A little over 10,000 years ago, North America was home to the largest beaver that ever existed.

And, yeah, I know I talk to you all the time about giant extinct animals, like the biggest frog, the longest snake, the hugest dinosaurs.

But it’s important to me personally that you understand how big this beaver was.

It was more than twice as heavy and many, many times taller and longer than a modern beaver.

It rivaled the size of a small black bear!

Its name was Castoroides and, of course, it’s extinct now.

Because, you’d definitely know if it was still around.

But, in the not-too-distant past, this giant rodent lived as far north as Alaska and the Yukon Territory, and as far south as Florida.

And back then, beavers used to be a lot more diverse.

For example, not all beavers were swimmers.

There were also terrestrial beavers that dug burrows and twisty tunnels.

So, why are we left with just two species today -- the American and the Eurasian beavers?

And why isn’t one of them the giant beaver?!

Well, the answer to both questions might involve behavior.

Castoroides stuck around for about a million years.

And just like modern beavers, it was semiaquatic -- it lived both on the land and in the water.

The difference is that today’s beavers do a pretty special thing - one that the giant beaver probably didn’t, or couldn’t, do.

And that one change in its behavior may have been its undoing.

Beavers make up a family of rodents called the Castoridae.

And so far, about 30 genera have been identified in the fossil record.

Scientists think beavers originally evolved in North America toward the end of the Eocene Epoch, with the earliest known species scampering around Wyoming 35 million years ago.

There’s been some debate about what its niche was, but it might’ve been semiaquatic.

And by the Early Oligocene Epoch, fossils from China tell us that beavers were quick to establish themselves in Eurasia.

Between about 35 and 24 million years ago, different lineages of specialized beavers started to appear.

For example, one group was mostly fossorial - they were digging animals.

We know this in part because they had classic burrowing traits like big forelimbs and front claws, as well as a short tail and neck.

And many would’ve even used their teeth for digging too.

Take Palaeocastor, a small beaver from what’s now Nebraska.

During the Oligocene and Miocene Epochs, it made huge, spiraled dens in the earth.

Later, those burrows filled in with sediment, which solidified, leaving trace fossils called “Devil’s Corkscrews,” which we’ve actually done a whole episode on!

Now, the digging beavers stayed pretty small, only measuring about 20 to 30 centimeters long.

And they tended to live in open habitats, places where trees and woody plants weren’t really common.

Which wouldn’t have worked out so well for modern beavers, whose diets include a lot of wood.

They are part of another important beaver subgroup, one that evolved by the end of the Oligocene.

Unlike Palaeocastor and its burrowing cousins, these animals were semiaquatic.

Paleontologists think that the common ancestor of Castoroides and living beavers was a semiaquatic woodcutter that lived in the early Miocene 20 to 24 million years ago.

And some of the “semiaquatic” beavers were probably good at burrowing as well as swimming, just like today’s beavers are.

And while experts aren’t sure if Castoroides dug burrows, they know it was a swimmer.

The oldest fossils of this giant beaver that we know of come from Florida, between 1.6 million and 1 million years old.

Back then, Florida was a combination of mixed-tree woodlands and forested swamps, but wetlands weren’t limited to the American south.

In the giant beaver’s heyday, the ice sheets of the Northern Hemisphere expanded and retreated multiple times.

And it looks like the giant beaver came and went with those warm periods.

So, when the ice sheets advanced, places like Yukon Territory became cold, dry grasslands.

But during the warmer interglacial periods, forests bounced back.

And the water from melting ice sheets produced wetlands up north.

The fossil record shows Castoroides repeatedly invaded the Arctic during those warmer, wetter periods.

Which makes sense, because it was clearly built for a semiaquatic lifestyle.

It had relatively short legs and big back feet that were probably webbed.

But on land, it would’ve had a pretty awkward walk.

And by awkward, I mean probably adorable Because I mean, the thing was big, up to 2.5 meters long, and weighing about 100 kilograms.

Luckily, Castoroides could monitor its surroundings without leaving the water.

Like hippos, crocodiles and modern beavers, its eyes were near the top of its skull.

This allowed it to look out for predators on land while staying safe in the water.

So, Castoroides sounds a lot like modern beavers.

And fossils show that it lived alongside the modern American beaver.

Remains of both species sometimes appear at the same dig sites.

So why aren’t the giant beavers still around?

Like all rodents, Castoroides had big incisors on its upper and lower jaws that never stopped growing.

Rodents constantly gnaw on things to keep these from getting too long.

And Castoroides had incisors that were bigger and curvier than those of living beavers.

The giant beaver’s incisors could grow to be over 20 centimeters long!

But they didn’t have the chisel-like tips that modern beavers have.

With their shorter, sharper teeth, today’s beavers have no trouble chopping down trees - which makes them excellent landscapers, as well as builders.

With mud and logs, they build sophisticated lodges.

The ones they use in the winter are especially good at retaining heat, even though they have air vents.

And their dams are also, just, really impressive.

A beaver dam can be anywhere from one meter to 850 meters long.

And the tallest are 5 meters high!

And it’s hard to overstate just how much these structures deeply affect the environments around them.

That’s what makes American and Eurasian beavers textbook examples of Ecosystem Engineers.

An ecosystem engineer is any organism that physically changes its habitat in a way that affects other species, by altering the availability of certain resources.

And beaver dams do just that.

They can change the flow of streams and rivers, and often cause ponds to form, creating habitats for fish, amphibians, and other creatures.

Plus beavers also leave their mark on forests.

Some beavers cut down trees as far as 200 meters away from their home pond.

This has a huge influence on which plants grow where.

Now, the two modern beavers belong to the genus Castor which likely evolved sometime in the past 10 million years.

And their dam- and lodge-building behaviors may have evolved as a way for them to survive hard winters at higher latitudes.

So, because both species are builders, some experts think the very first beaver dams and lodges might’ve been built around this time.

But it looks like woodworking wasn’t really Castoroides’ thing.

Based on a study of stable isotope signatures found in its teeth and bones, which can help tell us what an animal ate, it looks like Castoroides mostly ate freshwater plants, like water lilies, pondweeds, and sedges.

So, diet-wise, it had more in common with today’s muskrat than it did with either living beaver.

And if Castoroides didn’t chow down on logs, then it probably wasn’t using those things to make lodges or dams.

And that would’ve left the giant beaver at the mercy of Earth’s changing climate.

When the most recent glacial period ended around 11,700 years ago, things got warmer and drier, and the swamps and lakes that Castoroides depended on became more scarce.

Slowly but surely, its range got smaller and smaller It was eventually driven out of the Great Plains, as wetlands and forests gave way to arid grasslands.

Meanwhile, in Florida, there was a temporary rise in scrubland and prairie habitats, which coincided with the giant beaver’s disappearance there.

The last known population of Castoroides died out in the Great Lakes region about 10,000 years ago.

But the Eurasian and American beavers are still with us, maybe because, unlike Castoroides, they can customize their habitats by building lodges and dams.

So when the climate changes or when resources grow scarce, they’re less vulnerable than some other species.

Like its giant ice age colleagues, like the Columbian mammoth, the short-faced bear, and other mega-mammals, Castoroides had some impressive proportions.

But today’s beavers have engineering on their side.

And whether you’re big or small, if you can shape the world around you, it could mean