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Kallie Moore:
Around 300 million years ago,

at the dawn
of the Permian Period,

life on the newly-formed
supercontinent of Pangaea

was in the throes
of an ecological revolution.

A lifestyle
that had never existed

before invertebrates emerged--

one that's been extremely common
ever since: herbivory.

Now, it feels hard
to imagine life on land

without plant-eaters.

Herbivores eating plants

and then carnivores
eating the herbivores

seems like
such a foundational part

of how ecosystems work

that it must be a tale
as old as time, right?

Well, actually, no.

Just like every other
complex behavior,

eating plants had to start
at some point.

Michelle Barboza-Ramirez:
The early evolutionary
experiments in herbivory

probably began
just before the Permian did,

in the swamps of
the Late Carboniferous Period.

And by the Early Permian,

just a few million years later,

it had become clear
that these experiments were

a resounding success.

Plant eating was spreading fast.

One of the first
big herbivores

to ever live on land

was Edaphosaurus.

Now, Edaphosaurus may look like

some kind of strangely-built
proto-dinosaur,

but it's actually
more closely related

to you and me

than to any dino
or any other kind of reptile.

It belonged to a group
informally known as pelycosaurs

that exploded in diversity

as the Late Carboniferous
transitioned

into the Early Permian.

Gabriel-Philip Santos:
By some estimates,

pelycosaurs represent around 70%

of all known amniotes
from this time,

far outnumbering
the early reptiles

on the sauropsid side
of the family tree.

Long before the era
of the dinosaurs,

when the reptile side would rise

to ecological prominence,

the Early Permian

saw the mammal side

reign over the land.

And herbivores like Edaphosaurus

were ecological pioneers.

From their time onwards,

wherever there were a lot
of plants around on land,

there were species
that made a living
by eating them.

The first herbivores
from the era of ancient life

left an ecological legacy
that has stood the test of time,

but many of the species
themselves would not.

Because
the Early Permian would end

with most of the pelycosaurs
vanishing

under mysterious circumstances

and a new group
rising to replace them.

This drastic evolutionary shift

would be a major milestone

in the story of our ancient
proto-mammal ancestors,

marking another step
towards us true mammals.

Oh, my heavens!

This is
a baby Dimetrodon toe.

Shut the front door.

Ah!

But this shift
would also come to be known

as one of the Period's
biggest enigmas, too,

because right around
this pivotal moment

in our evolutionary history,

the fossil record
suddenly goes dark.

The basic structure
of modern terrestrial ecosystems

is often depicted
as a simple pyramid.

Plants are at the base,

with the herbivores
who feed on the plants

one step up from there.

Above them are carnivores

who feed mostly
on the herbivores,

as well as on each other, sometimes.

Finally, you have apex predators
who sit right at the peak

with no one above them,

and who eat, well,
whoever they want.

But this "classic"
food chain structure

didn't emerge all at once,

and it hasn't always been
the ecological rule.

Instead, it developed in phases

over hundreds
of millions of years,

only coming together fully
in the Early Permian Period,

just under
300 million years ago.

Because, while we take it
for granted today,

this kind of
multi-layered food chain

is actually really complex,

and a whole lot had to happen
for it to form.

First, plants had to colonize
the land, which they did

by at least
470 million years ago,

in the Ordovician Period.

Then, animals had to follow.

Invertebrates
were the first animals

to make the jump,

becoming established on land
by the Silurian Period,

more than 420 million years ago.

But they probably weren't eating
much living plant material

in those days--

they were carnivores
who hunted and scavenged

and/or detritivores who fed
on dead, decomposing biomass.

Even when the early four-legged
amphibious vertebrates

began to crawl out of the water

around 375 million years ago

in the Late Devonian Period,

they too had
a basically all-meat diet.

And through most
of the Carboniferous Period

that followed,

the dominant amphibians
that flourished

in the tropical swamps

still lived
in this dog-eat-dog world--

or frog-eat-frog world,
if you will.

But as the Carboniferous
came to a close

and gave way
to the Early Permian,

herbivory suddenly exploded
onto the scene.

Blake de Pastino:
We're in Seymour, Texas,

in North Central Texas,

not far off
from the Oklahoma border.

These towns tend to have
these small museums

that most folks outside
of, say, the county,

don't know about.

But you see the exhibits,
and you get a taste

of, like, what the local
natural history is.

The Whiteside is unique,
because it is the place--

in North America, at least--

where you can get
hands-on encounters

with the Permian Period

and discover
your connection to it.

Help me understand--

that's what
I'm wondering about--

is, like, how does
a group of organisms

acquire the ability
to digest plant material

and make a living
off of it?

So, in the Early Permian,

even in
the Late Carboniferous,

we see this ecological
arms race happening,

where we see conifers
starting to appear,

and then as these conifers
are exploding,

animals start to develop
adaptations

to be able to process
these new plants.

About 300 million years ago,

early terrestrial animals
began eating living leaves,

roots, and seeds,

changing the ecological game
forever.

Herbivory evolved independently
in multiple lineages

around this same time

and with some similar
evolutionary effects

on the species themselves

because a carnivore
can't just wake up one morning

and start eating plants
full-time.

If you're a meat-eater,
you're shedding teeth,

so they lose their teeth
when feeding.

So, if you're
a plant-eater,

having teeth fall out of your
mouth is not gonna help you

when you're chewing up
conifers and roots

and things like that.

So, animals
that are herbivores

start to develop

these really
deep-rooted teeth

that are thicker enamel,
stronger,

so that they can adapt
to eating thicker things

and not lose their teeth
consistently.

Several of the earliest
herbivore lineages developed

much bigger and wider
barrel-shaped bodies

as they made the switch
to a plant-based diet,

like our pelycosaur friend
Edaphosaurus, for example.

Their expanded guts

house trillions
of symbiotic microbes

that help them break down

the otherwise indigestible
parts of their food,

like cellulose,

which makes up part of
the cell walls of plants.

Now, the fossil record
can't directly tell us

how exactly the ancestors
of these ancient animals

independently gained
their microbial partners,

but scientists have
a few ideas.

Animal species that foraged
in dead organic matter

would have regularly
ingested microbes

that were breaking down
plant litter,

and some of these microbes

could have survived
in their guts,

eventually putting
their cellulose-digesting skills

to work for the benefit
of their animal hosts.

And animal species
that fed on insects

could also have acquired

their cellulose-digesting
microbes

from the guts
of insects that they ate.

However it happened,

this partnership got started
multiple times

in multiple vertebrate lineages
in the Late Carboniferous,

giving them a boost
on their journey toward

a fully vegetarian lifestyle.

And developing
a huge gut

to use as a bacterial
fermentation chamber

is an adaptation
that many big land herbivores

have repeatedly evolved
ever since,

to squeeze
all the nutrients they can

out of a plant-based diet.

Think Triceratops
in the Mesozoic,

or bison and rhinos today.

And Edaphosaurus
wasn't alone

in ballooning up
around this time,

as his ancestors made the switch

from eating meat--
like insects--to plants.

Many other vertebrate lineages

also started living large
on land,

as they independently
transitioned to herbivory.

These included Cotylorhynchus,
which was another pelycosaur

from the synapsid side
of the family tree

that belonged
to a different subgroup

called the caseids.

Caseids like Cotylorhynchus
made the switch to herbivory

independently from
their edaphosaurid cousins,

but the two pelycosaur lineages

developed some similar traits
along the way,

like big, sprawling,
barrel-shaped bodies

with relatively tiny heads.

Beyond just pelycosaurs,

another early herbivore
called Diadectes

is also known from this period,

filling a similar niche.

But while these earliest
herbivores converged

on some similar physical traits,

like becoming big and bulky,

they also evolved
some totally different ones

on their independent journeys
toward vegetarian life.

Take their teeth, for example.

Edaphosaurus had dense clusters
of peg-like teeth

both in the lower jaw

and on the roof of their mouth,

called dental batteries.

We can tell from the amount
of wear on its teeth

that it was using them

to grind and crush
plant material

and continually replacing them
over the course of its life.

But the leaf-shaped teeth
of Cotylorhynchus show

a completely different
dental setup.

Instead of massive
dental batteries,

it had fewer teeth

that show no signs of wear
from contact with each other,

and they don't seem to have been
regularly replaced either.

Rather than crushing
and grinding plant material

between rows and clusters
of cheap, replaceable teeth,

Cotylorhynchus
must have been doing

something very different.

Some scientists think
that, after cropping off

a mouthful of plant material,

it used a big, tough tongue
to press the food up

against several large
and slender teeth

on the roof of its mouth,

which broke it up
enough for digestion.

And other lineages
of early herbivores
from this time

also had their own
unique combination

of dental adaptations.

There's no one single way of
being an herbivore, after all.

And different lineages
found themselves evolving

different solutions
to similar problems,

as they made the transition.

This is one of
the most famous bonebeds

in the entire world.

This is the world-famous
Craddock Bonebed.

Evolutionarily,
this is essentially

where your roots
come from as mammals.

This is an episode
in the history of life

that will have
deep ramifications

for all land dominant animals
from here on out.

Everything we know
about land dominance

is because of the critters

that you guys are going
to be digging today.

It's
a pretty amazing place.

So, life on land had gained

its first big
plant-munching herbivores

that now roam
the Early Permian landscape,

including right here,

in what are now
the Red Beds of Texas,

and which have been
a feature of our planet

in some form or another
ever since.

But alongside them evolved
another link in the food chain.

Earth's first large terrestrial
apex predators had arrived.

They were also pelycosaurs,

but their ancestors
had remained meat-eaters,

growing in size in parallel

with their newly herbivorous
edaphosaurid and caseid cousins.

And perhaps the most iconic
of these was Dimetrodon.

Crazy to think
that this is, you know--

when you think about
Permian and Dimetrodons,

wherever you are
in the world,

if you see a Dimetrodon
skeleton in a museum,

there's a good chance
it came from right here.

Wow, that's
incredible.

Dimetrodon is
the world's first-known

fully terrestrial carnivore

to fill the role
of apex predator,

the first to rise
to the very top

of the new complex food chain

that had emerged on land.

So, today, we are
at the Craddock Bonebeds.

This is
an amazing field site

that I'm so excited
we got to come visit.

You're looking at this stuff

that is hundreds
of millions of years old,

weathering out
of the side of a rock,

a rock that tells you
what environment you're in,

and bones that tell you
what animals

were in that environment.

It's--it's time travel, right?

Geologists, we say

that when you're looking
at the rocks,

when you're looking
at stratigraphy,

you can read them
like chapters of a book.

All of a sudden, you get to see
the world not just as it is now,

but as it has been
throughout eons

and as it might be
in the future.

And being able
to connect to that,

just--it's unlocking
all these new layers

to the world that I live in.

One of the things
I was going to ask,

like,
one of the things I find

really just kind of compelling
and fun about paleontology

is just, like,
the farther down you dig,

typically, the farther back
in time you go.

So, where in time are we,
between, like,

that hilltop and here?

Yeah.
So we're Lower Clear Fork,

which is Lower Permian,

roughly
287 million years ago.

So, 287 million years,
we're, you know,

reaching the apex
of size for Dimetrodon.

So, Dimetrodons
have been around

for about
20 million years now,

and their sizes
have been getting bigger

over the course

of the last
15, 20 million years.

And in the Lower Clear Fork
where you guys are,

we're at the last breath
of Dimetrodon.

So, we are seeing
the last big push
of Dimetrodon.

They get to
their biggest size.

We see Dimetrodon grandis,
which is the largest,

and then, you know,
wait about 10 million years,

and, you know,
they're all gone.

So, this is the last act.

This is their peak.

Wow.

Oh, my gosh! Ahh!

What did you find?

I don't know, a tiny little--
something diagnostic.

Holly Simon: Y'all,
you got a toe bone.

This is a ba--
a baby Dimetrodon toe.

Shut the front door.

Ah! What?!
That's awesome.

So, that's
a little toe.

We love toes.
Wow.

We love toes.
I love toes now too.

Well, unlike my co-hosts
and colleagues,

I'm not a scientist.

My background
is in science writing,

science journalism.

Most science writers
would tell you

that we do what we do

out of an abiding curiosity
about the world,

and that's probably
the case for me.

I am not
a skilled fossil hunter.

I don't have
those kinds of eyes,

but my colleagues do.

When I go on a dig with them,

it's humbling how easily
they find fossils.

It's most likely
a Dimetrodon rib.

My first Permian bone
was a Dimetrodon?

Yep, absolutely.

The very first thing
you found was Dimetrodon.

So, the caliche rind
on this tells you, you know--

There's another one.

Wow, they really are
just everywhere.

And once MB and Chris
started finding fossils,

then I kind of picked up on
what I was looking for.

And then you realize
how many there are.

And you can see the details.

You can see the porosity
of the bones, you know?

Oh, yeah.

This is important.

Permian is important.

You've got to take care
of the Permian bones.

Fossils stick.
Rocks don't stick.

And by the end of the day,

I was able to tell, generally,

what part of the Dimetrodon
this fossil was from.

And that was impressive,
considering that, like,

at the beginning of the day,

I didn't know a bone
from a stick.

That's a claw.

That's
a Dimetrodon claw.

Ooh, yeah,
look at that. Chh.

That is a killing claw.

They've only been
on the planet

for 20 million years,

and they've already
evolved claws

that can do everything
they need it to

in a matter of seconds.

Wow.

Super cool.

And I found it.

That's right.

Yeah.

Amazing--amazing job.

You're the one that
picked it up first, so...

It was the T-Rex
or the tiger of its time.

And just like
with the earliest herbivores,

Dimetrodon moving into a new,
groundbreaking ecological niche

came with some radical
physical adaptations.

Dimetrodon developed
a big, powerful skull

with deep jaws lined
with multiple types of teeth

adapted for different aspects
of its predatory lifestyle.

These included
large, sharp canines

for striking and stabbing prey,

as well as shearing teeth

for slicing meat
into digestible chunks.

Having differentiated teeth
is a trait found

in many modern
and ancient animals,

but this was
yet another innovation

that first appeared
in Dimetrodon

and its relatives.

The differences in these teeth
aren't as obvious as they are

in carnivorous mammals today,

but they had to start somewhere.

So, for the past
couple of weeks,

we've been slowly digging
this shelf,

and Holly's been finding
some incredible skeletons

and parts of bones,

and so, we just need
to expand that shelf
a little bit more,

and what's so lovely
about digging in Permian clays

is that
it's very easy to dig,

which makes it easy
to expose bones very quickly.

So, we're gonna expand
this shelf

and find
some more incredible stuff

and see what you guys
come up with.

Wow, okay, here it goes.

Oh, it is a huge bone.

Geez, yep,
it's still going,

which is exciting.

What...?

What is it?

It's a joint.

Oh, man, that looks
like a tooth-socket.

Oh, that is
a tooth-socket.

Holy shoot!

That's a skull.
Holy crap.

Oh, my God.

Look at that,
that's a tooth-socket.

Yeah.

Oh, geez,
that's a big skull.

You've been digging
Permian for 4 minutes,

and you found
a Dimetrodon skull.

It's amazing how fossiliferous
this site is--

and that's a real word,
by the way, "fossiliferous."

It's just, like,
chock full of stuff.

There's a rib here

in front of a spine there,
in front of a skull there.

It's fantastic.

And the mudstone
is kind of crumbling away.

It makes it really easy
to get at the bones

and dig through them.

Mudstone is a type of rock
that forms from,

as the name would have it,
mud, which means

that it's in a really gentle
sort of environment.

I mean, think about
where you would find mud

when you're out
in the world today, right?

You're by a lake bed,
you're in areas

where there's not
a lot of energy.

And for fossils, that's amazing.

That means you don't have
big waves

or anything that's gonna come
and disturb these corpses

that are left behind

that we can find
300 million years later

as fossils.

So, can you tell me more
about Dimetrodon's role

in the ecosystem here?

The food pyramid,
he's obviously the king.

He eats everything
and anything.

He's just recycling
all the organisms

in this ecosystem,

putting it back
into the ecosystem.

So, ultimately, you know,
his role here

is to keep
the population down.

You know,
you're population control.

The Early Permian
saw an array of pelycosaurs,

living different kinds
of lifestyles,

dominate the landscape

and flourish within
the ecological revolution

that was unfolding.

But as this time
came to a close

around 273 million years ago,

their reign ended
under suspicious circumstances.

Dimetrodon goes extinct

right at the end
of the Early Permian,

along with Edaphosaurus
and Diadectes.

There's a gap.

The fossil record
of land animals

seems to go virtually blank
for about 5 million years.

And when it picks back up
in the Middle Permian,

most of the once abundant
pelycosaurs have disappeared.

In their place is a new,
thriving group of synapsids

called the therapsids.

Therapsids evolved from
a lineage of pelycosaurs

that seemed to have
emerged diversified

and taken over ecosystems
during those 5 million years,

while almost all of the older
pelycosaur lineages vanished.

And this turnover was
a crucial plot twist

in the story of the rise
of mammals later on,

because therapsids had
a host of new traits

compared to
their pelycosaur ancestors

and relatives.

Therapsids were often
faster and more agile,

with their legs positioned

more directly
under their bodies

and less sprawled
at their sides,

giving them
a greater range of motion

than their pelycosaur ancestors.

Their skulls had changed too,

becoming stronger
and more robust,

with teeth that were
increasingly specialized

for different roles.

They may even have had
better hearing,

a more sensitive sense of smell,

and a faster growth rate.

With Dimetrodon,

it's all about
the jaw joints,

and so, jaw joints
are extremely important

at this point.

With the therapsids,

something interesting
is really happening.

These joints
are starting to shrink,

and those joints
are going to move up

into the back
of the skull

and turn into the eardrum.

So, once you start to shrink
those jaw joints back here,

you're making room up here,

which means you're getting
a bigger brain.

So, with your brain
getting larger,

your joints getting smaller
and moving into your ears,

you're adapting
extremely quickly

to the things that
are happening around you.

You're not only becoming
an apex predator,

but you're redefining

how you are becoming
a predator.

In short, they were becoming
increasingly mammal-like.

But how and why this transition

from pelycosaurs to therapsids
actually occurred

has been a long-standing mystery

in the field of paleontology.

This void
in the fossil record

is known as Olson's Gap,

after Everett C. Olson,

the researcher
who first proposed

this ecological changeover.

And the debate
over what it represents

basically boils down
to two competing ideas.

Hypothesis one
is that Olson's Gap

is the result
of geological bad luck.

We are simply missing
any good fossil sites

that cover this chapter of time.

It's annoying, but it happens.

The problem is that
our fossil sites

from either side of the Gap

also come from completely
different places too.

The Early Permian fossil sites

that record the reign
of the pelycosaurs

almost all come
from North America
and Western Europe,

which, at the time,
were at Pangea's equator,

whereas
the Middle Permian fossils,

when the therapsids
first show up,

mostly come from
higher latitude areas

in what's now Russia
and South Africa.

So, while the disappearance
of the pelycosaur

from the fossil record

and the appearance
of the therapsids

might seem dramatic,

under the Olson's Gap scenario,

this could just be because
our fossil record shifts

to different environments
in different places

over the course of this time.

This gives the illusion
of a stark shift,

but perhaps we're just
glimpsing fragments

of a longer, more gradual
and complex transition.

There's a lot
of scientists out there

that say there is no gap.

And there are a lot
of scientists that say,

yeah, there is a gap.

And it's all looking at
the different fossil records

all over the world.

In hypothesis two,

Olson's Gap should
really be thought of

as "Olson's Extinction."

According to this idea,

the global shift in life on land

observed between
the Early and Middle Permian

isn't just a result
of sampling bias.

It actually represents
a genuine mass extinction

that caused a global turnover
in terrestrial animal life.

Paleontologists
are still debating

whether the gap
or the extinction hypothesis

has more support,

but we do have a chance

at solving this mystery,
at least.

With more digging
and more discoveries

in places that have
historically been undersampled,

we may yet turn up fossils
that end up filling the Gap.

A good rule of thumb is,
when you see a bone

in the side of a hill,
like this...

Stop digging.
Right.

You don't want to dig
underneath of the bone,

because it could collapse
and fall out of the wall.

So, a good rule of thumb

is we want to dig
on top of the bone,

so we can uncover it
before we take it out.

When I came here,
I understood the science

behind what we were doing here.

I had written
and read about the Permian.

I knew what had happened there,
so I understood it

in sort of an abstract,
cognitive sense.

But what I didn't know
what to expect was

the experience of actually
going on a dig in the Permian,

which I've never
encountered before.

I've never had the opportunity
to do this before.

And I got to hold
in my hand the bones

of what seems like

my most direct
terrestrial ancestor, right?

Like, the animal
that conquered land,

figured out how to make
a living on land

and become an apex predator.

And, like, I am related
to this organism.

And that is something
I've never experienced before.

The other thing
that never gets old

is, like, I'm holding
part of an animal

that lived a quarter
of a billion years ago.

And you are
the first human beings

to ever see
these bones, you know?

Hey, man.

There's 7-plus
billion people
on the planet,

and you're
the first ones

in this entire
population on Earth

to see these bones.

So, the Early Permian Period
was a time of firsts

for planet Earth.

Life on the supercontinent
of Pangaea gained

its first big herbivores,
its first big apex predators,

and a new ecological framework

that's stuck around ever since.

And Permian life
may have even faced

its first major challenge too.

But out of it emerged
the therapsids.

And while the origins
of the therapsids

may be blurry and mysterious,

the rest of their story
through the Permian

is a tale of enormous
ecological success,

diversification,
and evolutionary innovation.

And it's a story
that, in a sense,

is still unfolding to this day

through the last surviving
therapsid lineage--

us mammals.

But many twists and turns
were still in store

for these early therapsids

in the Middle Permian Period.

They were about
to enter a golden age,

but at the Permian's end,

they, along with every
other living thing on Earth,

would have to face and survive

the most severe mass extinction
of all time.