How a Sinkhole Revealed a Lost World

In May of 2000,

a road construction project outside the town of Gray in eastern Tennessee revealed something

completely unexpected.

In the middle of what was supposed to be a roughly 480 million year

old bedrock from the Paleozoic Era were fossils of plants,

invertebrates, and vertebrates, like mammals that lived hundreds of millions of years later.

Scientists soon realized this was a window into a prehistoric lake

that existed about 5 million years ago in the shadow of the Appalachian Mountains.

In addition to revealing a world of toads, turtles, and tapirs,

the site also contained some surprising species generally known from Eurasia – including a

close relative of the red panda.

But how did a red panda relative end up in North America?

What can this tell us about how long ago –

and how many times – North America was connected to Europe

and Asia?

And how was this lakeside snapshot even preserved in the first place?

Well it turns out,

those old rocks are actually what made it possible for us to glimpse into

the world of the red panda of East Tennessee.

Around 480 million years ago, in the Paleozoic Era,

shallow seas covered what’s now Tennessee.

Like in much of eastern North America at the time, including the Appalachian area,

these seas alternated between advancing and retreating, evidenced by marine fossils.

And these fossils were left behind in carbonate rocks like limestone

that were made from their remains,

eventually forming the Knox Group rock formation.

This shallow ocean environment continued to lay down carbonate rocks

and fossils intermittently throughout the rest of the era,

until the sea retreated once more in the Pennsylvanian Period about 320 million years

ago,

when the landscape then transformed into one of vast swamps

that would later become the state’s coal deposits.

However, something happened after this period: the fossil record kind of disappeared.

By the Mesozoic Era, around 250 million years ago,

the supercontinent of Pangea began to break up

and the Appalachian Mountains were no longer being built.

In Tennessee, the Western Interior Seaway connected with the Gulf

and covered the western portion of the state.

This resulted in parts of the Late Cretaceous being preserved in the fossil record,

between 100 and 66 million years ago.

Not long after, the Seaway retreated and over the next 30 million years,

parts of the state were occasionally submerged by deltas,

as well as marine water from the Gulf,

preserving some fossils until after the Eocene, around 33 million years ago.

At this point, the fossil record completely ceased,

and there’s a huge gap that extends for about 30 million years,

up until the Pliocene epoch.

The reason for this loss of the fossil record is probably due to the

amount of erosion versus deposition in the environment.

Areas where there is greater deposition than erosion tend to have more sedimentary rock,

the rock we find fossils in.

See, environments like those at the base of mountains, or in shallow seas,

tend to lead to more deposition than erosion, resulting in more sedimentary rock.

And by the start of the Cenozoic, 66 million years ago,

after the Appalachian mountains were built and the seas retreated,

Tennessee and much of the surrounding area lost both of these depositional features,

meaning there was more erosion.

But just because there’s no record of this time,

that doesn't mean that animals didn't live there…

And thanks to the incredible preservation at Gray,

we have some insight into one snapshot in time from this otherwise large gap.

5 million years ago,

the area was a warm lake environment surrounded by forest,

with a climate similar to Tampa Bay, Florida today.

And while grasslands had become dominant across the globe,

Gray was a closed forest environment filled with hickory and oak trees.

Which probably made it a refuge for non-grassland animals in North America.

It housed lake animals that are found today in warmer temperatures – like alligators.

On land, tapirs roamed alongside the prehistoric rhino Teleoceras, elephant-like mastodons,

and saber-toothed cats.

Some of these animals are notable browsers and mixed-feeders,

unlike the grazers found in western North America at this time.

Gray even records tiny animals such as lizards, snakes, moles, and rodents.

But also, some unexpected creatures were present – like a relative of the European badger,

the plant known as Moonseed, and even a primitive relative of the red panda.

This panda-relative, named Pristinailurus

and also known as Bristol’s Appalachian panda after its discoverer,

had similarities to the red panda we know today – like in its overall build.

But it was much larger in size.

It’s estimated that Pristinailurus weighed between 8 and 15 kilograms,

compared to the average living red panda at 5 kilograms, for example.

We know this because,

unlike the otherwise sparse fossil record of the red panda

and its relatives found mostly in Eurasia,

Gray preserves two nearly complete skeletons.

And this rich fossil material gives us insight into the evolution of the red

panda family.

The Gray finds reveal that both Pristinailurus and its close relative,

the modern red panda,

probably descended from an older relative shared with the mountain lion sized Simocyon,

but with a twist.

While Simocyon was from a line of red pandas

that ate a more of a carnivorous diet,

Pristinailurus had taken a step toward the modern red panda’s more herbivorous diet.

Pristinailurus did have teeth designed for crushing plants,

but it also retained the ancestral ability to eat meat.

This means it was more omnivorous than its modern relative

which relies heavily on plants such

as bamboo.

The “false thumb” of Pristinailurus was also smaller than the modern red panda,

meaning it probably spent more time on the ground rather than in the trees.

But Pristinailurus and other typically Eurasian species found at Gray suggested immigration events from

Europe to North America

that scientists didn’t know about.

See, we know connections to Eurasia allowed animals like cats to arrive in North

America during the Miocene,

around 18 million years ago.

Additionally, later Pleistocene migrations introduced mammoths to North America around 1.5 years ago

and bison less than 200,000 years ago.

But Pristinailurus implies another immigration event in-between these two,

sometime between the Miocene and Pliocene.

So how does a relative of today’s red panda get from Eurasia to North

America?

During the middle Miocene, around 15 million years ago,

deciduous forests stretched throughout the northern hemisphere,

allowing red panda relatives and other forest-loving species to spread.

But by the time of Gray, many of these forests collapsed,

leaving isolated pockets of forests, and in turn,

isolated animals from one another like Pristinailurus in Tennessee.

But how do we even know all this?

What is it about the Gray site

that allows it to preserve such a rare record of this otherwise invisible period

of time?

Well the reason can actually be found in the older rock record from the

Paleozoic that surrounds it!

Carbonate rock like the limestones

and dolostones of the Knox Group has a unique property in

that it dissolves when met with a weak acid,

such as in acid rain.

And over millions of years since the rock was laid down in the Paleozoic,

acidic rain had been slowly dissolving that bedrock.

In Tennessee and much of eastern North America,

some of these limestone bedrocks consisted of thinly-bedded and highly fractured areas.

…Which allowed acidic water to go deeper into the bedrock

and pool in certain places.

This pooling would continue to erode the rock in those areas underground,

creating larger cavities for water, leading to what are called karsts.

Karsts are a type of landscape where water interacts with the subsurface,

traveling for long distances underground and creating a series of caves and waterways.

As the erosion continues, the bedrock dissolves and cavities can subside or collapse.

This leads to sinkholes forming on the surface, which then fill with debris, soil,

vegetation, and water.

The process is what led to the creation of the ancient lake

that existed in Gray 5 million years ago,

as the Gray sinkhole was filled in by water,

attracting the animals that lived nearby.

Over time, the lake sediments buried any flora

and fauna that ended up in there too,

becoming isolated from sunlight and oxygen.

This created the exceptional preservation of everything from animals

and plants to the soil of the ancient Gray environment.

Eventually, the sinkhole continues this process until it is sealed, becoming a time-capsule.

And after a few more million years of erosion –

and a construction project – opened the “time-capsule” back up,

it revealed fossils from that time in pristine condition.

Since its discovery in 2000,

Gray has revealed a world that was largely a mystery in eastern North America.

One that had not only tapirs browsing on the ground,

but red pandas climbing around the tree canopy.

A world where organisms from the expanding western grasslands met with closed-forested refugee taxa

– and where North American species met with newly arrived Eurasian ones.

And this entire world would have remained a mystery to us today

if it wasn’t for the bedrock surrounding the sinkhole.

It’s interesting how the rock from the past can still be used to preserve

a time separated from it by hundreds millions of years.

But this process tells us about the previously unknown migrations

while giving us a glimpse into the world of the red panda from eastern

Tennessee.

Now what about the other - unrelated - Panda.

How does a bear --

which is a member of the order Carnivora -- evolve into an herbivore?

Despite how it looks,

nothing about the history of the giant panda is black and white.

Find out more in our episode, “The Fuzzy Origins of the Giant Panda”.

We also gotta thank this month's impactful Eontologists!

Addie, Annie & Eric Higgins, Carl Woelfel, Jackie Scott-Ralston, Jake Hart, John Davison Ng,

JuanM, Melanie Lam Carnevale, Nico Robin, Raphael Haase AND S.T.E.V.E!

By becoming an Eonite at patreon.com/eons,

you can get fun perks like access to behind-the-scenes content,

including clips from Eons shoots!

And as always thanks for joining me in the Ken Barns studio.

Subscribe at youtube.com/eons for more fantastic fossils.

Despite how it looks, nothing about the history of the giant pland...

Planda?

Giant panda is black and white...

Planda!

That's like a new app.

"Get your life together with Planda!"

I don't think that can go in bloopers, just kidding.