The secret weapon that let dinosaurs take over the planet

We've all heard about
how the dinosaurs died.

The story I'm going to tell you

happened over 200 million years
before the dinosaurs went extinct.

This story starts at the very beginning,

when dinosaurs were just
getting their start.

One of the biggest mysteries
in evolutionary biology

is why dinosaurs were so successful.

What led to their global dominance
for so many years?

When people think about
why dinosaurs were so amazing,

they usually think about the biggest
or the smallest dinosaur,

or who was the fastest,

or who had the most feathers,

the most ridiculous armor,
spikes or teeth.

But perhaps the answer had to do
with their internal anatomy,

a secret weapon, so to speak.

My colleagues and I,
we think it was their lungs.

I am both a paleontologist
and a comparative anatomist,

and I am interested in understanding

how the specialized dinosaur lung
helped them take over the planet.

So we are going to jump back
over 200 million years

to the Triassic Period.

The environment was extremely harsh,

there were no flowering plants,

so this means that there was no grass.

So imagine a landscape
filled with all pine trees and ferns.

At the same time,
there were small lizards,

mammals, insects,

and there were also carnivorous
and herbivorous reptiles,

all competing for the same resources.

Critical to this story

is that oxygen levels

have been estimated to have been
as low as 15 percent,

compared to today's 21 percent.

So it would have been crucial
for dinosaurs to be able to breathe

in this low-oxygen environment,

not only to survive

but to thrive and to diversify.

So, how do we know
what dinosaur lungs were even like,

since all that remains of a dinosaur

generally is its fossilized skeleton?

So the method that we use

is called extant phylogenetic bracketing.

This is a fancy way of saying
that we study the anatomy,

specifically in this case
the lungs and skeleton,

of the living descendants of dinosaurs
on the evolutionary tree.

So we would look at the anatomy of birds,

who are the direct
descendants of dinosaurs,

and we'd look at
the anatomy of crocodilians,

who are their closest living relatives,

and then we would look at
the anatomy of lizards and turtles,

who we can think of like their cousins.

And then we apply these anatomical data
to the fossil record,

and then we can use that
to reconstruct the lungs of dinosaurs.

And in this specific instance,

the skeleton of dinosaurs

most closely resembles
that of modern birds.

So, because dinosaurs were competing with
early mammals during this time period,

it's important to understand
the basic blueprint of the mammalian lung.

Also, to reintroduce you
to lungs in general,

we will use my dog Mila of Troy,

the face that launched a thousand treats,

as our model.

(Laughter)

So this story takes place
inside of a chest cavity.

So I want you to visualize
the ribcage of a dog.

Think about how the spine
and vertebral column

is completely horizontal to the ground.

This is how the spinal vertebral column

is going to be in all of the animals
that we will be talking about,

whether they walked on two legs

or four legs.

Now, I want you to climb inside
of the imaginary ribcage and look up.

This is our thoracic ceiling.

This is where the top surface of the lungs

comes into direct contact
with the ribs and vertebrae.

This interface is where
our story takes place.

Now I want you to visualize
the lungs of a dog.

On the outside, it's like
a giant inflatable bag

where all parts of the bag
expand during inhalation

and contract during exhalation.

Inside of the bag, there's a series
of branching tubes,

and these tubes are called
the bronchial tree.

These tubes deliver the inhaled oxygen

to, ultimately, the alveoli.

They cross over a thin membrane
into the bloodstream by diffusion.

Now, this part is critical.

The entire mammalian lung is mobile.

That means it's moving
during the entire respiratory process,

so that thin membrane,
the blood-gas barrier,

cannot be too thin or it will break.

Now, remember the blood-gas barrier,
because we will be returning to thsi.

So, you're still with me?

Because we're going to start birds
and it gets crazy,

so hold on to your butts.

So the bird

is completely different from the mammal,

and we are going to be using birds

as our model to reconstruct
the lungs of dinosaurs.

So in the bird,

air passes through the lung,
but the lung does not expand or contract.

The lung is immobilized,

it has the texture of a dense sponge,

and it's inflexible

and locked into place on the top
and sides by the ribcage

and on the bottom
by a horizontal membrane.

It is then unidirectionally ventilated

by a series of flexible,
bag-like structures

that branch off of the bronchial tree

beyond the lung itself,

and these are called air sacs.

Now, this entire extremely delicate setup

is locked into place

by a series of forked ribs

all along the thoracic ceiling.

Also, in many species of birds,

extension arise from the lung

and the air sacs,

they invade the skeletal tissues,

usually the vertebrae, sometimes the ribs,

and they lock the respiratory
system into place,

and this is called vertebral pneumaticity.

The forked ribs and
the vertebral pneumaticity

are two clues that we can hunt for
in the fossil record,

because these two skeletal traits

would indicate that regions
of the respiratory system of dinosaurs

are immobilized.

This anchoring of the respiratory system

facilitated the evolution
of the thinning of the blood-gas barrier,

that thin membrane over which oxygen
was diffusing into the bloodstream.

The immobility permits this

because a thin barrier is a weak barrier,

and the weak barrier would rupture
if it was actively being ventilated

like a mammalian lung.

So why do we care about this?

Why does this even matter?

Oxygen more easily diffuses
across a thin membrane,

and a thin membrane is one way
of enhancing respiration

under low-oxygen conditions,

low-oxygen conditions
like that of the Triassic Period.

So, if dinosaurs did indeed
have this type of lung,

they'd be better equipped to breathe

than all other animals,

including mammals.

So do you remember the extant
phylogenetic bracket method,

where we take the anatomy
of modern animals

and we apply that to the fossil record?

So, clue number one
was the forked ribs of modern birds.

Well, we find that in pretty much
the majority of dinosaurs.

So that means that the top surface
of the lungs of dinosaurs

would be locked into place,

just like modern birds.

Clue number two is vertebral pneumaticity.

We find this in sauropod dinosaurs
and theropod dinosaurs,

which is the group that contains
predatory dinosaurs

and gave rise to modern birds.

And while we don't find evidence
of fossilized lung tissue in dinosaurs,

vertebral pneumaticity gives us evidence
of what the lung was doing

during the life of these animals.

Lung tissue or air sac tissue
was invading the vertebrae,

hollowing them out
just like a modern bird,

and locking regions
of the respiratory system into place,

immobilizing them.

The forked ribs

and the vertebral pneumaticity together

were creating an immobilized,
rigid framework

that locked the respiratory
system into place

that permitted the evolution
of that super-thin,

super-delicate blood-gas barrier
that we see today in modern birds.

Evidence of this straight-jacketed
lung in dinosaurs

means that they had
the capability to evolve a lung

that would have been able to breathe

under the hypoxic, or low-oxygen
atmosphere of the Triassic period.