字幕表 動画を再生する 英語字幕をプリント 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 means “coal-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!