January 30, 2006 | David F. Coppedge

How Fossils Form: We Don’t Rightly Know

Fossils have been such a mainstay of evolutionary theory for at least two centuries, one would think we have a pretty good picture of the process.  An article by Sid Perkins in Science News1 is revealing: “Only in the past decade or so have people begun to study in detail what happens to organisms after death,” the article states.  That’s surprising in a world where death is pervasive.
    Some research teams have actually done science projects on taphonomy, the study of fossilization.  They have buried everything from birds to rhinos and exhumed them a few years later to see what happens.  In most cases, much of the animal is gone.  Consider what happens to birds.  Perkins shows a picture of Archaeopteryx and puzzles not why only 10 have been found, but why any have been preserved at all:

Most carcasses that harden into fossils, including those of birds, were deposited in a body of water and then buried by sediment, says David A. Krauss, a paleobiologist at City University of New York.  However, the irksome fact that dead birds float conflicts with this observation.  In fact, bird carcasses float for quite a while, according to the results of experiments conducted by Krauss and his colleagues.  The researchers reported their findings last October at a meeting of the Society of Vertebrate Paleontology in Mesa, Ariz.
    In their tests, conducted outdoors during the summer, Krauss and his colleagues placed carcasses of doves, swallows, and blackbirds in tanks filled with water.  Every one of the dozen birds floated.  By the end of the third day, a thick film of bacteria had formed on the carcasses.  Soon thereafter, the birds’ remains became infested with bugs and maggots.  Over the next 3 to 4 weeks, the carcasses decayed, lost some feathers, and began to fall apart—but they still floated.
    Only after decomposition breached the birds’ internal air sacs and permitted water to flow into those cavities did the body parts finally sink, says Krauss.  At that point, the remains certainly wouldn’t have made informative fossils. (Emphasis added in all quotes.)

They did get better results when sand or silt in the water impregnated the feathers, but still, the remarkable preservation of Archaeopteryx and many of the bones in China is startling considering the odds.
    Other than bones and teeth, which already contain minerals, most of an animal’s carcass is soft tissue which decomposes rapidly.  Buried eggs, for instance, are rapidly consumed by bacteria, yet fossils are known.  Rare are the chemical conditions, such as acidity, which allow for preservation.  Scientists are still not sure which conditions can avoid the usual decompositional processes.
    In 1983, a team from U of Wisconsin-Madison buried a white rhino that had died in a local zoo.  They checked how the fossilization was going after about eight years, and then a few years later.  The limbs had detached and much of the flesh on the legs had disappeared, but surprisingly, some of the muscles from forelimbs and shoulders looked fresh, “like [they] came out of a butcher’s shop” a team member commented.  Most of the skin was missing and a coating of a hard, grainy substance called “grave wax” coated the body cavity.
    In short, very little is known about fossilization.  Plants and animals have a variety of tissues, so there is “no single route” to becoming a fossil.  “In a sense, fossilization is as much a process of elimination as an active means of preservation,” Perkins stated, referring to how taphonomy needs to take into account what vanishes as much as what remains.  In one memorable line, he said, “Most organisms live, die, and disappear without leaving any hints that they ever existed.”


1Sid Perkins, “Modern science investigates the initial stages of how fossils form,” Science News, Week of Jan. 28, 2006; Vol. 169, No. 4 , p. 56.

No, we are NOT going to say “billions of dead things buried in rock layers laid down by water all over the earth,” but think about this; it must require unusual conditions to get an Archaeopteryx and all the fine detail of preservation of feathers and soft tissues seen in many famous museum pieces, especially those coming out of the Liaoning Province of China.  Think of the dramatic fossil graveyards around the world where large mammals, dinosaurs or whales are buried in huge expanses of strata.
    Perkins speaks glibly about this 570-million-year-old embryo, or that 150 million year old Archaeopteryx, and such, but how can he know?  All this research is relatively new, and shows rapid decay within less than 20 years.  How is it possible to claim what a fossil will look like in millions of years?  Think of how little remains when coroners exhume famous remains like Jesse James to look for clues in some historical mystery, and yet fossil embryos and jellyfish have been found, and things as ephemeral as footprints, raindrops and even dinosaur vomit.
    This is a good area of research for a budding young scientist who has some decades of useful career ahead to work.  Since so many claims are made based on assumptions of long ages, it would be useful to know the requirements for fossilization with actual field testing, and get some numbers on how long various features can be expected to last even if completely mineralized.  We wonder if Harry Truman has fossilized at Mt. St. Helens under the ash at Spirit Lake.  Probably so; he was a pretty hard cuss already.

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