WEBVTT - BrainStuff Classics: Why Don't All Skeletons Become Fossils? 0:00:01.920 --> 0:00:06.520 Welcome to brain Stuff production of I Heart Radio. Hey 0:00:06.559 --> 0:00:09.360 brain Stuff, Lauren bog Obam here with a classic episode 0:00:09.360 --> 0:00:12.440 from our previous host, Christian Sagar. In this one, we 0:00:12.520 --> 0:00:15.800 wanted to explore why there have been billions of living 0:00:15.840 --> 0:00:18.560 things on this earth across billions of years that life 0:00:18.600 --> 0:00:22.239 has existed, but only a very few have left direct 0:00:22.480 --> 0:00:28.400 fossil evidence. Hey brain Stuff, it's me Christian Sager. You know, 0:00:28.680 --> 0:00:31.400 when I'm digging a six ft hole in the middle 0:00:31.400 --> 0:00:34.080 of the desert, I start to wonder where are all 0:00:34.120 --> 0:00:37.360 the dead animals? Shouldn't we be waiting knee deep in 0:00:37.479 --> 0:00:40.519 fossils every time we go outside. I know that's morbid, 0:00:40.720 --> 0:00:44.080 but you can probably guess that not every animal that 0:00:44.200 --> 0:00:48.560 dies leaves behind fossil evidence. But why is that? Well, 0:00:48.760 --> 0:00:51.840 just to get our terms straight, a fossil is any 0:00:51.920 --> 0:00:55.800 physical remnant left behind by an organism that died long ago. 0:00:56.280 --> 0:00:59.680 In many cases, fossils might only be things like preserved 0:00:59.720 --> 0:01:03.639 foot prints or nest sites. But today we're looking at 0:01:03.680 --> 0:01:07.959 direct remains of animal bodies, like bones. The likelihood that 0:01:08.040 --> 0:01:12.839 any particular animal body will become fossilized is amazingly small. 0:01:12.920 --> 0:01:15.880 It's actually less than one percent so let's look at 0:01:15.920 --> 0:01:20.200 the stations of the obstacle course to fossilization. First, there's 0:01:20.240 --> 0:01:24.000 body type. Fossilization has a strong preference for animals with 0:01:24.200 --> 0:01:28.800 hard body parts like bones, teeth, and shells. Animals with 0:01:28.880 --> 0:01:32.280 soft bodies like slugs and jellyfish, well, they usually just 0:01:32.360 --> 0:01:36.120 decompose completely and disappear after death, and except in a 0:01:36.440 --> 0:01:41.080 very few rare cases like freezing, dry momification and peat 0:01:41.160 --> 0:01:44.839 bog preservation, the same thing happens to the soft tissues 0:01:44.920 --> 0:01:50.040 on all animal bodies skin, organs, eyeballs, et cetera. They 0:01:50.080 --> 0:01:54.360 all make excellent meals from microorganisms and are thus consigned 0:01:54.360 --> 0:01:58.640 to the ravages of rot. The second main hurdle defossilization 0:01:59.080 --> 0:02:02.040 is exposure. To become a fossil, you need to be 0:02:02.160 --> 0:02:05.280 one of the rare animal bodies that is rapidly buried 0:02:05.400 --> 0:02:09.200 soon after the animal dies. This is most likely to 0:02:09.280 --> 0:02:12.080 happen in or near the site of a moving body 0:02:12.120 --> 0:02:15.400 of water, like a river or a floodplain, where runoff 0:02:15.639 --> 0:02:19.560 floodwaters or regular flow may quickly cover a dead body 0:02:19.560 --> 0:02:24.000 in sediment. It might also happen in arid desert settings, 0:02:24.000 --> 0:02:27.640 where wind can quickly bury animal remains in sand dunes. 0:02:28.040 --> 0:02:32.320 If the remains are not rapidly buried, scavenging animals are 0:02:32.360 --> 0:02:36.240 likely to scatter and then consume them. After all, nature 0:02:36.320 --> 0:02:39.000 hates to pass up a free lunch, and even a 0:02:39.040 --> 0:02:43.119 clean skeleton left out exposed to the elements will eventually 0:02:43.160 --> 0:02:48.520 be erased by the ravages of the weather. That's decalcification, erosion, 0:02:48.840 --> 0:02:52.799 and corrosion. But let's say your bones are lucky enough 0:02:52.840 --> 0:02:56.239 to be rapidly buried somehow. The next big hurdle is 0:02:56.280 --> 0:03:00.320 a sediment itself. A nice dry sand or alkaline mud 0:03:00.400 --> 0:03:02.640 might be a good place to become a fossil, but 0:03:02.720 --> 0:03:05.280 if your bones are buried in soil with a higher 0:03:05.320 --> 0:03:09.440 temperature and higher acidity, your prospects are a lot slimmer. 0:03:09.760 --> 0:03:13.080 Acidic environments, meaning soils with a low pH tend to 0:03:13.160 --> 0:03:17.440 dissolve hydro zappatite, a calcium phosphate mineral that is a 0:03:17.560 --> 0:03:21.639 main structural ingredient in our bones. So many soil types 0:03:21.680 --> 0:03:25.880 on Earth will simply destroy all the bones they swallow. 0:03:26.160 --> 0:03:30.160 But even in friendly sediment, over a long enough period 0:03:30.200 --> 0:03:33.720 of time, bones can break down the organic proteins and 0:03:33.800 --> 0:03:38.680 bones like collagen, eventually decompose and the inorganic molecules, and 0:03:38.720 --> 0:03:43.640 bones can be crushed, dissolved, or otherwise destroyed by physical 0:03:43.680 --> 0:03:47.720 force over the centuries. So if you want your actual 0:03:47.840 --> 0:03:50.960 bone structure to survive, you have to be lucky enough 0:03:51.040 --> 0:03:55.840 to undergo a little transformation. Most really ancient bones we find, 0:03:55.880 --> 0:03:59.480 such as dinosaur bones, aren't the unaltered original bones that 0:03:59.560 --> 0:04:02.640 were bare eed millions of years ago. Instead, they're either 0:04:02.880 --> 0:04:09.000 a minerally modified versions of those bones or be stone photocopies. 0:04:09.480 --> 0:04:14.119 Two processes represent the majority of these cases, perma mineralization 0:04:14.280 --> 0:04:19.599 and replacement. In perma mineralization, mineral rich water seeps into 0:04:19.640 --> 0:04:22.159 the buried bones and fills the pores of the bones 0:04:22.360 --> 0:04:26.720 with its mineral content. These minerals form crystals inside the bones, 0:04:26.800 --> 0:04:30.279 causing them to modify and harden over time. Sometimes this 0:04:30.360 --> 0:04:34.920 process is also called petrification. In replacement, the original bones 0:04:34.960 --> 0:04:39.200 can be completely dissolved but still leave fossil copies as 0:04:39.240 --> 0:04:43.640 the mineral in the groundwater completely replaces the shape of 0:04:43.680 --> 0:04:47.800 the bones over long periods of time. So let's say 0:04:47.880 --> 0:04:51.679 you're the rare dead animal that wins the fossilization lottery, 0:04:51.720 --> 0:04:55.120 and you just happen to pass all these tests. You 0:04:55.160 --> 0:04:57.840 still have to be found. The total surface of the 0:04:57.880 --> 0:05:01.599 Earth is almost two hundred millions square miles, and even 0:05:01.839 --> 0:05:04.400 for a guy like me, there's only so much time 0:05:04.440 --> 0:05:11.200 to dick. Today's episode was written by Joe McCormick and 0:05:11.200 --> 0:05:13.520 produced by Tyler Clang. For more on this and lots 0:05:13.520 --> 0:05:15.720 of other topics, was it how stuff works dot com. 0:05:15.760 --> 0:05:18.480 Brain Stuff is production of iHeart Radio. For more podcasts 0:05:18.480 --> 0:05:20.440 in My heart Radio, visit the i heart Radio app, 0:05:20.600 --> 0:05:23.240 Apple Podcasts, or wherever you listen to your favorite shows.