this recovery varies, but it is always short. The fossil is protected until it is exposed, but it also is invisible. As soon as it is exposed, wind and water attack it, and they can destroy it quickly. The best fossils are found when someone spots an exposed bone that turns out to be part of a buried skeleton, and therefore still well preserved. But many fine fossils have been washed away because no one happened to see them when they were first exposed, or the people who saw them didn't realize what they were seeing.

And as if that were not enough, even the processes of evolution contribute to the lack of transitional fossils the creationists love to cite. Evolutionary changes tend to occur in small, isolated communities, and the new animals then move into the general territory and supplant their relatives there. The transition seems to be abrupt everywhere except the small area where the change occurred. And we seldom are lucky enough to find those small areas where the changes occurred, even if they were fossilized.

Geologists call a distinctive body of rock that serves as a convenient unit for study and mapping a "formation," and name it for some spot where it is exposed. The problems and rewards of the fossil record are well illustrated by the Morrison Formation, named for a small town in the Front Range just west of Denver where it crops out. It was formed in the late Jurassic period, around 150,000,000 years ago, in the Morrison Basin, a vast subtropical area that extended from central New Mexico north to Saskatchewan. It covered about 750,000 square miles, including northeastern Arizona, eastern Utah, Colorado, northwestern Kansas, western Nebraska, Wyoming, most of Montana and more that half of North and South Dakota, plus small corners of Texas, Oklahoma, Idaho, Manitoba, and Alberta [ILLUSTRATION FOR FIGURE 1 OMITTED]. It was a hot, humid area of meandering streams and tangled forests, with seasonal dry spells. It was a dinosaur paradise.

When the first dinosaur fossil hunters came west in the 1870s, many of the great deposits they found were in the Morrison Formation. Cation City, Colorado, and Como Bluff and Bone Cabin, Wyoming, were among the early quarries that produced wonders people had never seen before: huge sauropods including the camarasaurs, apatosaurs and long diplodocuses, and the gigantic brachiosaurs; strange stegosaurs with their double row of plates down their backs, and the carnivores that preyed on them, allosaurs and ceratosaurs. Later sites of finds include Dinosaur National Monument, Utah, where many corpses piled up on a sandbar in a sluggish river, and the Cleveland-Lloyd quarry in central Utah. This was a predator trap where five or six huge sauropods got stuck in a mud hole, drawing hundreds of carnivores, which in turn got stuck in the mud and added to the carrion smell, which drew more carnivores.