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  • Way back, about 310 million years ago, a lizard-like creature took shelter in a hollow stump in

  • what's now Nova Scotia.

  • It was small, not much bigger than your hand, and it hunted for insects in an ecosystem

  • that was way different from anything you'll find in Canada today.

  • It lived in a warm, swampy forest, full of giant ferns and tree-like plants that reproduced

  • using spores, not seeds.

  • But this lush environment ultimately caused the death of that little creature; its fossil

  • remains were found preserved in that stump, buried by sediment from a river that overflowed

  • its banks.

  • We call that little creature Hylonomus, and it's the world's earliest known reptile.

  • Now, this also makes it one of the earliest amniotes, animals whose eggs contain a special

  • membrane, called an amnion, that allows them to survive on dry land.

  • Today, amniotes include all reptiles, birds, and mammals.

  • And Hylonomus was not alone.

  • It shared its tropical forests and humid swamps with amphibians like Cochleosaurus, a giant,

  • semiaquatic ambush predator - like, the amphibian version of today's crocodiles.

  • But then, about 305 million years ago, something happened.

  • The climate had been shifting for several million years, from steamy and tropical to

  • drier and more seasonal.

  • And that shift took a toll on the swamps and rainforests, which slowly began to disappear.

  • By around 299 million years ago, most of those humid forests were gone.

  • And along with them disappeared a host of early amphibians, like Cochleosaurus, that

  • couldn't cope with the newly dry conditions.

  • This extinction event is sometimes called the Carboniferous Rainforest Collapse, and

  • it set the stage for a takeover that would be a crucial turning point in the history

  • of terrestrial animal life.

  • Because, the collapse of the rainforests was followed by the spread of cooler, drier landscapes

  • that were less hospitable to the big, dominant amphibians.

  • And yet, one group of tetrapods was poised to take advantage of that new terrain: the

  • first amniotes -- animals that, like Hylonomus, could lay their eggs on land.

  • And these creatures would turn out to be our very early ancestors.

  • So, the disappearance of the rainforests was just as much an environmental catastrophe

  • then as it would be today.

  • But if it weren't for that time when the rainforests collapsed - in an extinction event

  • that you probably haven't heard of - our ancestors might never have made it out of

  • the swamps.

  • The Carboniferous Rainforest Collapse is named after that window of geologic time when it

  • took place: the Carboniferous Period.

  • Carboniferous literally meanscoal-bearing.”

  • And we call it that because, over millions of years, all of that vegetation in those

  • dense, humid forests turned into peat, and then coal.

  • The Carboniferous was given its name in 1822 by two dapper English geologists.

  • But it's only been within the last 20 years or so that researchers have started to take

  • a closer look at what happened at its end.

  • So we're still piecing together the story of the Rainforest Collapse.

  • But it looks like changes in climate played a big part.

  • We can see evidence of this in places that are rich in coal today, like the Appalachian

  • and Illinois Basins of North America.

  • Because, coal is actually full of the spores that those ancient plants used to reproduce.

  • So scientists can study those spores in the coal to figure out what kinds of plants lived

  • in a certain place at a certain time.

  • And, tracking changes in the types of spores over time reveals that the fossil record of

  • plants seems to shift back and forth between wetter and drier-adapted species throughout

  • the latter half of the Carboniferous, between about 323 million and 299 million years ago.

  • And the coal beds themselves start to become thinner and thinner, which further suggests

  • that the climate was getting drier.

  • Shorter cycles of hot wetlands meant there was less swamp vegetation to decay into peat

  • and then coal.

  • Now, some experts think that a short but intense glacial phase caused the collapse, based on

  • records of sea level change and changes in the types of fossil soils we see at the time.

  • Plus, a third factor was also at work: large-scale volcanism.

  • One group of paleontologists has suggested that the eruption of two big areas of volcanic

  • activity - one in what's now northwest Europe and one in Mongolia - may be linked to the

  • rainforest collapse.

  • Regardless of what climate changes exactly caused the rainforest collapse, its effects

  • were significant for both plants and animals.

  • But it's clear from the fossil record that this was an especially tough time to be a

  • plant.

  • Because, as the rainforests dwindled, they also went through lots of changes.

  • The dominant trees of the Carboniferous were the lycopsids, also known as scale-trees,

  • tall, tree-like plants that actually aren't closely related to modern trees.

  • And as the collapse began, they died off pretty quickly and abruptly.

  • A few lycopsids are still around today, like the clubmosses, but their heyday is long over.

  • Then, the lycopsids were mostly replaced by another kind of wetland plant, the tree-ferns

  • - which are also not trees.

  • They're just ferns that grow trunks, like trees do, and they're still around today.

  • This switch from lycopsid forests to tree-fern forests is important, because they're different

  • kinds of habitats.

  • Lycopsid forests had more open canopies, letting the sun shine in on the flooded forest floor,

  • while tree-fern forests were darker, with more closed canopies.

  • And, speaking as someone who moved from New Mexico to Montana in the winter, I can tell

  • you that most organisms don't cope well with abrupt changes in their habitats.

  • Anyway, after undergoing changes like these for millions of years, by around 300 million

  • years ago, the equatorial coal forests were definitely on their way out.

  • This is one of only two mass extinctions of plants known from the fossil record.

  • And we also know that around nine families of amphibians and amphibian-like tetrapods

  • went extinct during this time, like the baphetids, which were big, fish-eating aquatic predators.

  • What happened next depends on who you ask.

  • But either way, the results turn out to be the same: the rise of the amniotes.

  • One group of experts has suggested that the collapse left behind isolated patches of rainforest

  • where separate populations of tetrapods -- including amniotes -- diversified into new species.

  • The scientists tested this by studying differences in tetrapod diversity from the early Carboniferous

  • all the way through the middle of the following period, the Permian.

  • What they found was that, worldwide, tetrapods remained diverse, but their communities shrank;

  • so, there were more different kinds of tetrapods, but fewer of them.

  • According to this model, the collapse caused the extinction of a lot of the dominant amphibian

  • groups.

  • But it also created new opportunities in those patches of rainforest for the amniotes to

  • diversify and thrive.

  • And it was those early amniotes that moved into new dietary niches, becoming carnivores

  • and also the first large herbivores, like the sail-backed Edaphosaurus.

  • Now, a second hypothesis argues that the collapse of the rainforest actually made communities

  • of tetrapods more connected with each other, which ended up helping amniotes in a different

  • way.

  • This model focuses on how closely-related different tetrapod species were, in relation

  • to where they lived.

  • And it finds that species that were far-apart were still more closely related than expected,

  • suggesting that there must've been movement between communities.

  • So, instead of tetrapods being trapped in patches of rainforest surrounded by newly

  • dry land, there might've been a more gradual transition from wet to dry.

  • And this would've created larger, more connected habitats that the amniotes could've taken

  • advantage of, equipped with new features that allowed them to live completely terrestrial

  • lives.

  • Either way, both hypotheses agree that the diversification of the amniotes was a major

  • outcome of the collapse.

  • Because, the amphibians that dominated the Carboniferous were tied to the swampy environments

  • of the coal forests, but the amniotes weren't.

  • And this is probably because of two unique traits that set the amniotes apart from the

  • amphibians.

  • They didn't have to lay their eggs in water; they could do it on dry land.

  • And they had scales that helped them retain moisture in more variable climates.

  • These adaptations made it possible for them to expand into all that new, dry habitat,

  • and evolve into the weird forms we see in the Permian Period.

  • These were things like the sail-backed Dimetrodon and the barrel-bodied, tiny-headed herbivore

  • Cotylorhynchus.

  • And they included the amniotes that would eventually give rise to us, some 300 million

  • years later.

  • Like, I haven't met you, but I can tell you that, even though you don't lay eggs,

  • you my friend are an amniote.

  • If not for the collapse of the rainforests where little Hylonomus once lived, way back

  • in the Carboniferous, amniotes might not have had the chance to take over from the giant

  • amphibians.

  • It was an ecological catastrophe in its time, but this little known extinction event shaped

  • the world we know today -- including by making us possible.

  • Thanks for watching PBS Eons, which is produced by Complexly.

  • If you'd want to keep imagining the world complexly with us, check out Animal Wonders

  • hosted by Jessi Knudsen Castañeda.

  • Animal Wonders is an animal rescue and education facility that cares for close to 100 exotic

  • animals and non-releasable wildlife.

  • Every week on the Animal Wonders YouTube channel, Jessi features different animals and shares

  • what it's like to keep them happy and healthy.

  • Recently, Jessi and the Animal Wonders team took in Tigli the arctic fox.

  • If you'd like to learn all about Tigli's story and find out how he's getting along

  • with the other foxes at Animal Wonders, there is a link in the description to a video all

  • about that.

  • Big scaly high fives to this month's Eontologists: Patrick Seifert, Jake Hart, Jon Davison Ng,

  • Sean Dennis, Hollis, and Steve!

  • Become an Eonite at patreon.com/eons and help us keep sharing stories from the ancient past!

  • And thank you for joining me in the Konstantin Haase Studio.

  • Go to youtube.com/eons and subscribe!

Way back, about 310 million years ago, a lizard-like creature took shelter in a hollow stump in

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When the Rainforests Collapsed

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    joey joey posted on 2021/05/01
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