Rock Talk
By Larry Larason

How to make a fossil
Some thoughts on the death of a cat

A few years ago one of our cats was run over. She was not a beautiful animal, just a plain gray tabby. But she was one of the best cats we ever had -- polite, friendly, a good companion. She might complain when a kid used her like an inanimate toy, but she never scratched a human. She saved her claws for her prey. She was an avid hunter; her kills dismayed me sometimes, but that is a cat’s nature, after all.

I gathered her body into a plastic bag and started to put it in the trash cart. But that was too disrespectful for a special cat. I buried her in the back yard, instead. Our little granddaughter was visiting at the time. We often took her on rockhounding trips, so she was used to geology talk. When I buried the sad bag of remains she wondered, “Will the cat be a fossil?” I assured her that the cat would rest where I interred it for a long, long time. I still ponder her question occasionally. Will the cat become a fossil?

 Lots of fossils have been unearthed, but think of all the billions of creatures that have lived over geologic time. How many became fossils? The percentage must be well past the decimal point. Special circumstances are required for an animal to be fossilized.

A physicist once told me that he could never have become a biologist. When I asked why, he said, “Because they work with squishy stuff!” I’m a little squeamish, too. I almost prefer jerky to a rare steak. My point here is that if you are also squeamish, you don’t need to stop reading, because I’m not going to write about the squishy parts, only about bones.

 We usually think of fossils embedded in tough sedimentary rock, but there are lots of ways animals can be fossilized. We find mammoths frozen in permafrost. Tiny creatures may be trapped and covered in tree sap that becomes amber. A unique process of fossilization is seen at the La Brea tar pits in Los Angeles where animals sank into tar. [Talk about squishy parts!] Several similar sites have recently been explored in South America. We don’t have these options around Gallup, however.

Let’s look at two fossil occurrences in some detail. Near Ghost Ranch in northern New Mexico a bone bed was found to have many nearly intact skeletons of Coelophysis. This is the state dinosaur of New Mexico, by the way. It lived during the Triassic Period, more than 206 million years ago. In many ways this early saurian resembled a crocodile but walked on two legs. Paleontologists wondered what had caused the mass die-off. Finally someone noticed that there were pieces of charcoal embedded in the rock containing the bones. So, it seems that a wild fire overcame the animals, or they smothered in the smoke. Shortly afterward a flash flood washed the partially decomposed bodies into piles and buried them in mud.

In eastern Nebraska intact skeletons of several species are found around what was a waterhole 12 million years ago. The animals include camels, saber-toothed deer, turtles, and teleoceros -- an ancestral rhinoceros with short legs, a barrel chest and a short nose-horn. What killed them? It was a volcano which exploded in Idaho. Even though the caldera is not among the top twenty world-wide, it erupted a tremendous amount of ash, which drifted eastward to fall on the plains. The animals inhaled the glassy shards while grazing. With damaged lungs they seem to have expired at the waterhole where they drank in an effort to clear the ash out of their throats. As more ash fell and drifted they were buried. This site is now protected as Ashfall Fossil Beds State Park.

There’s a common element in both these situations: burial soon after death. Bone is about 20 percent protein. Left in the open, in sunlight and weather, the protein is rapidly broken down and the bones become chalky, so they deteriorate quickly. Burial is necessary to preserve bones long enough for them to become mineralized.

Another consideration in the fossilization process is whether the bones are buried in a depositional or eroding environment. Even if buried soon after death, erosion will expose the bones again if deposition does not continue to cover them. Gallup,where I live, is eroding. That’s probably not news to some of my city-mates with retaining walls on their property. The Four Corners Region has been eroding for a long time. Except for lava flows, there has been no significant deposition in this region for several million years. My house is backed up near the edge of the canyon, so my cat is definitely buried in an eroding environment. The process is slow, of course; I figure the cat’s grave should be stable for a thousand years or so.

Gallup has passed the century mark. Some famous cities like Rome have survived for several centuries. The original buildings are ruins now, of course. But it’s not only buildings that are ephemeral. Think of all the ghost towns in the West. Will our community survive for another thousand years? There’s no reason to think it won’t – if they solve the water problem and continue repaving I-40. But, my house will last only about one hundred years -- maybe. After that someone may tear it down to build a new home. They will probably re-grade the lot at that point. So the cat’s grave will be gone.

Another issue to consider is the definition of “fossil.” Fossils are remains of an organism that lived during a previous geological period. We speak of fossil mammoths because they lived during the Pleistocene. The Pleistocene [aka the Ice Age] lasted almost 2 million years. [It could be argued that because our planet still has polar ice caps that the Ice Age is ongoing.] The period before that, the Pliocene, lasted 4.5 million years. Our present period, the Holocene, is only about 10,000 years old. It began when the continental glaciers melted. How long will the Holocene last? We are all locked in to our own time, so it is impossible to guess what events will cause future geologists to declare our present period to be over and a new one [the Neocene?] to have begun. Still, one might expect it to be a million years, at least, before that happens. We also have to assume there will still be a civilization to care about such things so far in the future. Even if Gallup survives till then, the lot where my house is located will have crumbled away. The body of my cat will have been exposed long before that.

My granddaughter is still young enough to think of the cat becoming a fossil as a neat idea. I won’t spoil it for her. But I know the cat’s skeleton will not remain buried into the next geologic age. However, it will rest where I put it until long after I am gone. Maybe that’s all that matters.