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Why Don't All Skeletons Become Fossils?

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Channel: BrainStuff - HowStuffWorks
Categories: Archeology / Paleontology   |   Biology   |   Chemistry   |   Science  
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Skeletons of ancient animals sometimes become fossilized. But if this happened to every animal that ever lived, wouldn’t we be swimming in a sea of fossils right now? BrainStuff explains why only a fraction of animal bones turn into fossils.

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Hey Brainstuff -- it’s me, Jonathan. You know, sometimes in a lull between events at CES I’m digging a six-foot hole in the middle of the desert outside Las Vegas and I start to wonder: Where are all the dead animals? Shouldn’t we be wading knee-deep in fossils every time we go outside?

You can probably guess that not every animal that dies leaves behind fossil evidence.

But why is that?

Just to get our terms straight, a fossil is any physical remnant left behind by an organism that died long ago. In many cases, fossils might only be things like preserved footprints or nest sites. But today, we’re looking at direct remains of animal bodies, like bones.

The likelihood that any particular animal body will become fossilized is amazingly small, way less than 1%. You might as well buy a lottery ticket.

So let’s look at the stations of the obstacle course to fossilization:

First, there’s body type. Fossilization has a strong preference for animals with hard body parts, like bones, teeth and shells. Animals with soft bodies, like slugs and jellyfish, will usually just decompose completely and disappear after death.

The second main hurdle to fossilization is exposure. To become a fossil, you need to be one of the rare animal bodies that is rapidly buried soon after the animal dies. This is most likely to happen in or near the site of a moving body of water, like a river or a flood plain, where runoff, floodwaters or regular flow may quickly cover a dead body in sediment.

It might also happen in arid desert settings, where wind can quickly bury animal remains in sand dunes. If the remains are not rapidly buried, scavenging animals are likely to scatter and consume them -- after all, nature hates to pass up a free lunch. We're alike in that way. And even a clean skeleton left out exposed to the elements will eventually be erased by decalcification, erosion and corrosion.

But let’s say your bones are lucky enough to be rapidly buried somehow. The next big hurdle is the sediment itself. A nice dry sand or alkaline mud might be a good place to become a fossil. But if your bones are buried in soil with a higher temperature and higher acidity, your prospects are a lot slimmer.

Acidic environments (meaning soils with a low pH) tend to dissolve hydroxyapatite -- a calcium phosphate mineral that is a main structural ingredient in our bones. So, many soil types on Earth will simply destroy all the bones they swallow.

I'm counting on it. (I kid, I kid)

But even in friendly sediment, over a long enough period of time, bones can break down. The organic proteins in bones, like collagen, eventually decompose, and the inorganic minerals in bones can be crushed, dissolved or otherwise destroyed by physical forces over the centuries.

So if you want your actual bone structure to survive, you have to be lucky enough to undergo a little transformation. Most really ancient bones we find, such as dinosaur bones, aren’t the unaltered, original bones that were buried millions of years ago, but either minerally modified versions of those bones, or “stone photocopies.”

Two processes represent the majority of these cases: permineralization, and replacement.

In permineralization, mineral-rich water seeps into the buried bones and fills the pores of the bones with its mineral content. These minerals form crystals inside the bones, causing them to modify and harden over time. Sometimes this process is also called “petrification.”

In “replacement,” the original bones can be completely dissolved, but still leave fossil copies, as the minerals in the groundwater completely replace the shape of the bones over long periods of time.

So let’s say you’re the rare dead animal that wins the fossilization lottery, and you just happen to pass ALL these tests? You still have to be found.

The total surface of the Earth is almost 200 million square miles, and even for a guy like me, there’s only so much time to dig.


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