Speaker 1: Welcome to brain Stuff from how stuff works. Hey, brain stuff,
Speaker 1: it's me Christian Seger. You know, when I'm digging a
Speaker 1: six foot hole in the middle of the desert, I
Speaker 1: start to wonder where are all the dead animals? Shouldn't
Speaker 1: we be waiting knee deep in fossils every time we
Speaker 1: go outside. I know that's morbid, but you can probably
Speaker 1: guess that not every animal that dies leaves behind fossil evidence.
Speaker 1: But why is that? Well, just to get our terms straight,
Speaker 1: a fossil is any physical remnant left behind by an
Speaker 1: organism that died long ago. In many cases, fossils might
Speaker 1: only be things like preserved footprints or nest sites. But
Speaker 1: today we're looking at direct remains of animal bodies, like bones.
Speaker 1: The likelihood that any particular animal body will become fossilized
Speaker 1: is amazingly small. It's actually less than one percent. So
Speaker 1: let's look at the stations of the obstacle course to fossilization. First,
Speaker 1: there's body type. Fossilization has a strong preference for animals
Speaker 1: with hard body parts like bones, teeth, and shells. Animals
Speaker 1: with soft bodies like slugs and jellyfish, well, they usually
Speaker 1: just decompose completely and disappear after death, and except in
Speaker 1: a very few rare cases like freezing, dry mommification and
Speaker 1: peat bog preservation, the same thing happens to the soft
Speaker 1: tissues on all animal bodies, skin, organs, eyeballs, et cetera.
Speaker 1: They all make excellent meals from microorganisms and are thus
Speaker 1: consigned to the ravages of rot. The second main hurdle
Speaker 1: defossilization is exposure. To become a fossil, you need to
Speaker 1: be one of the rare animal bodies that is rapidly
Speaker 1: buried soon after the animal dies. This is most likely
Speaker 1: to happen in or near the site of a moving
Speaker 1: body of water, like a river or a floodplain, where
Speaker 1: runoff floodwaters or regular flow may quickly cover a dead
Speaker 1: body in sediment. It might also happen in arid desert
Speaker 1: settings where wind can quickly bury animal remains in sand dunes.
Speaker 1: If the remains are not rapidly buried, scavenging animals are
Speaker 1: likely to scatter and then consume them. After all, nature
Speaker 1: hates to pass up a free lunch, and even a
Speaker 1: clean skeleton left out exposed to the elements will eventually
Speaker 1: be erased by the ravages of the weather. That's decalcification, erosion,
Speaker 1: and corrosion. But let's say your bones are lucky enough
Speaker 1: to be rapidly buried somehow. The next big hurdle is
Speaker 1: the sediment itself. A nice dry sand or alkaline mud
Speaker 1: might be a good place to become a fossil, but
Speaker 1: if your bones are buried in soil with a higher
Speaker 1: temperature and higher acidity, your prospects are a lot slimmer.
Speaker 1: Acidic environments meaning soils with a low pH tend to
Speaker 1: dissolve hydro zappatite, a calcium phosphate mineral that is a
Speaker 1: main structural ingredient in our bones. So many soil types
Speaker 1: on Earth will simply destroy all the bones they swallow.
Speaker 1: But even in friendly sediment, over a long enough period
Speaker 1: of time, bones can break down. The organic proteins and
Speaker 1: bones like collagen, eventually decompose, and the inorganic molecules and
Speaker 1: bones can be crushed, dissolved, or otherwise destroyed by physical
Speaker 1: force over the centuries. So if you want your actual
Speaker 1: bone structure to survive, you have to be lucky enough
Speaker 1: to undergo a little transformation. Most really ancient bones we find,
Speaker 1: such as dinosaur bones, aren't the unaltered original bones that
Speaker 1: were buried millions of years ago. Instead, they're either a
Speaker 1: minerally modified versions of those bones or be stone photocopies.
Speaker 1: Two processes represent the majority of these cases, perma mineralization
Speaker 1: and replacement. In perma mineralization, mineral rich waters seep into
Speaker 1: the buried bones and fills the pores of the bones
Speaker 1: with its mineral content. These minerals form crystals inside the bones,
Speaker 1: causing them to modify and harden over time. Sometimes this
Speaker 1: process is also called petrification. In replacement, the original bones
Speaker 1: can be completely dissolved but still leave fossil copies, as
Speaker 1: the mineral in the groundwater completely replaces the shape of
Speaker 1: the bones over long periods of time. So let's say
Speaker 1: you're the rare dead animal that wins the fossilization lottery
Speaker 1: and you just happen to pass all these tests, you
Speaker 1: still have to be found. The total surface of the
Speaker 1: Earth is almost two hundred million square miles, and even
Speaker 1: for a guy like me, there's only so much time
Speaker 1: to dig. Check out the brain stuff channel on YouTube,
Speaker 1: and for more on this and thousands of other topics,
Speaker 1: visit how stuff works dot com.