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  • One day, about 12 million years ago in the heart of North America, herds of rhinos were

  • grazing on wide-open grasslands, while turtles swam in a nearby watering hole.

  • Early dogs hunted in the underbrush, and long-legged cranes searched the water for food.

  • I like to think that it started out as a nice day.

  • But it definitely ended pretty terribly.

  • The air quickly filled with volcanic ash, dimming the sky and falling to the ground

  • in clumps, forming drifts like snowbanks.

  • The ash coated everything, including the plants the animals  ate, causing their teeth to

  • become scratched and worn as if they’d been chewing mouthfuls of sand.

  • And they unavoidably inhaled it - the sharp, glassy edges slicing at their lungs.

  • After days or weeks of suffering, the rhinos and other animals eventually succumbed, and

  • were slowly buried in ash.

  • The watering hole where that took place is now known as the Ashfall Fossil Beds  in...

  • Nebraska.

  • Yes, Nebraska -- where there are no volcanoes within a thousand kilometers.

  • So what caused this really bad day in the Miocene?

  • Where did the ash come from?

  • The answer rests inside one of the most dynamic, transformative, and potentially dangerous

  • features in North America, a feature that’s also responsible for some of the continent’s

  • most amazing fossil deposits:

  • A volcano we now call Yellowstone.

  • The Yellowstone volcano is actually a supervolcano.

  • That means it’s more explosive than your run of the mill volcano, and the effects of

  • its eruptions are much greater.

  • One of its eruptions, 2.1 million years ago, was about 6,000 times as big as the eruption

  • of Mt St Helens in 1980.

  • Today, Yellowstone slumbers beneath the ground in Wyoming.

  • But 12 million years ago, it was in Idaho.

  • And 16 to 14 million years ago, it was on the border of Oregon, Idaho and Nevada, making

  • a brief stop in its long, slow migration across the continent.

  • The volcano itself isn’t actually moving.

  • Instead, it’s fed by a mantle plume – a massive body of magma that originates deep

  • inside the earth before upwelling toward the crust.

  • That plume holds relatively still, while the continent above moves over it.

  • Geologists refer to the volcanoes and craters made by these-plumes as hotspots.

  • As North America is slowly pushed across the Yellowstone plume by plate tectonics, it creates

  • a string of volcanic scars above where the plume once was.

  • And no matter where it’s been, every few hundred-thousand years, the supervolcano erupts,

  • churning out remarkable amounts of lava and ash.

  • But, as big and impressive and even destructive as those eruptions have been, none of them

  • have actually caused any major extinction events.

  • In fact, for paleontologists, what makes the Yellowstone plume so special is not just the

  • magnitude of its eruptionsit’s the fact that those eruptions have turned out

  • to be really, really good at preserving fossils.

  • So how does a volcano preserve fossils?

  • After all, lava plus animals equals, wellfire and burning.

  • Unless youre Chris Pratt, in which case youre somehow just fine.

  • So how can something like that be responsible for saving fossils rather than destroying

  • them?

  • Well, let’s talk about those Nebraskan animals first, because not only are they amazing,

  • it's also where I did my undergraduate summer internship back in 2006.

  • Now 12 million years ago, the Yellowstone mantle plume was under Idaho, and the volcano

  • there erupted, sending ash flying 1600 kilometers away, probably reaching as far as the Atlantic

  • Ocean.

  • The ash killed and buried the animals, preserving them at Ashfall Fossil Beds in faraway Nebraska,

  • creating the only fossil formation made entirely of falling ash.

  • And although the event sounds sudden, the process at Ashfall was painfully slow for

  • those involved.

  • When the ash started to fall, smaller animals, with their smaller lungs, were quickly overwhelmed

  • and died.

  • This is why the bottom of the Ashfall fossil beds contains a layer of small animals, like

  • turtles and birds.

  • Their remains were then trampled on and scavenged, as other larger, hardier animals continued

  • to visit the watering hole.

  • And next came a layer of medium-sized animals, including horses and small camels.

  • They wouldve survived longer than the turtles and birdsfor weeks, rather than for hours

  • or days.

  • As a result, the skeletons of  these animals show signs of bony growths, which are caused

  • by the long-term inhalation of ash.

  • An affliction known today asMarie’s Disease”.

  • Then, on top of all this, is a layer of more than 100 individuals of the rhino species called

  • Teleoceras.

  • Being bigger than all the other animals, the rhinos were able to live longer.

  • Their remains, too, show evidence of Marie’s Disease, and they may have lasted weeks or

  • even months, only to finally succumb to fever, pain, and irreversible lung damage.

  • Now, as painful as they were, the deaths of these animals were still scientifically invaluable.

  • Ashfall preserved so many articulated rhino skeletons that scientists are now able to

  • tell males from femalesmales were larger had bigger lower tusks

  • And the way the rhinos were preserved even taught us a lot about their behavior.

  • Because their skeletons were found together and mostly uncrushed, it seems that the rhinos

  • died at about the same time, as a herd.

  • That herd was made mostly of juveniles and adult femalestheir mothers, some of whom were

  • pregnant with their fetuses preserved.

  • There were only a few males in the formation, which suggests that there probably weren’t

  • many males in the herd.

  • Now, some modern rhinos form herds, but their herds are typically about a tenth the size

  • of the one discovered at Ashfall.

  • And that sort of knowledge about herd structure is almost impossible to get in the fossil

  • record, because it requires that a lot of animals die together, and very quickly.

  • Otherwise, it’s hard to say if animals died in a group, or if lots of solitary animals

  • died in the same area over time.

  • But it’s not only volcanic ash that can preserve amazing fossils.

  • 14 million years ago, lava from the Yellowstone Plume coursed through what’s now Washington

  • state, filling rivers and lake systems.

  • And while lava normally burns soft tissue - as anyone who’s playedthe floor is

  • lavawill know - this lava was cooled enough by the water that it left a particularly brilliant

  • fossil.

  • Lava spilled down into a lake and covered a dead, bloated rhino called Diceratherium,

  • which was either on the shore of the lake or was already floating in the water.

  • And remarkably, the lava cooled so quickly that the rhino became entombed in rockstill

  • floating upside down.

  • Eventually the tissue inside the rock rotted away.

  • And what’s left today is an imprint of a rhino’s body, upside-down, with fossil bone

  • fragments found inside.

  • It’s truly a one-of-a-kind specimen.

  • But though these sites are amazing, the Yellowstone plume’s greatest claim to fossilization

  • fame is a bit moreindirect.

  • In addition to preserving individual fossils, it’s also responsible for preserving the

  • formations that contain fossils.

  • For example, the John Day Formation in Oregon is famous for its fossil deposits, containing

  • animals like 30 million year old dogs and sabertoothed cat-like animals called nimravids.

  • These rich fossil formations are made of ancient dirt, or paleosols.

  • And the paleosols of the John Day formation are pretty soft and prone to erosion.

  • When theyre exposed to water, they -- as well as the fossils they contain -- are typically

  • weathered away.

  • But!

  • 15 million years ago, those gently eroding paleosols were coated in a thick layer of

  • lava from the Yellowstone Plume, which at the time was sitting under where Oregon meets

  • Idaho and Nevada.

  • And lava is much harder than the paleosols, so this new layer protected the fossil-bearing

  • rocks below -- at least, until a river slowly wore down the lava and exposed the fossils.

  • But it’s not just one formation that’s been protected by the Yellowstone plume in

  • this way.

  • The volcano’s many eruptions have sent lava flowing all over the Pacific Northwest, and

  • hot clouds of ash formed hard, glassy layers that spread even further south, into Nevada

  • and California.

  • These eruptions preserved many important and impressive fossil formations, like the Ellensburg

  • Formation in Washington and the Virgin Valley formation in Nevada.

  • And there are still plenty of parts of the continent that are covered in thick layers

  • of Yellowstone’s lava or ash.

  • So there are probably a lot of other fossil deposits that we don’t even know are there!

  • Needless to say, the Yellowstone supervolcano isn’t done yet.

  • As you might imagine, because it’s so big and potentially life-threatening, Yellowstone

  • is monitored verrrry closely by geologists.

  • I mean, ash clouds blocking out the sun and lava flows covering several states might be

  • great for fossils, but it’s not so great foranything that’s currently alive.

  • Now, of course, volcanic eruptions aren’t exactly regular.

  • But geologists have found that Yellowstone’s most major eruptions tend to occur about every

  • 600,000 to 800,000 years.

  • And its last major eruption was 640,000 years ago, while its last eruption of any size was

  • a smaller outpouring of lava only 70,000 years ago.

  • So the Yellowstone supervolcano’s next eruption might happen kind of soon -- but only in geologic

  • terms!

  • And in any case, don’t worry about what might happen the next time that thing blows.

  • Instead, think of all of the exquisite Holocene fossil deposits that will be preserved for

  • future paleontologists, with the rich variety of North American life frozen in time, just

  • like those rhinos were that fateful day 12 million years ago.

  • Hope that makes you feel better.

  • Thanks for joining me today!

  • And extra-big thanks to our two eontologists, David Reed Rasmussen andSteve.

  • Thank you so much for your support!

  • If you’d like to join them, head over to patreon.com/eons and pledge for some neat

  • n nerdy rewards.

  • Now, let me know what you want to learn about!

  • Leave me a comment, and as always, be sure to go to youtube.com/eons and subscribe!

One day, about 12 million years ago in the heart of North America, herds of rhinos were

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超级火山是如何使新生代最酷的化石形成的(How a Supervolcano Made the Cenozoic’s Coolest Fossils)

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    joey joey 發佈於 2021 年 05 月 01 日
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