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We've all heard about
how the dinosaurs died.

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The story I'm going to tell you

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happened over 200 million years
before the dinosaurs went extinct.

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This story starts at the very beginning,

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when dinosaurs were just
getting their start.

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One of the biggest mysteries
in evolutionary biology

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is why dinosaurs were so successful.

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What led to their global dominance
for so many years?

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When people think about
why dinosaurs were so amazing,

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they usually think about the biggest
or the smallest dinosaur,

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or who was the fastest,

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or who had the most feathers,

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the most ridiculous armor,
spikes or teeth.

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

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a secret weapon, so to speak.

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

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I am both a paleontologist
and a comparative anatomist,

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and I am interested in understanding

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

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So we are going to jump back
over 200 million years

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to the Triassic period.

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The environment was extremely harsh,

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there were no flowering plants,

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so this means that there was no grass.

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

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At the same time,
there were small lizards,

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mammals, insects,

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and there were also carnivorous
and herbivorous reptiles --

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all competing for the same resources.

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Critical to this story

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is that oxygen levels have been estimated
to have been as low as 15 percent,

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compared to today's 21 percent.

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

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in this low-oxygen environment,

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not only to survive

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but to thrive and to diversify.

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

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since all that remains of a dinosaur
generally is its fossilized skeleton?

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The method that we use is called
"extant phylogenetic bracketing."

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

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specifically in this case,
the lungs and skeleton --

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of the living descendants of dinosaurs
on the evolutionary tree.

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So we would look at the anatomy of birds,

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who are the direct
descendants of dinosaurs,

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and we'd look at
the anatomy of crocodilians,

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who are their closest living relatives,

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

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who we can think of like their cousins.

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

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

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And in this specific instance,

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the skeleton of dinosaurs most closely
resembles that of modern birds.

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

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

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Also, to reintroduce you
to lungs in general,

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we will use my dog Mila of Troy,

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the face that launched a thousand treats,

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as our model.

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(Laughter)

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This story takes place
inside of a chest cavity.

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

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Think about how
the spinal vertebral column

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is completely horizontal to the ground.

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This is how the spinal
vertebral column is going to be

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in all of the animals
that we'll be talking about,

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whether they walked on two legs

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or four legs.

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

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This is our thoracic ceiling.

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This is where the top surface of the lungs
comes into direct contact

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with the ribs and vertebrae.

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This interface is where
our story takes place.

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

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

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where all parts of the bag
expand during inhalation

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and contract during exhalation.

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

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and these tubes are called
the bronchial tree.

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These tubes deliver the inhaled oxygen
to, ultimately, the alveolus.

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

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Now, this part is critical.

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The entire mammalian lung is mobile.

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

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so that thin membrane,
the blood-gas barrier,

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cannot be too thin or it will break.

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

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So, you're still with me?

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

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so hold on to your butts.

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(Laughter)

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The bird is completely different
from the mammal.

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And we are going to be
using birds as our model

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to reconstruct the lungs of dinosaurs.

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So in the bird,

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

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The lung is immobilized,

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it has the texture of a dense sponge

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and it's inflexible and locked into place
on the top and sides by the ribcage

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and on the bottom
by a horizontal membrane.

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It is then unidirectionally ventilated

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by a series of flexible,
bag-like structures

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that branch off of the bronchial tree,

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beyond the lung itself,

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and these are called air sacs.

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Now, this entire extremely delicate setup
is locked into place

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by a series of forked ribs

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all along the thoracic ceiling.

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Also, in many species of birds,

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extensions arise from the lung

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and the air sacs,

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they invade the skeletal tissues --

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usually the vertebrae,
sometimes the ribs --

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and they lock the respiratory
system into place.

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And this is called
"vertebral pneumaticity."

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The forked ribs and
the vertebral pneumaticity

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

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because these two skeletal traits

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would indicate that regions
of the respiratory system of dinosaurs

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are immobilized.

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This anchoring of the respiratory system

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

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

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The immobility permits this
because a thin barrier is a weak barrier,

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

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like a mammalian lung.

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So why do we care about this?

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Why does this even matter?

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Oxygen more easily diffuses
across a thin membrane,

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and a thin membrane is one way
of enhancing respiration

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under low-oxygen conditions --

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low-oxygen conditions
like that of the Triassic period.

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

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they'd be better equipped to breathe
than all other animals,

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including mammals.

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So do you remember the extant
phylogenetic bracket method

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where we take the anatomy
of modern animals,

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and we apply that to the fossil record?

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

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

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

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would be locked into place,

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just like modern birds.

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Clue number two is vertebral pneumaticity.

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We find this in sauropod dinosaurs
and theropod dinosaurs,

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which is the group that contains
predatory dinosaurs

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and gave rise to modern birds.

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

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vertebral pneumaticity gives us evidence
of what the lung was doing

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during the life of these animals.

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Lung tissue or air sac tissue
was invading the vertebrae,

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hollowing them out
just like a modern bird,

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and locking regions
of the respiratory system into place,

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immobilizing them.

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The forked ribs

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and the vertebral pneumaticity together

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were creating an immobilized,
rigid framework

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that locked the respiratory
system into place

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that permitted the evolution of that
superthin, superdelicate blood-gas barrier

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that we see today in modern birds.

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Evidence of this straightjacketed
lung in dinosaurs

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means that they had
the capability to evolve a lung

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that would have been able to breathe

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

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This rigid skeletal setup in dinosaurs
would have given them

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a significant adaptive advantage
over other animals, particularly mammals,

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whose flexible lung couldn't have adapted

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to the hypoxic, or low-oxygen,
atmosphere of the Triassic.

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This anatomy may have been
the secret weapon of dinosaurs

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that gave them that advantage
over other animals.

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And this gives us an excellent launchpad

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to start testing the hypotheses
of dinosaurian diversification.

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This is the story of
the dinosaurs' beginning,

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and it's just the beginning of the story
of our research into this subject.

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Thank you.

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(Applause)