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.

DAVID A. THOMAS

reationists often point out that the fossil record has gaps, and they seem to think that those parts we do Chave 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 pre- served. 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

2 6 November/December 1998 SKEPTICAL INQUIRER Allosaurus sculpted by the author using bones found in the Morrison lew Mexico '-T , Oklahoma Formation. * Texas or in the mountains are soon found by scavengers, such as hye- nas or ceratosaurs, and soon reduced to bone chips. Most ani- mals 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 arc caught in a sandstorm. If the current in the river is fairly Figure 1. Extent of the Morrison Formation. strong, even those few animals that die in the water are soon torn apart and their bones scattered over acres of river bottom. The transition seems to be abrupt everywhere except the small It is estimated that perhaps one animal in a thousand is fos- area where the change occurred. And we seldom are lucky silized, likely a generous estimate. enough to find those small areas where the changes occurred, The second condition necessary for an animal to be fos- even if they were fossilized. silized is that it must be buried in a depositional area: that is, Geologists call a distinctive body of rock that serves as a con- more and more layers of mud or gravel must be laid down over venient unit for study and mapping a "formation," and name it it. If the area is subject to erosion—and nearly all land surfaces for some spot where it is exposed. The problems and rewards of are—the fossil will soon be washed out and destroyed. the fossil record are well illustrated by the Morrison Formation, The third step is that this depositional area must at some named for a small town in the Front Range just west of Denver time become an erosional area, so that wind and water wear it where it crops out. It was formed in the late Jurassic period, down and uncover the buried remains. around 150,000,000 years ago, in the Morrison Basin, a vast The fourth step necessary for the recovery of a fossil is that subtropical area that extended from central New Mexico north when the fossil is uncovered, someone knowledgeable has to to Saskatchewan. It covered about 750,000 square miles, includ- walk along that ridge, or study the face of that cliff, and locate ing northeastern Arizona, eastern Utah, Colorado, northwestern the fossil and recover it. The time frame for this recovery varies, Kansas, western Nebraska, Wyoming, most of Montana and but it is always short. The fossil is protected until it is exposed, more that half of North and South Dakota, plus small corners but it also is invisible. As soon as it is exposed, wind and water of Texas, Oklahoma, Idaho, Manitoba, and Alberta (see Figure attack it, and they can destroy it quickly. The best fossils are 1). It was a hot, humid area of meandering streams and tangled found when someone spots an exposed bone that turns out to forests, with seasonal dry spells. It was a dinosaur paradise. be part of a buried skeleton, and therefore still well preserved. When the first dinosaur fossil hunters came west in the But many fine fossils have been washed away because no one happened to see them when they were first exposed, or the peo- David A. Thomas is a professional sculptor, and has produced ple who saw them didn't realize what they were seeing. detailed scientific sculptures of dinosaurs and other ancient life And as if that were not enough, even the processes of evo- forms for museums all over the world. He is an avid fossil enthu- lution contribute to the lack of transitional fossils the cre- siast. He described his study of a Paluxy River trackway showing ationists love to cite. Evolutionary changes tend to occur in a theropod attack on a sauropod dinosaur in a feature article in small, isolated communities, and the new animals then move the December 1997 Scientific American. Address: Alcazar into the general territory and supplant their relatives there. Corporation, 3205 Alcazar NE, Albuquerque, A/A/ 87110.

SKEPTICAL INQUIRER November/December 1998 27 canyonlands of Utah and northern Arizona. The Rio Grande rift has cut a swath through it. Volcanic action in the Yellowstone area has destroyed it in that region. The present-day map of the Morrison Formation (Figure 2), reveals that about 75 percent of it is still buried, and nearly all of the rest is washed out and destroyed. That portion of the Morrison Formation available to paleontologists—the part that has produced the wonders described above—appears to be about one percent of the total formation, perhaps less. But those pieces we do have of this Morrison puzzle form a coherent whole. Of course parts are missing, but just as the public library's collection of books is incomplete but still gives a good picture of modern civilization, so too do those parts of the Morrison formation corroborate and extend each other to give a good broad overview of life at that time and place. Camarasaurs and allosaurs are found in all areas. The Cleveland-Lloyd quarry gave us new carnivores, like stoke¬ sosaur and marshosaur, to go with the allosaurs and cer- atosaurs. Dry Mesa gave us another big predator, torvosaur. And the finds are continuing: workers at Dinosaur National Monument in recent years turned up a nearly complete thero¬ pod (unfortunately missing the head) they have been calling "not an allosaur," which had a furcula, like a bird's wishbone, intact in the skeleton. Paleontologists in Canon City are preparing a beautifully preserved and nearly complete skeleton of a stegosaur. Utah workers are excavating a camarasaur at Mussentuchit Wash, southwest of the San Rafael Swell. Figure 2. Sites and deposits of the Morrison Formation. We are never going to have a complete picture of Morrison 1870s, many of the great deposits diey found were in the life. We're not going to strip thousands of feet of overburden Morrison Formation. Canon City, Colorado, and Como Bluff off the high plains! But wc don't have to do that to have a and Bone Cabin, Wyoming, were among the early quarries that good, broad understanding of what life was like in Morrison produced wonders people had never seen before: huge sauropods time. Since the quarries are in different levels of the Morrison including the camarasaurs, apatosaurs and long diplodocuses, Formation, representing different parts of Morrison time, we and the gigantic brachiosaurs; strange stegosaurs with their dou- even have a good grasp for the way the animals evolved ble row of plates down their backs, and die carnivores that preyed through time. on them, allosaurs and ceratosaurs. Later sites of finds include And that understanding is more real, more valuable and Dinosaur National Monument, Utah, where many corpses piled much more profound than anything the creationists could up on a sandbar in a sluggish river, and the Cleveland-Lloyd ever acquire with their all-or-nothing insistence on some quarry in central Utah. This was a predator trap where five or six impossible ideal world. huge sauropods got stuck in a mud hole, drawing hundreds of It's not true that, "If we don't know everything, we don't carnivores, which in turn got stuck in the mud and added to the know anything!" We know a lot about the Morrison Formation carrion smell, which drew more carnivores. and the thousands of formations that make up this wonderful More recent finds in the Morrison Formation include the world. We are missing some details, but die broad outlines are Dry Mesa quarry in western Colorado, which produced the clear. And new finds are adding to our knowledge every day. super giants ultrasaurus and supersaurus, and the seismosaur That's where the fun is; that's what keeps the fossil hunters wan- site in New Mexico, where much of a diplodocus-related ani- dering the ridges looking for those little telltale scraps of bone. mal up to 140 feet long was lodged on a sandbar. The creationists' emphasis on the gaps of die fossil record is What a productive stratum! It has given us a large part of just another example of the nit-picking they do to try to cover what we know about the late Jurassic plants and animals. their lack of evidence. They do not share science's commitment But nearly all of the eastern half of the Morrison Formation to a free and open search for truth. Since they have no facts to is still buried under the prairies of the high plains. Most of the present, creationists are reduced to searching for debater's tricks quarries are west of the Front Range in Colorado, and the to try to shore up their arguments, including the absurdity that Rocky Mountain uplift has destroyed much of the Morrison. evidence against "evolution"—that is, against biology and geol- It has been stripped off of the Colorado mountains, the San ogy—is somehow evidence for their position. In so doing, they Rafael Swell in Utah, and many other areas including the miss the reality, and the drama and excitement and fun, of the Black Hills. The Colorado River has washed it away in die history of life on this fantastic planet. O

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