WEBVTT - Why Don't All Skeletons Become Fossils?

0:00:02.080 --> 0:00:07.400
<v Speaker 1>Welcome to brain Stuff from how stuff works. Hey, brain stuff,

0:00:07.440 --> 0:00:10.799
<v Speaker 1>it's me Christian Seger. You know, when I'm digging a

0:00:10.840 --> 0:00:13.160
<v Speaker 1>six foot hole in the middle of the desert, I

0:00:13.200 --> 0:00:16.600
<v Speaker 1>start to wonder where are all the dead animals? Shouldn't

0:00:16.600 --> 0:00:19.360
<v Speaker 1>we be waiting knee deep in fossils every time we

0:00:19.440 --> 0:00:22.520
<v Speaker 1>go outside. I know that's morbid, but you can probably

0:00:22.560 --> 0:00:27.600
<v Speaker 1>guess that not every animal that dies leaves behind fossil evidence.

0:00:27.640 --> 0:00:30.880
<v Speaker 1>But why is that? Well, just to get our terms straight,

0:00:30.920 --> 0:00:34.760
<v Speaker 1>a fossil is any physical remnant left behind by an

0:00:34.880 --> 0:00:38.959
<v Speaker 1>organism that died long ago. In many cases, fossils might

0:00:39.040 --> 0:00:43.120
<v Speaker 1>only be things like preserved footprints or nest sites. But

0:00:43.200 --> 0:00:47.400
<v Speaker 1>today we're looking at direct remains of animal bodies, like bones.

0:00:47.800 --> 0:00:51.960
<v Speaker 1>The likelihood that any particular animal body will become fossilized

0:00:52.120 --> 0:00:55.959
<v Speaker 1>is amazingly small. It's actually less than one percent. So

0:00:56.120 --> 0:01:00.640
<v Speaker 1>let's look at the stations of the obstacle course to fossilization. First,

0:01:00.720 --> 0:01:04.720
<v Speaker 1>there's body type. Fossilization has a strong preference for animals

0:01:04.720 --> 0:01:09.479
<v Speaker 1>with hard body parts like bones, teeth, and shells. Animals

0:01:09.480 --> 0:01:12.959
<v Speaker 1>with soft bodies like slugs and jellyfish, well, they usually

0:01:13.000 --> 0:01:16.760
<v Speaker 1>just decompose completely and disappear after death, and except in

0:01:16.840 --> 0:01:21.520
<v Speaker 1>a very few rare cases like freezing, dry mommification and

0:01:21.640 --> 0:01:25.119
<v Speaker 1>peat bog preservation, the same thing happens to the soft

0:01:25.160 --> 0:01:30.360
<v Speaker 1>tissues on all animal bodies, skin, organs, eyeballs, et cetera.

0:01:30.680 --> 0:01:34.559
<v Speaker 1>They all make excellent meals from microorganisms and are thus

0:01:34.600 --> 0:01:38.480
<v Speaker 1>consigned to the ravages of rot. The second main hurdle

0:01:38.520 --> 0:01:42.640
<v Speaker 1>defossilization is exposure. To become a fossil, you need to

0:01:42.680 --> 0:01:45.640
<v Speaker 1>be one of the rare animal bodies that is rapidly

0:01:45.680 --> 0:01:49.840
<v Speaker 1>buried soon after the animal dies. This is most likely

0:01:49.920 --> 0:01:52.559
<v Speaker 1>to happen in or near the site of a moving

0:01:52.600 --> 0:01:55.680
<v Speaker 1>body of water, like a river or a floodplain, where

0:01:55.760 --> 0:01:59.800
<v Speaker 1>runoff floodwaters or regular flow may quickly cover a dead

0:02:00.040 --> 0:02:04.280
<v Speaker 1>body in sediment. It might also happen in arid desert

0:02:04.360 --> 0:02:08.440
<v Speaker 1>settings where wind can quickly bury animal remains in sand dunes.

0:02:08.880 --> 0:02:13.120
<v Speaker 1>If the remains are not rapidly buried, scavenging animals are

0:02:13.200 --> 0:02:17.040
<v Speaker 1>likely to scatter and then consume them. After all, nature

0:02:17.160 --> 0:02:19.799
<v Speaker 1>hates to pass up a free lunch, and even a

0:02:19.840 --> 0:02:23.959
<v Speaker 1>clean skeleton left out exposed to the elements will eventually

0:02:24.000 --> 0:02:29.320
<v Speaker 1>be erased by the ravages of the weather. That's decalcification, erosion,

0:02:29.680 --> 0:02:33.640
<v Speaker 1>and corrosion. But let's say your bones are lucky enough

0:02:33.639 --> 0:02:37.080
<v Speaker 1>to be rapidly buried somehow. The next big hurdle is

0:02:37.080 --> 0:02:41.120
<v Speaker 1>the sediment itself. A nice dry sand or alkaline mud

0:02:41.200 --> 0:02:43.480
<v Speaker 1>might be a good place to become a fossil, but

0:02:43.520 --> 0:02:46.120
<v Speaker 1>if your bones are buried in soil with a higher

0:02:46.120 --> 0:02:50.239
<v Speaker 1>temperature and higher acidity, your prospects are a lot slimmer.

0:02:50.560 --> 0:02:53.920
<v Speaker 1>Acidic environments meaning soils with a low pH tend to

0:02:53.960 --> 0:02:58.280
<v Speaker 1>dissolve hydro zappatite, a calcium phosphate mineral that is a

0:02:58.360 --> 0:03:02.480
<v Speaker 1>main structural ingredient in our bones. So many soil types

0:03:02.520 --> 0:03:06.679
<v Speaker 1>on Earth will simply destroy all the bones they swallow.

0:03:07.000 --> 0:03:10.960
<v Speaker 1>But even in friendly sediment, over a long enough period

0:03:11.000 --> 0:03:14.520
<v Speaker 1>of time, bones can break down. The organic proteins and

0:03:14.600 --> 0:03:19.480
<v Speaker 1>bones like collagen, eventually decompose, and the inorganic molecules and

0:03:19.560 --> 0:03:24.440
<v Speaker 1>bones can be crushed, dissolved, or otherwise destroyed by physical

0:03:24.520 --> 0:03:28.519
<v Speaker 1>force over the centuries. So if you want your actual

0:03:28.639 --> 0:03:31.800
<v Speaker 1>bone structure to survive, you have to be lucky enough

0:03:31.840 --> 0:03:36.680
<v Speaker 1>to undergo a little transformation. Most really ancient bones we find,

0:03:36.720 --> 0:03:40.320
<v Speaker 1>such as dinosaur bones, aren't the unaltered original bones that

0:03:40.360 --> 0:03:44.320
<v Speaker 1>were buried millions of years ago. Instead, they're either a

0:03:44.440 --> 0:03:49.880
<v Speaker 1>minerally modified versions of those bones or be stone photocopies.

0:03:50.320 --> 0:03:54.920
<v Speaker 1>Two processes represent the majority of these cases, perma mineralization

0:03:55.080 --> 0:04:00.480
<v Speaker 1>and replacement. In perma mineralization, mineral rich waters seep into

0:04:00.480 --> 0:04:02.960
<v Speaker 1>the buried bones and fills the pores of the bones

0:04:03.160 --> 0:04:07.520
<v Speaker 1>with its mineral content. These minerals form crystals inside the bones,

0:04:07.600 --> 0:04:11.120
<v Speaker 1>causing them to modify and harden over time. Sometimes this

0:04:11.200 --> 0:04:15.760
<v Speaker 1>process is also called petrification. In replacement, the original bones

0:04:15.760 --> 0:04:20.000
<v Speaker 1>can be completely dissolved but still leave fossil copies, as

0:04:20.040 --> 0:04:24.480
<v Speaker 1>the mineral in the groundwater completely replaces the shape of

0:04:24.480 --> 0:04:28.640
<v Speaker 1>the bones over long periods of time. So let's say

0:04:28.680 --> 0:04:32.520
<v Speaker 1>you're the rare dead animal that wins the fossilization lottery

0:04:32.520 --> 0:04:35.919
<v Speaker 1>and you just happen to pass all these tests, you

0:04:35.960 --> 0:04:38.640
<v Speaker 1>still have to be found. The total surface of the

0:04:38.680 --> 0:04:42.520
<v Speaker 1>Earth is almost two hundred million square miles, and even

0:04:42.640 --> 0:04:45.200
<v Speaker 1>for a guy like me, there's only so much time

0:04:45.240 --> 0:04:53.360
<v Speaker 1>to dig. Check out the brain stuff channel on YouTube,

0:04:53.440 --> 0:04:55.599
<v Speaker 1>and for more on this and thousands of other topics,

0:04:55.760 --> 0:05:10.720
<v Speaker 1>visit how stuff works dot com.