Eons

About five million years ago, a new predator arrived in America.

It had made its way from the south, crossing the still-forming Isthmus of Panama and onto the coastal plains of Texas and then Florida.

The largest of these creatures stood 3 meters tall and weighed 150 kilograms.

And in its old home, it was the uncontested apex predator, armed with all kinds of awesome adaptations that it used to kill its prey - - sometimes in pretty weird ways.

And this invader wasn't alone.

It was part of one of the biggest mass migrations in Earth's history.

It seemed more than ready to hold its own against North American predators like wild dogs and sabre-toothed cats.

But one thing set this killer apart -- it wasn't some toothy mammal or ravenous reptile.

It was a giant, flightless, carnivorous bird.

And it came to be known by one of the coolest and most richly earned nicknames in all of paleontology: the terror bird.

The story of the terror bird invasion actually begins nearly 145 million years ago, at the start of the Cretaceous Period.

This is when the last remnants of the supercontinent Gondwana -- which would later become Africa and South America -- went their separate ways.

South America became an island continent, drifting through the ancient ocean while its inhabitants lived on in isolation.

There were fantastic, gigantic versions of today's armadillos and sloths.

And there were the terror birds, which rose to become the continent's top predator after the extinction of the dinosaurs.

Over the course of about 60 million years, this group of carnivorous birds -- now known as Phorusrhacidae -- diversified into as many as 25 different species, ranging in height from 1 to 3 meters, with a variety of body types and lifestyles to match.

Some species may have been scavengers, but others were definitely predators.

And they were built to kill They had massive beaks tipped with sharp hooks - - kind of like what you find on modern raptors - - adapted for delivering powerful stabs and ripping flesh from bone.

Their neck vertebrae also suggest that they could swivel their heads quickly, which would've helped them track and strike at their prey.

And some had strong, stout leg bones that seemed better suited for kicking than for running.

Researchers think they could use their powerful legs to crack open the bones of its victims - - possibly to get to the marrow.

Plus, they had large, curved, compressed claws were perfectly suited for subduing and stabbing prey.

And unlike most other birds, many of the bones in terror birds' skulls were totally fused together.

This allowed them to use their own heads as weapons, by basically pecking stuff to death.

Other clues about how terror birds hunted come from birds we know today.

For example, many living predatory birds, like the secretary bird, kill with vicious kicks.

And terror birds' closest living relatives - - a pair of species that still live in South America called seriemas -- actually subdue their prey by picking it up and smashing it against the ground over and over again.

Which is not how I want to go Since these are the closest living analogues to terror birds, paleontologists think the extinct giants might have used the same techniques.

And so, yeah: Hence the name!

When fossils of these birds were first discovered in Argentina in the late 1880s, they were given the rather obscure name Phorusrhacos, which is thought by some to mean "bearer of scars."

But nearly a hundred years later, in 1978, after having studied these things for decades, paleontologist Larry Marshall dubbed them "terror birds," which proved to be catchier and easier to say.

Now, on its home turf, terror birds had plenty of prey to choose from, because most mammals in South America were herbivores.

But eventually, thanks to continental drift, South America got a new neighbor... North America.

This process took a long time, of course, but by at least 5 million years ago, a chain of islands had formed that linked the two continents for the first time.

And land masses weren't the only things on the move.

The meeting of the Americas marked the beginning of what scientists think may have been one of the greatest exchanges of animal life ever between two continents.

Today it's known as the Great American Biotic Interchange, when animals from both continents were suddenly free to migrate, bringing into contact all sorts of species that had never met before.

And a lot of what we know about this phenomenon, we know from the fossils of mammals.

For example, we know that, five million years ago, North America was home to deer, horses, cats, and bears ... but there were also camels, elephants, and tapirs.

And all of these groups moved south.

Meanwhile, in South America, there were marsupials, giant ground sloths, and huge cousins of the armadillo called glyptodons that moved north.

And in general, the mammals from North America were more successful in the south than the animals that made the reverse trip.

That's because, other than terror birds, there weren't a lot of large predators in South America.

So the North American animals -- from mice to canines -- did very well in their new home, and they diversified like crazy.

In fact, half of the mammal genera living in South America today are descended from North American immigrants.

But North America had way more big predators than the southern migrants were used to.

So, most animals that moved up from South America didn't last very long.

And this included the terror birds.

We know from fossils that at least one type of terror bird followed its prey north -- Titanis walleri, one of the largest terror bird species The earliest evidence of Titanis in the US has been found in Texas, in strata dating back 5 million years, to the late Paleogene Period.

Which is weird, because scientists think that the land bridge between North and South America probably wasn't complete until about 3 million years ago.

So, even though they couldn't fly, these giant birds must've somehow managed to float, or swim, or walk through the shallow waters that connected the islands between the two continents.

From there, Titanis roamed the open, coastal plains, eating anything it could chase down, kill, and swallow whole.

But in its new home, for the first time, Titanis had to deal with competition from other big predators -- like sabre-toothed cats and the ancestors of modern wolves.

Then, around the beginning of the Pleistocene Period, the outlook for the terror birds got even worse, when the climate began to change.

Temperatures grew colder, and glaciers began their march south.

Soon, beset by advancing winters and bigger predators, the last North American Titanis met its end around 2 million years ago, as the most recent Ice Age started to set in.

So, the invasion of the terror birds turned out to be brief -- less than three million years.

And because their time here was so short, they didn't leave much evidence behind.

The entire fossil record of Titanis in North America consists of just a few dozen bones and bone fragments -- mainly of the neck, legs, feet and toes -- found at only four sites in Florida and one in Texas.

But their migration was just one small wave of the Great American Biotic Interchange, which turned out to be a crucial chapter in the history of the Americas that changed life on both continents forever.

In the end, the time when terror birds came to North America is an important reminder of how big changes can create lots of awesome new opportunities for some of us ... while also creating tremendous pressure to either adapt.

Or disappear.