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Crash Course Geology explains how fossils form, noting that most organisms decay and only a rare few are preserved, usually through rapid burial in sediment followed by processes like permineralization, compression and carbonization, or preservation in amber, desert caves, or ice. The video stresses that fossils are rare, that erosion erases much of the record, and that fossils like Boris and the Notasaur reveal major events in life and Earth's history.
- Fossils are preserved remains or traces of organisms at least 10,000 years old, including bones, shells, skin, feathers, footprints, and nests.
- Most fossils form when organisms are quickly buried in sediment such as mud, sand, or ash, as happened to the Notasaur about 110 million years ago.
- In permineralization, minerals from groundwater seep into pores of buried tissue and replace it, turning bone and soft tissue into stone.
- Compression and carbonization flatten lightweight organisms like leaves, fish, and insects into thin carbon films.
- Trace fossils such as footprints preserve behaviors like walking or burrowing, including footprints of children made about 11,500 years ago in New Mexico.
- Rare whole-body preservation happens in amber, desert caves, tar pits, bogs, and ice, which slow decomposition.
- Fossilization is rare because rapid burial and hard body parts improve the odds, while erosion and weathering constantly destroy the fossil record.
- Fossils like Boris, a fishlike tetrapod with paddle-shaped feet, and Lagerstätten such as the Burgess Shale help reveal how life moved onto land and how life and Earth have changed over time.
- 0:00A Dinosaur's Last Meal
- 0:31What Counts as a Fossil
- 1:15How the Notasaur Was Buried
- 1:57Permineralization and Replacement by Minerals
- 3:08Compression and Carbonization
- 3:40Trace Fossils and Footprints
- 4:22Whole-Body Preservation Methods
- 5:43Why Fossilization Is So Rare
- 6:20Discovery of the Notasaur
- 6:56Boris and the Move to Land
- 8:11Lagerstätten and the Burgess Shale
- 9:22Fossils Reveal Earth's History
A Dinosaur's Last Meal
0:00Get ready because you're about to look a dinosaur in the face. This is a notasaur. In its belly, you can still see specks of its last meal. Burnt salad from the traces of melanin in its bumpy skin. We can wager that it was a reddish brown with a light underbelly. And those tobines even still look squishable even though they've turned to stone.
0:17Don't you just want to squeeze them? Turns out the worst day of this notaur's life was a very lucky day for making a fossil. Hi, I'm Sage and this is Crash Course Geology.
What Counts as a Fossil
0:31This is poop and it's a fossil. This is a bug bedazzled in fool's gold and it's a fossil. This baby cave lion unearthed from perafrost. This worm sperm. This ant with a parasitic mushroom growing out of its butt. All fossils. Fossils are preserved remains or traces of organisms that are at least 10,000 years old.
0:49But they aren't just shells and bones, ammonite necklaces, and T-Rex thighs. Fossils can also preserve skin, feathers, or flowers. They can freeze footprints, tunnels, and nests in time. Even whole baby mammoths, fossils are the closest we'll ever get to longgone organisms. And often, they're a snapshot of their last worst days.
1:09But even though everyone dies, very few fossilize. Actually, becoming a fossil is like winning the geological jackpot. You see, fossils require very specific conditions to exist at all. Most fossils form when living things are quickly trapped and buried in sediment, like mud, sand, or ash. That's what happened to our Notasaur friend.
How the Notasaur Was Buried
1:31I mean, I consider it a personal friend. About 110 million years ago, the notasaur died somehow. Its body floated down a river and then into the sea as it bloated with gas. Eventually, the body burst and sank belly up to the seafloor where mud swirled around and buried it. After that initial burial, there are a few different ways fossils can form.
1:53The first path is permineralization. That's when minerals from water in the ground leech into the pores of buried wood, bone, shells, or soft tissue, replace them, and leave behind a heavier, denser 3D mold. As sediment kept layering over our notasaur, minerals infused and replaced much of its armored skin, bone, and squishy bits, which eventually hardened into stone.
Permineralization and Replacement by Minerals
2:14Now, that's what I call rockhard abs. Often, as minerals replace the original tissue, they change color. And on rare occasions, they can actually opalize and get all glossy. Oo, shiny. Even so, it can be hard to tell permineralized fossils from regular rocks. So allow me, your internet science talk talk clown, to let you in on a professional secret that real paleontologists use in the field.
2:35You lick them. No, really. Bone is more porous than rock. So if it sticks to your tongue, it's probably fossilized bone. Dwayne, may I? Actually, I like a relationship as it is. By the way, if you think minerals are as neat as I do, you're going to want to check out the Sihow rocks box. They send you cool rocks right to your home.
2:56Check out the link below to learn more. Okay, so replacement by minerals. That's the first path to fossilhood. Another possible path is the old CNC, compression and carbonization, which usually happens to lightweights like leaves, fish, and insects. First, layers of sediment bury an organism until it's flatter than the acting in a bee movie.
Compression and Carbonization
3:14That's the compression part. Then, as the organic bits decompose, carbonization begins, sparking chemical reactions that transform its remains into a filmy carbon silhouette. Rest in peace. But not all burial by sediment involves preservation of body parts. Trace fossils preserve impressions of organisms behaviors from long ago, like walking, wiggling, or burrowing.
3:33And they're no less amazing than preserved skin and bones. Like 11,500 years ago in what's now New Mexico, fossils formed from the footprints of little kids who stopped to play in the fresh tracks of a giant ground sloth. And honestly, every time I think about this, I feel like crying. I love us. We're very cute animals.
Trace Fossils and Footprints
3:53And some of the rarest, wildest, most jaw-dropping fossils don't involve getting buried under sediment at all. In fact, there's quite a few methods to choose from. Let's hear a word from our sponsor. Are you dead or about to be? We here at the preservation station offer [music] bespoke fossilization services for the discerning organism.
4:13Whether it's an amber tomb or a desert cave [music] or a 10,000-year deep freeze, we'll match you with [music] a special environment that will preserve all or most of your body, so you can look almost as good as the day you died. [music] When it works, it really works. And when it doesn't, me, you're already dead.
Whole-Body Preservation Methods
4:37So don't delay. Slow the decomposition of your body today. Sign me up. Sometimes, in really specific cases, fossils preserve whole bodies, skin and all. Like when resin oozes off a tree and seals up a little bug before it can crawl away, hardening into tiny transparent geological coffins we call amber.
4:57Or when a creature dies in a desert cave and dries out, becoming a mummy. Some exceptional fossils have formed from organisms that landed in tar pits or bogs, while others have been found frozen in ice. that slows down decomposition a lot. Keeping skin, hair, even whiskers intact against all odds. And I do mean all odds.
5:15In the scheme of everything that's lived and died on our planet, fossilization is very rare. Usually, organisms rest in pieces which get broken down and recycled either by other organisms or geological processes. But two main factors increase the odds of winning the fossilization lottery. Number one, rapid burial by sediment soon after death.
5:36That makes it more likely the remains will settle in place undisturbed. And number two, burial of a tough body part. Hard bits like shells, bones, and teeth have better odds of fossilizing than soft tissue, which decomposes very easily. But it's not just extraordinary to become a fossil. It's extraordinary for a fossil to stick around long enough for us to find it.
Why Fossilization Is So Rare
5:58Erosion and weathering are constantly wiping the geological record clean like a big etch of sketch. Because Earth's surface is constantly changing, the fossil record will always be incomplete. So, the Notasaur fossil really beat the odds. First, it became a fossil. Then, it lay undisturbed for millions of years, just 250 ft below the surface.
6:18And then, it was discovered by chance in 2011 when some oil sands miners in Alberta, Canada, drilled into it by accident. As one of them put it, they were like kids on Christmas morning. I mean, I would be too if I got to dig up a dinosaur. Lucky ducks. When we are lucky enough to find fossils, we can learn a lot about how our planet and the life it supports have changed over time.
Discovery of the Notasaur
6:37Let's check out one of my geology rock stars. Roughly 400 million years ago, a fishy critter managed to leave the sea and walk on land. If it hadn't, you and I wouldn't be here right now. But which came first? Living on land or evolving legs? Paleontologist Jennifer Clack spent a lot of time thinking about that mystery when she wasn't busy riding her motorcycle.
Boris and the Move to Land
6:57By the 1980s, scientists understood how today's backboneed land animals, including humans, came to be. They evolved from the first tetropods, or creatures with four legs, which themselves evolved from the first backbone creatures that arose in ancient seas. But nobody understood exactly how the transition from living in water to breathing and walking on land happened.
7:18So, in 1987, Clack headed to Greenland with some colleagues and returned with the trove of early tetropod fossils, including a big mouthed beast she called Boris. It took years for Clack to extract Boris from the chunk of rock he was fossilized in, but what she discovered was incredible. At the back of Boris's skull, rod-like structures suggested the creature had arteries for breathing through both gills and lungs.
7:44Boris also had legs with feet shaped like paddles. They were too small to fully support the animals weight for walking, but they definitely were not fins. As Clack herself put it, Boris basically was a fish with fingers. With this fossil, Clack solved the mystery of how our fishy ancestors transitioned from the ocean to the shore.
8:03The legs and lungs came first, then came life on land. Fossils like Boris and the Notasaur are as close as we can get to witnessing the dramas of the living things that came before us. Sometimes really exceptional troves of fossils turn up like earth storage units except they hold way cooler stuff than a couch that doesn't fit in your apartment literally.
Lagerstätten and the Burgess Shale
8:24They're called logger which comes from the German word for storage and place. Loggerstaten can be full of lots of fossils or a few really wellpreserved fossils or both like in the Canadian Rocky Mountains. The Burgess shale rock formation teams with fossils from more than half a billion years ago during the Cambrian period.
8:43Some of those early critters are the relatives of today's animals, including us. Hi, great great great great great great grandpa. And others like hallucenia are so weird it took scientists over three decades just to figure out where the head was. I think I'm supposed to pet that end. Other fossils in the Burgess shale are evolutionary dead ends.
9:01Life forms that died out on mass near the end of the Cambria. We're not exactly sure why, but understanding which life survived and evolved in the aftermath of the past extinctions can help us understand the repercussions of ecological change today. Poor one out for the trilobyes, y'all. Fossils also tell us a story of Earth itself and how our life sustaining mother has changed over time.
Fossils Reveal Earth's History
9:23Like, we know that rocks in today's mountain ranges formed from ancient seafloors that were forced upward thanks to sea creature fossils found on mountain tops. And fern and reptile fossils found on opposite sides of the world helped us figure out that continents move. Fossils are like polaroid pictures of a dead thing's last day on Earth.
9:40And they're also clues to how life has changed and adapted over time in a big wild story that includes us and is still unfolding today. They invite us to imagine a time and place beyond the little slice we get to live and breathe in. So, let's hear it for the few, the elite, the fossilized. Shout out to the ant with a mushroom in its butt.
9:58Next time, grab your parker. We're talking about glaciers. See you then. Thanks for watching this episode of Crash Course Geology, which was filmed in our studio in Indianapolis, Indiana. Was made with the help of all these awesome people. If you want to help keep Crash Course free for everyone forever, you can join our community on Patreon.
10:17[bell]
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