Hidden inside ordinary-looking rocks, an astonishing trove of fossils reveals a dramatic new picture of how rat-sized creatures ballooned in size and began to evolve into the vast array of species—from cheetahs to bats to whales to humans—that rule our planet today.
Paleontologists Discover New Mammal Fossils Hidden in Rocks
Published: October 25, 2019
Narrator: In the bluffs outside of Denver, Colorado, paleontologist Tyler Lyson is searching for evidence of a pivotal moment in our evolution: the rise of mammals after the demise of the dinosaurs.
Tyler Lyson: The period of time right after the extinction of the dinosaurs, 66 million years ago, is one of the least understood moments in time in all of earth’s history.
At Corral Bluffs we find the rocks of the right age.
So we are looking for things like mammals, turtles, crocodiles, birds and plants and trying to combine all of that into a reconstruction of the environment.
Narrator: Tyler, an experienced dinosaur hunter, was ready to apply his fossil finding techniques to the site.
Ian Miller: …so far.
Lyson: Hardly, anyway.
The old search image was sort of classic paleontology where you go out, walk the bottom of a, of a gully or the base of a cliff and you’d find broken bits of bone.
Narrator: Then, you head head uphill, looking for bigger bones weathering out of the dirt. But at Corral Bluffs, the search for early mammals was coming up empty.
Lyson: I immediately started searching for bone and not finding anything. So I was pretty frustrated.
What could I do to, to find fossils here?
Narrator: The fossils were there. Tyler was just looking for them in the wrong way. Until he came across an old specimen in the Museum's vault. It was a mammal skull, embedded in a round rock.
Lyson: And that's when sort of the lightbulb went off and I was like, “Oh, well maybe concretions.”
James Hagadorn: Concretions are kinda like nodules—like an egg or a ball that you find in a rock, an ancient rock.
Narrator: James Hagadorn is curator of geology at the Denver Museum of Nature and Science. He's a colleague of Tyler's and has lots of experience with concretions.
Hagadorn: So if you have a skull or a tooth or even a bit of poop and it falls to the sea floor or it ends up on the bottom of a lake, um, and gets buried, it is compositionally different than the rest of the sand around it or the mud or whatever.
Narrator: With typical fossils, the organic matter would slowly be replaced by minerals, leaving petrified bones within the surrounding sediments. But with concretions, the minerals encase the organic matter as well, creating a distinctive rock within a rock, with a fossil inside.
Hagadorn: That kaleidoscope of minerals-get attracted to that tooth or skull or tree root or something, and they start to grow around it layer by layer in the sediment, kind of like a pearl in an oyster.
Narrator: The mineral casing can preserve otherwise fragile fossils, just like those of the early mammals Tyler is looking for.
Lyson: The concretions form around these bones and it creates this hard protective shell. And then these fossils in their nice protective casings, roll down the hill.
And then we crack it open and sure enough we started finding bone inside.
Male student: He just found a mammal skull.
Lyson: I just found a mammal skull!
And that was a complete game changer for this entire project.
Narrator: Tyler and the team split open concretion after concretion revealing hundreds of fossils, including dozens of mammals from the period right after the dinosaurs went extinct.
Lyson: People have been looking for fossils in Corral Bluffs for over a hundred years and they just didn’t look for concretions. They were looking for actual leaves or they were looking for bones.
Narrator: It was an honest mistake. The concretions here are formed by a mineral called apatite, which is not usually associated with terrestrial sites like the Denver Basin.
Hagadorn: Apatite is the same mineral that my teeth and bones are made out of and so are yours.
In marine settings, in ancient oceanic deposits, they’re pretty common. Prior to working on this project, I had never seen an apatite concretion in a terrestrial setting. It's super rare.
Narrator: Yet here in plain sight, apatite concretions have been hiding priceless specimens from one of the most important periods in mammal evolution.
Hagadorn: Tyler has a paleontological goldmine right there. I mean, there's just nothing like it.
Wouldn't it be cool if you could find more of them? Cause nature repeats itself.
Whatever processes led to the formation of those fossils and the preservation of 'em, ought to repeat itself elsewhere in the world.