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← The secret weapon that let dinosaurs take over the planet

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Showing Revision 7 created 01/08/2020 by Erin Gregory.

  1. We've all heard about
    how the dinosaurs died.
  2. The story I'm going to tell you
  3. happened over 200 million years
    before the dinosaurs went extinct.
  4. This story starts at the very beginning,
  5. when dinosaurs were just
    getting their start.
  6. One of the biggest mysteries
    in evolutionary biology
  7. is why dinosaurs were so successful.
  8. What led to their global dominance
    for so many years?
  9. When people think about
    why dinosaurs were so amazing,
  10. they usually think about the biggest
    or the smallest dinosaur,
  11. or who was the fastest,
  12. or who had the most feathers,
  13. the most ridiculous armor,
    spikes or teeth.
  14. But perhaps the answer had to do
    with their internal anatomy --
  15. a secret weapon, so to speak.
  16. My colleagues and I,
    we think it was their lungs.
  17. I am both a paleontologist
    and a comparative anatomist,

  18. and I am interested in understanding
  19. how the specialized dinosaur lung
    helped them take over the planet.
  20. So we are going to jump back
    over 200 million years
  21. to the Triassic period.
  22. The environment was extremely harsh,
  23. there were no flowering plants,
  24. so this means that there was no grass.
  25. So imagine a landscape
    filled with all pine trees and ferns.
  26. At the same time,
    there were small lizards,
  27. mammals, insects,
  28. and there were also carnivorous
    and herbivorous reptiles --
  29. all competing for the same resources.
  30. Critical to this story

  31. is that oxygen levels have been estimated
    to have been as low as 15 percent,
  32. compared to today's 21 percent.
  33. So it would have been crucial
    for dinosaurs to be able to breathe
  34. in this low-oxygen environment,
  35. not only to survive
  36. but to thrive and to diversify.
  37. So, how do we know
    what dinosaur lungs were even like,

  38. since all that remains of a dinosaur
    generally is its fossilized skeleton?
  39. The method that we use is called
    "extant phylogenetic bracketing."
  40. This is a fancy way of saying
    that we study the anatomy --
  41. specifically in this case,
    the lungs and skeleton --
  42. of the living descendants of dinosaurs
    on the evolutionary tree.
  43. So we would look at the anatomy of birds,
  44. who are the direct
    descendants of dinosaurs,
  45. and we'd look at
    the anatomy of crocodilians,
  46. who are their closest living relatives,
  47. and then we would look at
    the anatomy of lizards and turtles,
  48. who we can think of like their cousins.
  49. And then we apply these anatomical data
    to the fossil record,
  50. and then we can use that
    to reconstruct the lungs of dinosaurs.
  51. And in this specific instance,
  52. the skeleton of dinosaurs most closely
    resembles that of modern birds.
  53. So, because dinosaurs were competing with
    early mammals during this time period,

  54. it's important to understand
    the basic blueprint of the mammalian lung.
  55. Also, to reintroduce you
    to lungs in general,
  56. we will use my dog Mila of Troy,
  57. the face that launched a thousand treats,
  58. as our model.
  59. (Laughter)

  60. This story takes place
    inside of a chest cavity.

  61. So I want you to visualize
    the ribcage of a dog.
  62. Think about how
    the spinal vertebral column
  63. is completely horizontal to the ground.
  64. This is how the spinal
    vertebral column is going to be
  65. in all of the animals
    that we'll be talking about,
  66. whether they walked on two legs
  67. or four legs.
  68. Now I want you to climb inside
    of the imaginary ribcage and look up.

  69. This is our thoracic ceiling.
  70. This is where the top surface of the lungs
    comes into direct contact
  71. with the ribs and vertebrae.
  72. This interface is where
    our story takes place.
  73. Now I want you to visualize
    the lungs of a dog.
  74. On the outside, it's like
    a giant inflatable bag
  75. where all parts of the bag
    expand during inhalation
  76. and contract during exhalation.
  77. Inside of the bag, there's a series
    of branching tubes,
  78. and these tubes are called
    the bronchial tree.
  79. These tubes deliver the inhaled oxygen
    to, ultimately, the alveolus.
  80. They cross over a thin membrane
    into the bloodstream by diffusion.
  81. Now, this part is critical.

  82. The entire mammalian lung is mobile.
  83. That means it's moving
    during the entire respiratory process,
  84. so that thin membrane,
    the blood-gas barrier,
  85. cannot be too thin or it will break.
  86. Now, remember the blood-gas barrier,
    because we will be returning to this.
  87. So, you're still with me?

  88. Because we're going to start birds
    and it gets crazy,
  89. so hold on to your butts.
  90. (Laughter)
  91. The bird is completely different
    from the mammal.
  92. And we are going to be
    using birds as our model
  93. to reconstruct the lungs of dinosaurs.
  94. So in the bird,

  95. air passes through the lung,
    but the lung does not expand or contract.
  96. The lung is immobilized,
  97. it has the texture of a dense sponge
  98. and it's inflexible and locked into place
    on the top and sides by the ribcage
  99. and on the bottom
    by a horizontal membrane.
  100. It is then unidirectionally ventilated
  101. by a series of flexible,
    bag-like structures
  102. that branch off of the bronchial tree,
  103. beyond the lung itself,
  104. and these are called air sacs.
  105. Now, this entire extremely delicate setup
    is locked into place

  106. by a series of forked ribs
  107. all along the thoracic ceiling.
  108. Also, in many species of birds,
  109. extensions arise from the lung
  110. and the air sacs,
  111. they invade the skeletal tissues --
  112. usually the vertebrae,
    sometimes the ribs --
  113. and they lock the respiratory
    system into place.
  114. And this is called
    "vertebral pneumaticity."
  115. The forked ribs and
    the vertebral pneumaticity
  116. are two clues that we can hunt for
    in the fossil record,
  117. because these two skeletal traits
  118. would indicate that regions
    of the respiratory system of dinosaurs
  119. are immobilized.
  120. This anchoring of the respiratory system

  121. facilitated the evolution
    of the thinning of the blood-gas barrier,
  122. that thin membrane over which oxygen
    was diffusing into the bloodstream.
  123. The immobility permits this
    because a thin barrier is a weak barrier,
  124. and the weak barrier would rupture
    if it was actively being ventilated
  125. like a mammalian lung.
  126. So why do we care about this?

  127. Why does this even matter?
  128. Oxygen more easily diffuses
    across a thin membrane,
  129. and a thin membrane is one way
    of enhancing respiration
  130. under low-oxygen conditions --
  131. low-oxygen conditions
    like that of the Triassic period.
  132. So, if dinosaurs did indeed
    have this type of lung,
  133. they'd be better equipped to breathe
    than all other animals,
  134. including mammals.
  135. So do you remember the extant
    phylogenetic bracket method

  136. where we take the anatomy
    of modern animals,
  137. and we apply that to the fossil record?
  138. So, clue number one
    was the forked ribs of modern birds.
  139. Well, we find that in pretty much
    the majority of dinosaurs.
  140. So that means that the top surface
    of the lungs of dinosaurs
  141. would be locked into place,
  142. just like modern birds.
  143. Clue number two is vertebral pneumaticity.

  144. We find this in sauropod dinosaurs
    and theropod dinosaurs,
  145. which is the group that contains
    predatory dinosaurs
  146. and gave rise to modern birds.
  147. And while we don't find evidence
    of fossilized lung tissue in dinosaurs,
  148. vertebral pneumaticity gives us evidence
    of what the lung was doing
  149. during the life of these animals.
  150. Lung tissue or air sac tissue
    was invading the vertebrae,
  151. hollowing them out
    just like a modern bird,
  152. and locking regions
    of the respiratory system into place,
  153. immobilizing them.
  154. The forked ribs
  155. and the vertebral pneumaticity together
  156. were creating an immobilized,
    rigid framework
  157. that locked the respiratory
    system into place
  158. that permitted the evolution of that
    superthin, superdelicate blood-gas barrier
  159. that we see today in modern birds.
  160. Evidence of this straightjacketed
    lung in dinosaurs

  161. means that they had
    the capability to evolve a lung
  162. that would have been able to breathe
  163. under the hypoxic, or low-oxygen,
    atmosphere of the Triassic period.
  164. This rigid skeletal setup in dinosaurs
    would have given them
  165. a significant adaptive advantage
    over other animals, particularly mammals,
  166. whose flexible lung couldn't have adapted
  167. to the hypoxic, or low-oxygen,
    atmosphere of the Triassic.
  168. This anatomy may have been
    the secret weapon of dinosaurs
  169. that gave them that advantage
    over other animals.
  170. And this gives us an excellent launchpad
  171. to start testing the hypotheses
    of dinosaurian diversification.
  172. This is the story of
    the dinosaurs' beginning,

  173. and it's just the beginning of the story
    of our research into this subject.
  174. Thank you.

  175. (Applause)