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Fossils – How Life Turns to Stone · Talking Science

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So if you've ever walked on a beach and picked
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up one of those spiral -shaped stones, an ammonite
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fossil, you aren't just holding a cool -looking
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rock. No, not at all. You're really holding this
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biological miracle, honestly, that managed to
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survive tens of millions of years of just Earth's
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relentless recycling system. Exactly. And welcome
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to today's Deep Dive. We've got a really fascinating
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stack of paleontological research and geological
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surveys on the table today for you. It's a great
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collection of sources. It really is. And our
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mission today is to, well, to demystify how flesh
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actually turns to stone because we want to break
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down why the crowded shelves you see in natural
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history museums are actually a beautifully deceptive
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illusion. They really lie to you in a way. Right.
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And look at how scientists managed to reconstruct
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the story of deep time from an archive that is
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truthfully very patchy. And incomplete. Yeah,
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it's a profound thing to think about, really,
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when you're holding a piece of deep time. I mean,
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an ammonite swimming in an ocean, say, 66 million
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years ago, it had to beat odds that are almost
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mathematically impossible to calculate. Just
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to end up sitting on the palm of your hand today.
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Exactly. Okay, so let's unpack this. Because
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if you walk into any major natural history museum,
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you see room after massive room packed to the
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ceiling with, you know, dinosaur femurs and ancient
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shells and giant fossilized tree trunks. Oh yeah,
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the massive displays. Right, it makes fossilization
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look like this totally normal everyday occurrence.
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Like it just happens when things die. So why
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is finding a fossil considered such a mathematical
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miracle? Well, to really grasp the rarity of
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fossils, we have to look at the default setting
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of nature. And the default setting is total decomposition.
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Just rotting away. Basically, yeah. When an organism
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dies, whether it's a massive dinosaur or a fern
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or a tiny marine clam, the natural world immediately
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goes to work breaking it down to reuse the nutrients.
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Right, because nature doesn't waste anything.
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Exactly. Bacteria, fungi, insects, scavengers,
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they dismantle soft tissues in a matter of days
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or weeks. And even the harder biological hardware,
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you know, like bones or thick shells, they will
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eventually be weathered to dust by ultraviolet
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light from the sun or dissolved by acidic rain.
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Or just scattered across miles by larger scavengers,
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I imagine. Yeah, exactly. The default outcome
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for any living thing on Earth is that absolutely
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nothing recognizable remains. So the planet is
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just the ultimate ruthless recycler. It really
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is. So to become a fossil, an incredibly improbable
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sequence of events has to interrupt that default
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recycling process. Okay. The very first hurdle
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is that the organism needs to be buried almost
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immediately after death. It has to be sealed
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away before the scavengers and the... The aerobic
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bacteria, the ones that need oxygen, can finish
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their job. So it can't just lie out on the plains?
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No. Usually this means being rapidly entombed
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by mud in a flooding river delta or swallowed
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by a shifting sand dune or, you know, covered
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by a sudden heavy fall of volcanic ash. Okay,
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but just burying a bone in the backyard doesn't
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turn it into a fossil, right? There has to be
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more to it. Oh, much more. Once it's buried,
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it has to stay buried. And it has to be geologically
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protected. Because acidic groundwater percolating
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through the soil, it can easily dissolve a skeleton
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over just a few centuries. Wow. And the sediment
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it's buried in can't be, say, dug up by a river
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changing course a thousand years later. And crucially,
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it has to survive the literal churning and grinding
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of the Earth's tectonic plates over the following
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millions of years. Oh, right, because the Earth's
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crust is always moving. Yeah, subduction zones
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recycle whole chunks of the Earth's crust into
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magma, which obviously destroys any fossils inside
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them. Right, magma tends to do that. Yeah, it's
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pretty thorough. And then assuming it survives
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all of that, mineral -rich water has to slowly
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move through the surrounding rock and interact
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with the remains in a highly specific chemical
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way to preserve the structure. Okay, so becoming
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a fossil is basically like winning a geological
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lottery three times in a row. That's a great
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way to put it. You have to win the rapid burial
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lottery, the tectonic survival lottery, and then
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the perfect chemical bath lottery. Yes. And because
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so many steps in that chain usually fail, only
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a microscopic fraction of all the organisms that
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have ever lived have left any fossil trace at
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all. That is wild. And of those, an even smaller
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fraction have been fortunate enough to be pushed
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back up to the surface by erosion just in time
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to be found by a curious human being before the
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weather destroys them. Wow. This is also why...
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by soft bodied creatures like jellyfish, worms,
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slugs. They're massively underrepresented in
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the scientific record. They just don't have the
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hard shells or dense bones to survive even the
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very first step of that chaotic process. Right,
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of course. Well, if it's so incredibly rare,
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how does the actual flesh to stone transformation
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physically happen? Like if I'm holding a heavy
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stone ammonite, what was the actual recipe that
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made it? Well, there are a few different pathways,
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but one of the most common chemical recipes is
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a process called permineralization. Permineralization.
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Yeah. This typically happens with organisms that
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already have a porous internal structure. So
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think... porous bone wood or a shell okay after
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the organism is rapidly buried in sediment groundwater
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that is heavily saturated with dissolved minerals
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usually things like silica calcite or iron pyrite
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it slowly percolates through the tiny microscopic
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pores and empty cavities within that original
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bone or wood structure i'm trying to picture
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this it's essentially like um taking a dry kitchen
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sponge and letting it soak up liquid cement.
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The liquid gets into all the tiny holes, hardens,
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and now you have a sponge made out of concrete.
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That captures the mechanics of it perfectly.
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Over enormously long spans of time, the dissolved
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minerals in that groundwater precipitate out
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of the water. Right. They shift from a liquid
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solution into a solid mineral, filling in all
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those microscopic internal spaces. It turns the
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once organic structure into solid stone from
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the inside out. From the inside out. Yeah, and
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this process can be so incredibly delicate that
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if you put a piece of permineralized petrified
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wood under a microscope, you can still see the
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original individual cell walls of the tree, even
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though the wood itself is now solid rock. That
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is just wild. But you mentioned a few different
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pathways. What if the bone or the shell doesn't
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soak up the minerals? Then we might get a mold
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and a cast. a mold forms when a buried organism's
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hard parts completely dissolve away after burial
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oh they just vanish yeah say an acidic shift
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in the groundwater washes the actual shell away
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entirely But, crucially, the surrounding mud
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or sediment has already hardened enough to retain
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a perfect hollow cavity in the exact shape of
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the organism. Oh, I see. So it's essentially
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a rock with a dead animal -shaped hole trapped
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inside it. Right. It captures the external form
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of the creature in negative space, which is the
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mold. Now, if that hollow mold later fills up
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with a new influx of sediment or different minerals
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that harden, that new material takes on the original
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organism's shape as a positive 3D replica. Okay,
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got it. We call that repli - Right. So when it
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dries, you get a perfect physical replica of
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your hand, but there's no actual biological material
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left inside it. Exactly. Many of the beautiful,
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perfect shell fossils people buy in rock shops,
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they're actually casts. They're just stone replicas
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of a shape. That makes a lot of sense. Now, there's
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one more pathway mentioned in our sources. Carbonization
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or compression. Yes, carbonization. This one
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affects leaves and delicate ferns, they said.
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But wait, if you bury a delicate fern under tons
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of accumulating sediment in a swamp and put it
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under immense geological pressure, wouldn't you
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just annihilate the plant completely? Like how
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does squeezing it leave a fossil? You would definitely
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think it destroys it. But all organic life on
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Earth is carbon based. When soft plant tissue
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is buried under heavy sediment, particularly
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in low oxygen environments like, say, stagnant
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ancient swamps, it gets compressed completely
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flat. That immense geological pressure combined
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with rising heat causes the more volatile chemical
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components of the plant tissue, specifically
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the hydrogen, oxygen, and nitrogen, to literally
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get squeezed out and escape as gases. So the
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volatile gases just leave and the only thing
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left behind is the carbon. Exactly. It leaves
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behind a remarkably thin, dark, two -dimensional
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film of pure carbon on the surface of the rock.
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This film traces the original outline of the
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leaf in just exquisite detail. Wow. Yeah, you
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can often see the... pattern of the leaf's veins,
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perfectly preserved as this dark carbon shadow.
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And to scale that up, this exact same chemical
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compression process, when applied to massive,
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thick accumulations of swamp matter over millions
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of years, that is the recipe for coal. Wait,
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really? Coal's just mashed up carbonized fossils?
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Basically, yes. A vast coal seam is essentially
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a giant, crushed, large -scale record of carbonized
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prehistoric plants. That really reframes how
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you look at a lump of charcoal. Okay, we have
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bones turning to stone from the inside, shells
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dissolving to leave plaster -like replicas, and
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ferns being pressed into carbon shadows. But
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what about behavior? Because the text mentioned
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this. Can you fossilize an action? You actually
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can. Through something called trace fossils or
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ichnofossils. Ichnofossils. Right. These don't
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record the physical body of the animal. They
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record physical evidence of the animal interacting
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with its environment. This includes footprints
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preserved in ancient mud that later baked in
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the sun. It includes burrows dug by prehistoric
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worms, tooth marks scraped across the bone of
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a scavenged carcass, and fossilized droppings,
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which scientists call coprolites. Coprolites.
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I love that we came up with that. highly dignified
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academic word for fossilized dinosaur poop. It
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is very dignified. It is. But practically speaking,
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why do we care about a footprint? Like, if I
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want to study a T -Rex, wouldn't I rather just
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dig up an actual T -Rex thigh bone? Well, bones
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only tell us about the biological hardware. They
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show us the shape of the joints and the size
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of the muscles, but they are static. Think of
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a body fossil as a photograph of a suspect in
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a crime, whereas a trace fossil is the actual
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security footage of them committing the act.
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Oh, I love that analogy. Yeah, trace fossils
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show us the animal in motion. Ah, I see. So a
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bone tells you how long the leg was, but a footprint
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tells you how they actually used it. Exactly.
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No amount of staring at an isolated thigh bone
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will tell you definitively how fast an animal
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was running on a Tuesday 70 million years ago.
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But if you have a trackway of footprints, you
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can measure the spacing between the steps, compare
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it to the estimated hip height of the dinosaur,
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apply biomechanical physics, and calculate their
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actual walking or running speed. Wow. Just from
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the footprints. Yeah. And furthermore, if you
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find a trackway showing a dozen individuals of
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the same herbivore species moving in the exact
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same direction, stepping around the same obstacles,
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you suddenly have physical evidence for herd
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dynamics. You can't extract herd behavior from
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a scattered pile of bones. That is so cool. Okay,
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here's where it gets really interesting, though.
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Security footage is great, but earlier you said
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soft -bodied creatures like worms and jellyfish
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are massively underrepresented because they don't
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have hard parts to survive the lottery. Right,
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they usually rot away. Do we ever get lucky enough
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to find a place where the soft stuff actually
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made it through? We do, but they are the holy
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grails of paleontology. These sites are called
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Lagerstätten. Lagerstätten. Yeah, it's a German
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term translating to storage place or resting
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place. In geology, it refers to fossil sites
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featuring unusually complete preservation. We're
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talking about soft tissues, delicate internal
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organs, or entire vibrant ecosystems captured
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in mind -blowing detail. What creates a lagerstedt
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if normal fossilization requires a three -part
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lottery win? What does this require? It requires
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a perfect storm of environmental extremes. You
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need instantaneous burial, often coupled with
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an environment that has almost zero oxygen, anoxic
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conditions. Okay, because without oxygen... Without
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oxygen, the bacteria that normally rot soft tissue
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simply can't survive, so decomposition halts
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completely. You also need incredibly fine -grained
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sediment, like microscopic mud or volcanic ash,
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that can mold itself to the tiniest microscopic
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cellular detail. without crushing them it sounds
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like a perfectly sealed time capsule capturing
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a bustling ancient city right in the middle of
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a busy afternoon instead of just finding a stray
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brick from a ruined wall a million years later.
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That's a really good way to think about it. And
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a great example of one of these time capsules
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is the Burgess Shale in the Canadian Rocky Mountains.
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Oh, I've heard of that one. Yeah. This site preserves
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a snapshot of an underwater ecosystem from roughly
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half a billion years ago. It preserves soft -bodied,
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bizarre marine animals that had no shells, no
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bones, nothing hard at all. Half a billion years
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ago. Yes. Because they were buried in incredibly
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fine ox... oxygen -free mudslides. Scientists
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today can put these fossils under a microscope
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and study their delicate gills, their fragile
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digestive tracts, and even their complex eyes.
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Under any normal circumstance, those tissues
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would have rotted away in three days. Half a
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billion years ago, and we can still see their
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gills. That's stunning. It's incredible. Another
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spectacular lager state is located in the Liaoning
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province in China. The fossil beds there were
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formed by repeated volcanic eruptions. Like a
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Pompeii situation? Sort of, yeah. The fine -grained
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volcanic ash rained down into ancient lakes,
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burying dinosaurs almost instantly and sealing
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out the oxygen. Because ash is so fine, it preserved
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things mud couldn't. It's from leoning that we
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have spectacularly preserved dinosaurs that retain
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clear, undeniable physical impressions of feathers.
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Wow, feathers. Yeah, it provided the irrefutable
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evidence linking ancient theropod dinosaurs to
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the birds we see today. Okay, so let's talk about
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the timeline because this trips a lot of people
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up. We keep throwing around massive numbers.
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66 million years ago for the Emanite, 500 million
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for the Burgess Shale. How do scientists read
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the clocks of deep time? Like, how do we know
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a rock is exactly that old? Right. It's a huge
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question. Scientists use two complementary approaches.
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The first is relative dating, which establishes
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a sequence of events. It relies on the law of
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superposition. Superposition. Yeah. In an undisturbed
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sequence of sedimentary rock, the layers at the
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bottom were laid down first, meaning they are
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older than the layers deposited on top of them.
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Makes total sense. The deeper you dig, the further
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back in time you go. Exactly. They also use what
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are called index fossils to match these layers
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globally. Index fossils are specific species,
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like certain types of trilobites, that spread
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widely across the entire globe, but only existed
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for a very narrow, universally known window of
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time before going extinct. Okay, so they're like
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a highly specific marker. Yes. If you find a
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specific index fossil in a rock layer in North
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America and the exact same species in a layer
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in Europe, You know, those two completely different
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rock layers were formed at the exact same time.
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So relative dating is sort of like looking at
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a pile of mail on a desk. The letters at the
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bottom of the pile arrive first. The ones on
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top arrive last. Gives you an order of events.
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But how do we get the actual date? How do we
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read the postmark? For the precise postmark,
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we use absolute dating, primarily radiometric
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dating. This involves measuring radioactive isotopes.
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An isotope is just an unstable form of a chemical
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element that slowly decays into a different stable
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element at a highly consistent universally known
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rate, it's half -life. Right. So by measuring
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the ratio of the remaining unstable element to
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the newly formed stable element inside a rock,
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scientists can calculate exactly how much time
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has passed since that rock first formed. I have
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to jump in here because whenever radiometric
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dating comes up, the first thing people think
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of is carbon -14 dating. Do scientists use carbon
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-14 to date dinosaur bones? No. And this is a
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really widespread point of confusion. Yeah, I
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hear it all the time. Carbon -14 dating relies
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on a radioactive isotope of carbon that living
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things absorb from the atmosphere. But carbon
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-14 decays relatively quickly on a geological
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scale. It has a half -life of roughly 5 ,700
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years. Ooh, that's not long at all in geology
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time. Not at all. So by the time you get to about
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50 ,000 years, there is practically no detectable
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carbon -14 left to measure. It is fantastic for
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dating human archaeology, like ancient Egyptian
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artifacts or Neanderthal campfires, but it is
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mathematically useless for dating a T -Rex from
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66 million years ago. So for the dinosaurs, you
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need a clock that ticks much slower. Exactly.
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For ancient rocks, geologists use isotopes like
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potassium -40 or uranium -238, which have half
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-lives of hundreds of millions or even billions
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of years. Okay, but wait. Earlier we talked about
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permineralization. We said the original bone
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is replaced by new minerals from the groundwater
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over millions of years. Yeah. So if you try to
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radiometrically date the fossil itself, aren't
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you just dating the newer groundwater minerals,
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not the time the animal died? Yes. That is an
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incredibly important point, and it is the exact
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reason scientists usually do not date the fossils
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themselves. Sedimentary rock, the mud and sand
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that hold the fossils, is made of recycled particles
17:22
of older rocks, so dating it gives you a mess
17:24
of different ages. Oh, wow. So what do they do?
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Instead, scientists look for... layers of volcanic
17:30
ash or igneous rock located directly above and
17:33
below the fossil -bearing rock. Volcanic ash
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contains crystals that act as perfect radiometric
17:38
clocks starting the moment the magma cools and
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solidifies. Ah, I get it. So going back to the
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pile of mail, if you want to know when a specific
17:47
undated letter arrived, you don't test the letter
17:50
itself. You look at the dated grocery receipt
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sitting immediately above and below it in the
17:54
pile. If the receipt below is from Tuesday and
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the receipt above is from Thursday, you know
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the letter had to arrive on Wednesday. You bracket
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the time frame. That is a perfect analogy. Yes,
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by combining the relative position in the rock
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layers with the absolute dates of those volcanic
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layers, scientists build a genuinely reliable
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numerical timeline of Earth's history. So knowing
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how we construct this timeline brings us to how
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we deal with the gaps in it. We've established
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fossilization is a wildly rare lottery. The archive
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is patchy. And because the timeline has missing
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links, some critics argue those gaps undermine
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the concept of evolutionary transition. How does
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science navigate the missing information? It
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really comes down to understanding the inherent
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bias of the archive we're working with. The bias?
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Yeah. The fossil record heavily favors organisms
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with hard shells and skeletons, and it heavily
18:43
favors environments with lots of water and sediment.
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like river deltas or shallow seas. It is practically
18:49
useless at preserving life from dense upland
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rainforests or steep mountain slopes where erosion
18:55
destroys everything before it can be buried.
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That makes sense. So expecting a perfectly continuous
19:00
frame -by -frame record of every species that
19:03
ever lived completely misunderstands the physics
19:06
of fossilization. The gaps aren't a flaw in the
19:09
theory. They are a fundamental feature of geology.
19:12
Right. It's like watching a flipbook animation
19:14
where a third of the pages have been torn out
19:17
by water damage. You might be missing a couple
19:19
of transition frames between a jump, but you
19:21
can still clearly see the character moving from
19:23
point A to point B. Exactly. And more importantly,
19:26
the theory of evolution allows scientists to
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predict what kind of transitional fossils should
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exist in those missing pages and exactly what
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time period they should be found in. And has
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that worked? Oh, absolutely. One of the greatest
19:39
validations of this was the discovery of Tiktaalik
19:42
in the Canadian Arctic in 2006. What was the
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prediction there? Well, paleontologists knew
19:47
there had to be an evolutionary transition between
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ancient lobe -finned fish and the very first
19:52
four -limbed animals to walk on land. They used
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the dating methods we just discussed to identify
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sedimentary rocks of the exact right age, about
20:01
375 million years old, that were also formed
20:04
in the right environment, an ancient shallow
20:06
stream system. Okay. They mounted an expedition
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to those specific rocks, and they found Tiktaalik.
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It is technically a fish with gills and scales,
20:15
but it has distinctly limb -like bone structures
20:18
inside its fins, including a wrist. And it has
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a mobile neck that fish don't have. It was the
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perfect intermediate form, discovered exactly
20:26
where the timeline predicted it would be. They
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predicted the anatomy, the age, and the location
20:30
before they even swung a pickaxe. Yes. Another
20:33
classic example is Archaeopteryx from Germany,
20:36
which perfectly captures the transition between
20:38
reptiles and birds. Oh, I've seen pictures of
20:40
that one. Right. It has reptilian hardware teeth,
20:43
a flat sternum, and a long, bony tail. But it
20:47
is covered in fully developed aerodynamic feathers.
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Whenever the fossil record is actually well preserved
20:53
enough to test a prediction, it consistently
20:55
delivers the transitional forms we expect. So
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if gaps are the normal state of affairs, what
21:00
happens when you actually hit the jackpot and
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find a place where the fossil record didn't get
21:05
severely interrupted? Because the source material
21:08
highlights two globally significant sites for
21:11
this, both in Australia. Oh, Australia is a paleontological
21:14
gold mine. The first site is Riversley in remote
21:17
northwestern Queensland. Riversley preserves
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a remarkably rich record of mammal life stretching
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from about 25 million to 2 million years ago.
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What makes it a jackpot compared to normal sites?
21:27
It's the sheer continuity. The fossils are preserved
21:30
in a series of ancient freshwater limestone pools
21:32
and caves. It's been so productive that researchers
21:35
believe the deposits capture thousands of generations
21:37
of animals falling into these same cage systems
21:40
over enormous unbroken spans of time. Just generations
21:44
of animals falling in over and over. Because
21:47
it spans 23 million years continuously, it gives
21:50
us a high definition window into how Australian
21:53
fauna adapted as the continent literally drifted
21:55
north and its climate slowly dried out. Shifting
21:58
from lush, wet rainforests to the harsh, arid
22:02
outback we see today. Exactly. You can trace
22:05
the gradual evolution of modern kangaroos right
22:07
alongside terrifying extinct oddities like giant
22:10
flightless thunderbirds and the marsupial lion
22:13
phyllocolio. So Riversley gives us this beautiful
22:16
continuous story of relatively recent mammalian
22:19
evolution. But the other site in Australia goes
22:21
way deeper into time, right? Much deeper. If
22:23
you head south to the Flinders Ranges in South
22:25
Australia, you find the Etiokura Hills. The fossils
22:28
discovered here were so paradigm shifting that
22:31
they gave their name to an entire official chapter.
22:33
of geological time, the Ediacaran period, spanning
22:36
from about 635 to 541 million years ago. 541
22:41
million years ago. That's before the Cambrian
22:43
explosion. That is before animals evolved hard
22:46
shells, teeth, or skeletons. How on earth do
22:48
you get fossils of completely soft -bodied organisms
22:51
if they don't have hard parts to survive the
22:53
geological lottery? It comes back to the lack
22:55
of oxygen, but in a totally different way. In
22:58
the Ediacaran period, complex burrowing animals
23:01
and scavengers hadn't evolved yet. There was
23:03
nothing swimming or crawling around to eat the
23:05
carcasses. Oh, wow. A predator -free ocean. Yeah.
23:09
And the sea floor was covered in tough, leathery
23:13
mats of microbes. When these early soft -bodied
23:16
organisms died, they essentially sank into the
23:18
mud, got covered by sediment, and the microbial
23:21
mats helped seal them in like a protective skin,
23:24
allowing their shapes to be cast in sandstone.
23:26
What do these pre -Cambrian creatures actually
23:28
look like? They are profoundly bizarre. They
23:31
look like quilted fronds, ribbed air mattresses,
23:34
or flat segmented discs. So they have no idea.
23:36
Unidentifiable heads, no mouths, no digestive
23:39
tracts, and no legs that we can recognize. They
23:41
sound like Earth's weird alien rough drafts for
23:44
what complex life could be before biology locked
23:46
in the winning formula of having a head and a
23:48
tail. That's a great description. There is massive
23:52
ongoing scientific debate about where the Ediacomena
23:55
biota belong on the tree of life. Are they the
23:59
earliest ancestors of modern animals like jellyfish
24:02
and worms? Or do they represent a totally distinct,
24:05
failed evolutionary experiment, a separate kingdom
24:09
of life that went entirely extinct and left zero
24:12
descendants? That's fascinating. We just don't
24:14
fully know yet. And this highlights a crucial
24:17
aspect of paleontology. Honest science operates
24:20
through unresolved uncertainty. Finding fossils
24:22
that completely defy our current understanding
24:24
is what drives the field forward. I love that.
24:27
It's OK for science to look at a 600 million
24:29
year old rock and say, we are still figuring
24:31
this out. Absolutely. Well, we've traveled from
24:34
ancient Chinese ash beds to the shifting climates
24:37
of the Australian outback. But I think we need
24:39
to bring this back to the present moment because
24:40
fossilization isn't just a historical event that
24:43
stopped in the Ice Age. Not at all. It is an
24:46
ongoing, active planetary process. Right now,
24:50
at this exact moment, In a low oxygen swamp in
24:54
Louisiana or a muddy river delta in Bangladesh,
24:57
the right combination of rapid burial and protective
25:00
chemistry is coming together. So the lottery
25:02
is still being played. Exactly. The exact same
25:05
improbable multimillion year journey from living
25:09
tissue to stone is beginning today for some lucky
25:12
ferns, fish and maybe even a stray smartphone
25:15
dropped in the mud. So think back to that ammonite
25:18
we pictured picking up on the beach at the start
25:20
of our deep dive. That coiled stone used to be
25:23
a living, breathing creature swimming in a sea
25:25
that no longer exists, on a planet whose continents
25:28
have literally ripped apart and crashed back
25:30
together since it died. It required rapid burial,
25:33
protection from tectonic destruction, the perfect
25:35
chemical permineralization bath, and the sheer
25:38
luck of a human walking by just as erosion brought
25:41
it back to the sunlight. Every fossil is a message
25:43
that almost didn't survive the trip. It is a
25:46
genuine physical record of life doing what it
25:48
has always done, living, dying, and against all
25:51
odds, being remembered in stone. Beautifully
25:53
said. Which leaves me with one final kind of
25:55
mind -bending question for you to ponder as you
25:58
go about your week. If fossilization is happening
26:01
right now, actively capturing the present day,
26:04
oh, excuse me, catching the present day, what
26:07
perfectly ordinary everyday animals or what modern
26:10
human artifacts are currently winning the geological
26:13
lottery? What will become the puzzling index
26:15
fossils for whatever intelligent life is digging
26:18
through the Earth's sedimentary layers 240 million
26:20
years in the future? Thank you for joining us
26:23
on this deep dive into deep time for you today.
26:25
Keep wondering, keep exploring, and we'll see
26:27
you next time.