What Helped Mammals Survive the Asteroid Impact?
[An animated image of a long-necked dinosaur, seen from below, against a cloudy sky.]
JIN MENG (CURATOR, DIVISION OF PALEONTOLOGY): Think about this:
[An animated image of many dinosaurs running as a herd with a dragonfly-like insect in the foreground.]
JIN MENG: If we sit here sixty-six million years ago…
[An animated image of a closeup on two long-necked dinosaurs, with bird-like animals flying around them.]
JIN MENG: …and suddenly you feel the ground shake…
[An animated image of trees on fire.]
JIN MENG: …and the forest ignited…
[An animated image of a triceratops with a dragonfly-like insect on its horn and a small, furry creature next to it, plunged in darkness.]
JIN MENG: …and everything became so dark and dim.
[JIN MENG speaks to camera from a Museum hall.]
JIN MENG: What happened?
[Animation of an asteroid hurtling towards the Earth, seen from space, then crashing into the Earth’s surface.]
JIN MENG: There is a mountain-sized asteroid that just struck the planet Earth…
[A series of animated images showing a crocodile-like animal swimming underwater as the sky above the water’s surface blazes orange; ammonites lying motionless on the ocean floor; a giant animal lying dead with its jaw hanging open and many sharp visible with a surviving bird-like animal perched on top of it.]
JIN MENG: …and wiped out 75% of species living at that time.
[Animation of a triceratops looking up at the sky as it grows brighter and brighter before being engulfed in white.]
JIN MENG: Including, of course, non-avian dinosaurs.
[Animated image of a large, long-necked dinosaur with a small furry mammal next to it, followed by an image of a charred forest covered in smoke following the asteroid impact.]
JIN MENG: But mammals actually lived a long time ago before the extinction.
[Animation of a small mammal emerging from the barren ground and looking around.]
JIN MENG: And then the question is, why they survived?
[MUSEUM LOGO]
[JIN MENG speaks to camera from the Museum’s Hall of Primitive Mammals, with a variety of large fossil mammals mounted behind him.]
JIN MENG: My name is Jin Meng. I'm a curator and a paleontologist in the Division of Paleontology at the American Museum of Natural History.
[Three side-by-side photos of Jin Meng in various field sites at different ages.]
JIN MENG: I have been studying fossil mammals for over 40 years.
[Jin Meng motions to the large fossil mammal skeletons behind him.]
JIN MENG: What you see here in my background are all mammals that lived in the Cenozoic…
[On-screen text reads, “Cenozoic Era: 66 million years ago to present”]
JIN MENG: …the age of mammals. Many of them are very big.
But that's not the case in the beginning of mammal evolution.
[A shot of a small fossil mammal roughly the size of a cat, followed by a shot of a small skull displayed behind a magnifying glass.]
JIN MENG: Mammals originated about 200 million years ago.
[Three artist renderings of early mammals appear one by one under a label “ancestral mammals.” On-screen text identifies them as “Morgancucodon: lived about 205 million years ago”; “Dianoconodon: lived about 200 million years ago”; and “Feredocodon: lived about 168 million years ago.” They all somewhat resemble modern-day rats or mice but with clear differences.]
JIN MENG: So about two-thirds of mammalian evolution was in the Mesozoic along with dinosaurs.
[A Museum display depicting a scene 66 million years ago featuring a triceratops and other animals. Circles appear around three small animals that are identified as mammals. Text on screen reads “Mesozoic Era: 252 to 66 million years ago.”]
JIN MENG: If we look at today's mammals…
[A split-screen showing many different videos of various mammals living today with text reading “6,800 species of mammals”.]
JIN MENG: …we know that there are about 6,800 species of mammals.
[The videos fade to white as a diagram appears piece by piece as Jin Meng speaks.]
And of all those, only five species belong to monotreme from Australia.
[A line extends out from the text “6,800 species of mammals” to a circle containing a video of a small, spiky echidna. Underneath it, the word “monotreme” appears.]
JIN MENG: The rest belongs to one group: Theria.
[Another line extends from the text “6,800 species of mammals” in the other direction, connecting to the word “therians.”]
JIN MENG: Theria consists of marsupials and placental mammals.
[A line extends from the word “therians” to a circle containing a video of a kangaroo with the word “marsupials” underneath it. Another line extends to a circle containing a video of a chimpanzee with the words “placental mammals” underneath it.]
[An illustration depicting a landscape from the Mesozoic Era appears on screen. One by one, illustrations depicting Mesozoic-Era mammals appear on screen in circles of different colors: two in blue, one in green, one in yellow, and one in red.]
JIN MENG: In the Mesozoic, many mammals are all small, warm-blooded, covered with fur, and most of them are insectivores.
[All the illustrations except those in blue circles disappear, implying that those are Theria mammals and the rest are not.]
JIN MENG: But Theria mammals survived…
[Several other illustrations of Theria mammals in blue circles appear around the original two.]
JIN MENG: …and diversified after the extinction.
[JIN MENG speaks to camera.]
JIN MENG: So Theria mammals have two critical aspects relatively easily preserved as fossils.
[Footage of very small fossil Theria teeth mounted on small rods in glass tubes.]
JIN MENG: One is their teeth, another is their middle ear.
[A larger-than-life model of the lower jaw of a Theria mammal sits on a red background. As JIN MENG lists the types of teeth in the jaw, text appears on screen to identify them.]
JIN MENG: Theria mammals have incisors, canines, premolars and molars.
[Closeup of a larger-than life model of Theria molars.]
JIN MENG: And in particular the molars have like a triangular shape that combines two functions: one is cutting and one is crushing.
If I give you an apple, how do you eat it?
[A little girl bites into an apple and smiles as she starts to chew.]
JIN MENG: Your incisors cut the apple into a small piece, but you cannot swallow it.
[An x-ray view of a model skeleton chewing something with its molars.]
JIN MENG: You have to send that piece to the back and then let the molar chew it.
[A series of gliding shots across the teeth of the large mammals we saw earlier in the Museum’s hall.]
JIN MENG: The molar like this is unique for Theria mammals.
[A black-and-white artist rendering of a charred forest after the asteroid impact with a dinosaur skeleton in the foreground.]
JIN MENG: To survive a condition like after the impact, animals that have more general adaptations to different food have a higher chance to survive.
[A series of shots of modern-day Theria mammal skulls with teeth that evolved from this original unique structure. Text on screen identifies them as “modern hyena,” “modern raccoon,” and “modern red panda.”]
JIN MENG: So that pattern of tooth, it's one of the innovations in the evolution of Mammalia.
[An animation of sound waves entering a human ear and bones of the middle ear moving in response.]
JIN MENG: The second is the middle ear.
[Another artist rendering of a forest landscape destroyed by the asteroid impact.]
JIN MENG: After the impact, the day is dark and so hearing is probably more important than vision.
[A closeup of a small fossil skull. Text on screen identifies it as “Thrinaxodon: lived about 250 million years ago”.]
JIN MENG: If you look at mammal-like reptiles from before the extinction, this is an animal called a Thrinaxodon.
[The actual fossil skull is replaced by a model of a Thrinaxodon skull, with parts of the jaw painted different colors. Where the lower jaw meets the upper, we see three bones colored red, yellow, and blue.]
JIN MENG: Like reptiles, the lower jaw consisted of more than one bone.
[Text on screen identifies these bones as “bones as part of jaw joint.”]
JIN MENG: In mammals…
[A second model appears next to the first, this one identified as “opossum: alive today,” with much smaller red, yellow, and blue bones located in the back of the head, clearly not part of the jaw. It is clear that these are the same bones as in the first model, which have evolved to be much smaller and serve a different function.]
JIN MENG: …those bones gradually reduced in size, and then eventually transformed to the middle ear.
[Text on screen identifies these smaller bones as “bones as middle ear”.]
[Animations of a small, furry mammal in a post-asteroid-impact landscape.]
JIN MENG: Mammals with that kind of ear structure can hear high-frequency airborne sound to pick up the signals produced by, for example, insects.
So these animals have a much more capacity to deal, to cope with an environment that just came out from this catastrophic event.
[JIN MENG opens drawers in the Museum’s paleontology collections and holds up a fossil roughly the size of the tip of ballpoint pen mounted on a rod connected to a piece of cork.]
JIN MENG: Working on Mesozoic mammals is a challenge because Mesozoic mammals are rare, very difficult to find, and small. You have to look at them under the microscope.
[In his office, JIN MENG holds up a fossil of Mesozoic mammal jaw roughly an inch long in front of a computer screen showing a large image of a digital scan of the same jaw.]
JIN MENG: Now, with the technology of computed tomography, the CT scanner, we can get a very detailed, high resolution scan of these specimens.
[Within the CT scan, the outside of the jaw disappears and we see only the teeth inside it.]
JIN MENG: We can actually extract the tooth out of the jaw bone so that you can see the root condition.
We never knew that detail before we had this technology.
[A side-by-side image of an actual fossil skull and its CT scan.]
JIN MENG: So that gives us a lot of information about evolution that is not available 20 years ago.
[JIN MENG speaks to camera from his office.]
JIN MENG: Still in my heart, I believe that the roots of paleontology are in the field.
[A series of images of JIN MENG in various field locations from across several decades. In some he is using tools to excavate, in another he holds walking sticks and poses in front of a glaciers, in others he sits next to newly excavated fossils wrapped in white plaster, ready to transport.]
JIN MENG: I have been all over the world. And that's my dream. And I believe that that's what paleontologists should do. Because collecting fossils, that's the only direct evidence of evolution.
Discovery is forever. Discovery is forever.
About 99.9 percent of mammal species alive today—including humans—are from a mammal group, Theria, that survived the extinction of non-avian dinosaurs in the late Cretaceous and thrived over the next 66 million years. To understand what helped these animals succeed, paleontologists study ancient mammal fossils that are incredibly small, rare, and difficult to find.
Join Jin Meng, a paleontologist at the American Museum of Natural History, to explore what life was like in the immediate aftermath of the catastrophic asteroid impact—and how specialized teeth and ears, helped our very ancient relatives survive. See how Meng and colleagues are using technologies like CT scanning to unlock new information from delicate fossils that wasn’t possible even a few decades ago.