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.