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  • Imagine a world so warm that the ocean feels like a hot tub.

  • Huge volcanic eruptions have pumped the air full of globe-warming carbon dioxide.

  • And with the continents locked together from pole-to-pole in the supercontinent of Pangea,

  • the world is hot, flat, and very, very dry.

  • Early reptiles and mammal ancestors thrive in this sweltering land, dominating a landscape

  • that's still struggling to recover from the Permian extinction.

  • This is Earth, 250 million years ago, at the beginning of the Triassic Period.

  • But then, starting around 234 million years ago, the climate suddenly changed, for the

  • wetter.

  • The rains finally came to this hot, dry world.

  • And then they stayedfor two million years.

  • This period of intense rain killed off many of the early reptiles andconfused the

  • heck out of the geologists who found the flood deposits millions of years later.

  • This time is known as the Carnian Pluvial Episode, and it set the stage for a new group

  • of animals to take over the world:

  • the dinosaurs.

  • Evidence of just how hot and dry the world was at the start of the Triassic is trapped

  • in the land beneath our feet.

  • Rocks from that period are mostly swaths of red sandstones and soil deposits from dry

  • woodlands, with no sign of the coal swamps that had covered the world during much of

  • the Permian.

  • And one major reason that the world was so dry was the shape of Pangea.

  • With all the continents locked together, rain clouds couldn't move much past the coastlines,

  • and there were no big mountain ranges to break up the low, arid land.

  • Now, dinosaurs did exist in this dry, post-Permian world.

  • But they were still vying for their place among early reptiles and reptile-like mammal

  • ancestors.

  • The dominant carnivores back then were the early crurotarsans, a broad group of croc-like

  • animals that included reptiles like pseudosuchians and phytosaurs.

  • For example, there was Ornithosuchus, which had long hind legs and could actually stand

  • up when it wanted to run, which I imagine would've been both awesome and terrifying to

  • actually see.

  • But even stranger than these were the rhynchosaurs, herbivores with parrot-like beaks and, sometimes,

  • cheekbones to die for, all on a chubby lizard body.

  • And although there were no true mammals, there were dicynodonts -- the closest things to

  • them at the time.

  • They're actually more closely related to us than, say, Dimetrodon, despite being scaly,

  • four-legged creatures with bills and tusks.

  • All of these fascinating creatures were widespread for most of the Triassic, with one species

  • of dicynodontcalled Lystrosaurusbeing so common all over the world that its fossils

  • were actually used to help construct the idea of Plate Tectonics

  • But these animals, adapted as they were to life in a dry climate, were in for a big shakeup.

  • Most of what we know about the history of the climate comes from plant fossils and rock

  • types.

  • And in the early 1990's, two British geologists found rocks that didn't match the dry climate

  • of the Triassic that they knew.

  • Instead of finding red, slowly-deposited sand, they found thick layers of river rocks, sediments

  • from giant lakes, and evidence of coal swamps.

  • All of these were signs of massive rainfall, over the course of some two million years.

  • But stranger still, these traces of a suddenly wet climate turned out to be everywhere, from

  • England to the Americas to Israel, in regions that were far apart at the time.

  • That meant the rise in rainfall must have been world-wide.

  • Initially, other geologists were skeptical.

  • Couldn't these rocks just be explained by a lot of big local floods?

  • Well, over the next two decades, reports of more and more weird rocks kept trickling in,

  • and they kept pointing to a world that was getting wetter and wetter.

  • The rocks revealed coal deposits in Austria, traces of ancient lakes in Italy, wet soils

  • in Utah, and giant rivers in China.

  • And they all dated to the same window of time -- between 232 and 234 million years ago.

  • In time, this phenomenon came to be known as the Carnian Pluvial Episode, or CPE.

  • The Carnian is the name of the geologic age within the Triassic when this all happened.

  • And for what it's worth I seriously thought about naming this episode the Chronicles of Carnia

  • but I didn't because that would have been dumb.

  • Andpluvialmeans rain, and it rained a whole awful lot.

  • For example, one estimate suggests that the average annual rainfall in what's now Utah

  • almost quadrupled, reaching a peak of 1400 millimeters, or about 55 inches of rain a year.

  • For context, that's how much that a temperate rainforest gets today, like say, in the Pacific

  • Northwest.

  • And this would have happened over, and over, and over again, all around the world.

  • It was not one big flood; it was more like floods every year, all over the place, for

  • two million years.

  • And with all this rain, things were bound to change - and one of the biggest changes

  • was the sudden abundance of dinosaurs.

  • In rock dated to the start of the Carnian Pluvial Episode, dinosaurs account for about

  • 5% of the fossils of terrestrial vertebrates.

  • But by the end, they make up more than 90% of those fossils.

  • So what made the dinosaurs so suddenly successful?

  • Were they better off than their competitors in this newly wet world?

  • Or did other animals simply die, leaving them to rule the world by default?

  • The key might not have been the rain itself, but what the rain brought with it: a proliferation

  • of giant plants!

  • During this time, we begin to see lots of large conifers, and big coal-forming plants,

  • like the primordial-looking Bennettitales.

  • For herbivores, this change in food supply could have been a game changer.

  • For example, rhynchosaurs were abundant, but they were alsoshort.

  • And they couldn't stand on their hind legs to reach higher leaves.

  • This would've been fine in a dry environment, where plants tend to stay close to the ground.

  • But in a wet forest, rhynchosaurs would've only been able to eat smaller plants, or whatever

  • leaves and fruit fell to the ground.

  • Meanwhile, dicynodonts were herbivores, too, but they didn't have teeth.

  • And, they also didn't use gastroliths, the rocks that some animals - like birds - swallow

  • to help digest plant material.

  • Without teeth or gastroliths, dicynodonts would have had a hard time eating anything

  • fibrous, like wood.

  • And sure enough, fossils of dicynodont poop from this time have been found to contain

  • the digested remains of mostly soft ferns, with only very small amounts of wood.

  • By comparison, some plant-eating dinosaurs - which had both teeth and gastroliths - left

  • us poop fossils that are up to 85% wood!

  • Which is a lot of fiber.

  • So as the climate became wetter, soft small ferns were quickly replaced by tall woody

  • conifers, which the dicynodonts and rhynchosaurs didn't eat.

  • And without the rhynchosaurs and dicynodonts, then the carnivores -- those crurotarsans

  • -- would've lost a lot of their food supply.

  • So maybe, instead of being better-adapted to this new environment, dinosaurs were just

  • the only major group of reptiles left standing.

  • Or, y'know, squatting.

  • Even though we don't know exactly why the rain helped the dinosaurs, we do know that

  • dinosaurs became a lot more abundant during the Carnian Pluvial Episode.

  • And the dicynodonts, the rhynchosaurs and many of the early crurotarsans soon went extinct.

  • Now, there's still the question of: What made it rain for two million years in the

  • first place?

  • Well, right before the rains came, some 235 million years ago, a huge burst of volcanic

  • activity took place in Alaska and British Columbia.

  • Today it's known as the Wrangellian eruptions.

  • These eruptions lasted for more than 5 million years, churning out a layer of lava that got

  • to be 6 kilometers thick, and releasing enough CO2 to raise temperatures by about 3 to 10

  • degrees Celsius worldwide.

  • And, over a very long time, warmer temperatures can create a wetter climate, because they

  • can speed up the water cycle, driving more evaporation of surface water into the atmosphere,

  • among other things.

  • And in fact, because of this increase in atmospheric moisture, the CPE is also sometimes called

  • the Carnian Humid Episode.

  • But for what it's worth, my favorite name for this episode by far is The Wet Intermezzo.

  • Which I think is delightful so let's try to bring that term back, OK?

  • Anyway, after about a million years of volcanic activity, the atmosphere became so warm and

  • wet that rain could finally reach even Pangea's vast interior.

  • OK but then, how did it stop?

  • Well, when carbon levels are really high for a really long time, our planet can be pretty

  • good at getting some of the extra carbon back into the ground where it belongs.

  • Plants take it in and store it in their tissues; weathering and eroding rocks absorb it; and

  • the oceans soak it up to form carbonate rocks like limestone.

  • So as the Wrangellian eruptions slowed down, the carbon cycle was eventually able to stabilize,

  • excess CO2 was reabsorbed from the atmosphere, and the CPE gradually came to an end.

  • By the time the eruptions had completely ended 230 million years ago, the world had returned

  • to a classically hot, dry Triassic climate that only ended when Pangea began to break

  • up.

  • But even though the Carnian Pluvial Episode was short - only 2 million yearsits impacts

  • on life were permanent.

  • All that rainfall helped conifers spread and diversify, leading to the pine trees we know

  • today.

  • And while the start of the Triassic may have been the land of weird, croc-like-things running

  • on their hind legs by the time the rains had ended, the world

  • had fully entered the age of dinosaurs.

  • Thanks for joining me for this wet intermezzo!

  • Now, wouldn't you like to have all of natural history right there on your wall?

  • I would and now you can!

  • With the first-ever Eons poster, created by Franz Anthony.

  • Just go to DFTBA.com and links are in the description

  • Now, let me know what you want to learn about, because you know by now that we read your

  • comments!

  • And if you haven't already and I don't know why you wouldn't have

  • -- but if you havent, you should go to youtube.com/eons and subscribe. Thank you.

Imagine a world so warm that the ocean feels like a hot tub.

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That Time It Rained for Two Million Years

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    joey joey に公開 2021 年 05 月 01 日
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