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  • Thanks to CuriosityStream for supporting PBS Digital Studios.

  • It may not surprise you to learn that some kinds of fossils are more common than others.

  • Most dinosaurs, for instance, are known from only one or two specimens.

  • Meanwhile, some trilobite species are known from hundreds or thousands of specimens.

  • But one of the most abundant kinds of fossils on Earth, numbering in the millions of specimens,

  • came from something most people have never heard of.

  • And for decades, their identity was a mystery to paleontologists.

  • But geologists figured out that these mysterious fossils could basically be used to tell time

  • in the deep past!

  • Please allow me to introduce you to the conodonts.

  • From the time they were first discovered in the 19th century, and right up until the 1980s,

  • conodonts were only known from isolated hard parts, called conodont elements.

  • Some of them looked like little fans, or saw blades, or even barbed wire, but most of them

  • looked like cone-shaped teeth.

  • So they were given the name conodont, which meanscone tooth.”

  • These elements are all verrry tiny.

  • Most are under one millimeter, and none greater than five millimeters long.

  • And for a long time, paleontologists were totally stumped about what animals these things

  • belonged to, and where on the Tree of Life they belonged.

  • Some experts thought that, although they look kind of like teeth, their similarities to

  • teeth are pretty superficial.

  • But others pointed out that the fossils were made from a mineral called hydroxylapatite.

  • This is the same mineral that bones and teeth are made from in vertebrates.

  • So, this led many scientists to think that they were teeth, maybe from some kind of weird,

  • extinct fish.

  • But while paleontologists were spending a hundred years or so debating what conodonts

  • were, geologists were discovering what conodonts could do.

  • When a species is really abundant, widespread, distinctive, and found in a restricted span

  • of time, their remains can be used as Index Fossils.

  • These are fossils that help geologists correlate and date rock layers all over the world.

  • And conodonts were all of those things -- abundant, widespread, and composed of lots of distinctive

  • species.

  • So they turned out to be some of the most important index fossils from the whole Paleozoic

  • Era.

  • In fact, many subdivisions of the Paleozoic are actually defined by when certain conodont

  • species first or last appear.

  • For instance, you know when the Devonian Period ends and the Carboniferous Period begins,

  • because that's when you start finding fossils that belong to a species of conodont known

  • as Siphonodella sulcata.

  • Likewise, in the Triassic Period, nearly every age is defined by the first appearance of

  • various conodonts.

  • The extinction of Metapolygnathus parvus marks the end of the Carnian, for example, while

  • the start of the Rhaetian is defined by the first appearance of the genus Misikella.

  • So, geologists are able to use these fossils to basically tell time!

  • But conodont elements can do more than just that!

  • They're also, essentially, geological thermometers!

  • It turns out that conodont elements actually change color when they're heated up.

  • And no matter what species they belong to, they go through the same range of color changes

  • at the same temperatures.

  • Geologists have used these fossils to devise a six-point scale of how they change from

  • their natural tan color, to brown, then gray, then black, and finally white, as they get

  • hotter and hotter.

  • With this scale, called the Conodont Alteration Index, geologists can use the color of a conodont

  • that they find, to figure out how hot the surrounding rock once was.

  • And this can be really important in fields like petroleum geology, because it can reveal

  • whether sediments ever got hot enough for organic hydrocarbons to be converted into

  • oil and gas.

  • So conodont elements quickly became one of the handiest tools in the geologist toolkit.

  • But meanwhile, paleontologists still had no idea what kind of animals conodonts were.

  • In the 1970s, fossils of some vaguely fish-like creatures were found in Montana that were

  • thought to be conodont animals.

  • But further study revealed that the conodont elements found in those fossils were actually

  • in the animals' guts.

  • So it turned out that they weren't conodont animals; they were fossils of creatures that

  • ate conodont animals!

  • The mystery lingered until 1983, when fossils were discovered in 350 million year old sediments

  • from Scotland that brilliantly preserved the soft-bodied animals.

  • Andthey were kinda weird, with long eel-like bodies, tail fins, a stiff rod of tissue down

  • their backs, and giant bulging eyes.

  • Thanks to these beautiful specimens, we were able to learn that conodont animals were a

  • kind of fish after all!

  • The fossils revealed distinctive, zig-zag-shaped muscles, known as myomeres, which are still

  • present in fish today.

  • And the rod down its back was a notochord, an early precursor of the vertebral column

  • seen in vertebrates.

  • Both those features--myomeres and notochords--are found only in chordates.

  • And the fossils also revealed that the tiny, tooth-like conodont elements were concentrated

  • in the animal's mouth -- but also in its throat!

  • The elements were arranged in a complicated array of blades and points, like some kind

  • of horror movie monster.

  • Some experts think these spines and barbs may have been helpful in gripping and slicing

  • tiny prey.

  • But others suggest that they were used to filter plankton from the water.

  • Either way, the picture of these creatures was finally coming into focus!

  • Like modern lampreys and hagfish, conodonts were jawless fish, and they were one of the

  • earliest and most successful groups of vertebrates.

  • And they thrived all over the world throughout the Paleozoic Era, with many species schooling

  • in the open ocean like modern sardines, while others stayed closer to shore.

  • Throughout their history, conodonts were affected by several mass extinctions, including the

  • Great Dying at the end of the Permian Period 252 million years ago.

  • But it wasn't until the Triassic-Jurassic Extinction about 200 million years ago that

  • they were finally wiped out.

  • A quick rise in carbon dioxide caused the climate to warm up, while the acidity, salinity,

  • and oxygen levels in the oceans all began to change.

  • Their final extinction brought an end to 300 million years of conodonts' role as a cornerstone

  • of the world's ocean communities.

  • Sad, I know.

  • But their remains ended up being some of the most important fossils of the entire Paleozoic

  • Era.

  • They've helped geologists find oil and tell deep time, while allowing paleontologists

  • to understand a whole new type of animal life.

  • Thanks to their success and incredible abundance, they're among the most useful fossils in

  • the world.

  • Thanks to CuriosityStream for supporting PBS Digital Studios.

  • With CuriosityStream you can stream documentary films, and programs about science, nature,

  • and history, including exclusive originals!

  • For example, you could check out Planet Dinosaur, a three-part BBC series about the latest discoveries

  • from the days of the non-avian dinos.

  • CuriosityStream offers unlimited streaming, and for you Eons viewers -- because I like

  • your face! -- the first two months are free if you sign up at curiositystream.com/eons

  • and use the promo code EONS.

  • Thanks for joining me!

  • I'm always interested in what you want to learn about!

  • So leave me a comment below with your questions about ancient life!

  • And be sure to go to youtube.com/eons and subscribe.

  • Now I'm sure finding out how precise these tiny teeth fossils are was a bit shocking,

  • but what shouldn't be surprising is the preciseness of math.

  • Go learn the language of the universe with our sister channel Infinite Series and find

  • out what numbers are made of and if there's a way to divide by zero.

Thanks to CuriosityStream for supporting PBS Digital Studios.

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The Most Useful Fossils in the World

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    joey joey に公開 2021 年 05 月 01 日
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