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Conditions of Fossil Preservation: Rapid Burial, Hard Parts & the Elements

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  • 0:07 The Fossil Record
  • 1:05 Fossil Preservation
  • 2:37 Rapid Burial
  • 4:00 Hard Parts
  • 5:19 Effects of the Elements
  • 6:40 Lesson Summary
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Lesson Transcript
Instructor: April Koch

April teaches high school science and holds a master's degree in education.

Learn how fossils are formed and how varying conditions affect the preservation of organisms. What is the best way for a dinosaur to become fossilized? Find out here!

The Fossil Record

If I asked you to name five prehistoric animals in the next ten seconds, what would you say? Would you list the Tyrannosaurus rex or the wooly mammoth? Maybe you'd list the Stegosaurus, Brachiosaurus or the saber-toothed cat. But what about the smaller species? Do you think you'd list little guys like Archaeopteryx, Coelophysis or even the humble Compsognathus?

It's no secret that our fossil record is chock full of giants. We know way more about the massive, earth-shaking creatures of the past than we do the smaller guys. We know that there were tinier creatures living alongside the giant dinosaurs, but we don't see many fossils of them. Why is our fossil record so disproportionately full of larger species? Were the smaller guys really that rare in prehistoric days? Or is our fossil record just not showing us a clear picture of the past? Let's learn more about fossil preservation to find out what's really going on.

Fossil Preservation

Fossil preservation is the process by which the remains of an organism are transformed into rock or impressions within sedimentary rock. When you look at a fossil dinosaur bone, you're not actually looking at bone. You're looking at a rock in the shape of a bone. The minerals in the dinosaur's bones were replaced by other minerals. The cavities were filled with sedimentary material, and, over millions of years, the bone was turned into rock.

But not all dinosaur bones became fossils. Fossil preservation is tricky. It takes a lot of luck and special circumstances for any creature to get preserved as a fossil. When an organism dies, its body immediately begins to decompose. Bacteria and insects get right to work, breaking down the plant or animal's organic materials. Scavengers come running, grabbing arms and legs, dashing off with body parts to munch the meat off the bones. Fluctuating temperatures stretch and shrink the body's tissues. Rain and sun degrade skin and bones. Herds of animals trample the remaining structures, and beetles chew up whatever happens to be left.

How did we ever get any dinosaur bones preserved as fossils? With everything that happens to an animal after its death, it's a wonder we've been able to dig up and assemble as many prehistoric skeletons as we have! The secret to success in fossil preservation lies in the right combination of circumstances following the death of an organism. The first and most important circumstance is called rapid burial.

Rapid Burial

Let's say you have a cute little Compsognathus dinosaur who eats a toxic plant and dies at the edge of a lake. Now, that Compsognathus skeleton is destined for destruction very soon. The scavengers are coming and so are the insects, bacteria and weather fluctuations that will turn him into dust in no time.

But look! All of a sudden, his body falls into the lake. It sinks to the bottom, and now it's completely submerged. It's safe from the scavengers, the beetles and bacteria. It's safe from the weather because temperatures are fairly constant underwater. Maybe a few fish eat the meat off his bones, but nobody carries his arms and legs away. Within a year, his body gets covered by all the silt and sediment accumulating on the lake bed. From here on out, our Compsognathus is safe. It's only a matter of time before his bones are turned to fossils.

This is sort of a best-case scenario for rapid burial. A plant or animal that is buried in mud, silt or other protective substances very shortly after death is much more likely to be preserved as a fossil. Some of our fossilized dinosaurs were originally covered in sand, soil or dense foliage shortly after they died. Some ancient creatures were even preserved in a pit of tar! Rapid burial contributed to their being preserved as fossils because it protected them from the destructive forces of a terrestrial ecosystem.

Hard Parts

You've probably noticed that paleontologists know a lot about dinosaur bones but almost nothing about their skin. This is because the hard parts of organisms are the most likely to survive long enough to be fossilized. If you think about our most common fossils of prehistoric animals, they are typically bones, teeth, claws, shells, and the exoskeletons of arthropods. If you're talking plant life, you're looking mostly at wood and seeds. Most of the soft stuff, like body organs and leaf tissues, decomposes way too fast to be fossilized - even if it is buried rapidly under a lake. So if you were hoping to see Compsognathus' stomach in fossil form - sorry, it just can't make its way to a fossil that easily.

However, a few soft body parts can become fossilized as rock impressions or natural casts. For instance, impressions of skin, feathers, footprints and brains have been preserved as fossils. Also preserved are the bodies of some worms and jellyfish and the shapes of flowers, leaves and ferns. It doesn't always have to be a hard part in order to leave behind some trace of its existence. But in order to become a serious, three-dimensional fossil, it has to have been a hard part of an organism.

Effects of the Elements

Now, wait a second. What about mammoths? Don't we have some preserved soft tissues of mammoths somewhere? Well, yes. In fact, we don't just have fossils; we have actual skin and hair from mammoths that went extinct about 5,000 years ago. Notice that I said 5,000 years, not 65 million years! The timetable for mammoth preservation is nothing compared to dinosaurs. But either way, it is pretty cool that we have the skin and hair from mammoths! Any ideas on how these tissues were able to resist decomposition for 5,000 years?

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