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