Why Don't All Skeletons Become Fossils?

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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Episode Transcript

Speaker 1 (00:02):
Welcome to brain Stuff from how stuff works. Hey, brain stuff,
it's me Christian Seger. You know, when I'm digging a
six foot hole in the middle of the desert, I
start to wonder where are all the dead animals? Shouldn't
we be waiting knee deep in fossils every time we
go outside. I know that's morbid, but you can probably

(00:22):
guess that not every animal that dies leaves behind fossil evidence.
But why is that? Well, 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

(00:43):
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. It's actually less than one percent. 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

(01:04):
with hard body parts like bones, teeth, and shells. Animals
with soft bodies like slugs and jellyfish, well, they usually
just decompose completely and disappear after death, and except in
a very few rare cases like freezing, dry mommification and
peat bog preservation, the same thing happens to the soft

(01:25):
tissues on all animal bodies, skin, organs, eyeballs, et cetera.
They all make excellent meals from microorganisms and are thus
consigned to the ravages of rot. The second main hurdle
defossilization is exposure. To become a fossil, you need to
be one of the rare animal bodies that is rapidly

(01:45):
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 floodplain, 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.

(02:08):
If the remains are not rapidly buried, scavenging animals are
likely to scatter and then consume them. After all, nature
hates to pass up a free lunch, and even a
clean skeleton left out exposed to the elements will eventually
be erased by the ravages of the weather. That's decalcification, erosion,

(02:29):
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.

(02:50):
Acidic environments meaning soils with a low pH tend to
dissolve hydro zappatite, 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.
But even in friendly sediment, over a long enough period

(03:11):
of time, bones can break down. The organic proteins and
bones like collagen, eventually decompose, and the inorganic molecules and
bones can be crushed, dissolved, or otherwise destroyed by physical
force over the centuries. So if you want your actual
bone structure to survive, you have to be lucky enough

(03:31):
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. Instead, they're either a
minerally modified versions of those bones or be stone photocopies.
Two processes represent the majority of these cases, perma mineralization

(03:55):
and replacement. In perma mineralization, mineral rich waters seep 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

(04:15):
can be completely dissolved but still leave fossil copies, as
the mineral in the groundwater completely replaces 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

(04:35):
still have to be found. The total surface of the
Earth is almost two hundred million square miles, and even
for a guy like me, there's only so much time
to dig. Check out the brain stuff channel on YouTube,
and for more on this and thousands of other topics,
visit how stuff works dot com.