If you’ve ever been to, or lived in, or even flown over the central swath of North
America, then you’ve seen the remnant of what was a uniquely fascinating environment.
It’s all prairies and cities now, but long ago, it was rife with exotic, aquatic life.
It was full of ammonites, colossal clams, and toothy birds.
It had sharks with teeth like kitchen knives and marine reptiles with strangely elongated
necks.
And it had lots of fish, too, of course, including one species so ravenous that it was known
to swallow prey almost half the size of its entire body.
Or at least … it tried to.
This strange place was a shallow sea that split North America in two, from about 95
to 68 million years ago.
Scientists call it the Western Interior Seaway, and at its greatest extent, it ran from the
Caribbean Sea to the Canadian Arctic.
Now, the fact that there used to be an ocean where there’s now farmland might seem unusual
enough.
But for paleontologists, one of the most interesting things about this ancient sea is how it managed
to be so chock full of predators.
More than a dozen different kinds of sharks once swam over what’s now the Great Plains.
And so did lots of predatory reptiles like the sleek plesiosaurs and giant mosasaurs.
But in this watery world, large carnivores found a way to co-exist without driving each
other to extinction, swimming side-by-side for millions of years.
The question is: How?
And what happened to the ocean that once divided the continent?
The origins of the Western Interior Seaway go all the way back to the Jurassic Period,
when sea levels were about to rise, while the middle of North America was in the process
of sinking.
Starting about 180 million years ago, a plate of oceanic crust called the Farallon Plate
started approaching from the west, driving itself underneath the continental crust of
North America, along the Pacific coast.
The collision caused the North American plate to bend upward, creating mountains in some
places and, in others, the beginnings of a shallow basin.
The denser oceanic crust continued to work its way under the continent, all the way to
modern day Utah.
At some point, for reasons scientists still don’t fully understand, the Farrallon plate
started subducting at a shallower angle.
At this shallower angle, the Farallon plate pulled down on the continental crust, causing
what’s now the American Midwest to sink even farther.
And not long after that, sea levels started to rise.
From the mid-Cretaceous to the early Eocene, an increase in atmospheric greenhouse gasses
kept the planet unusually warm.
By one estimate, the average global temperature of the Cretaceous was six degrees Celsius
higher than it is today.
And warmer weather means higher sea levels.
So as North America flexed downward, ocean water slowly flowed in and by about 95 million
years ago in the late Cretaceous, the Arctic Ocean and the Gulf of Mexico were connected.
Now North America was a land divided in two: To the east, there was a large, isolated expanse
of land called Appalachia, where massive crocodiylians lived alongside early tyrannosauroids.
And to the west was Laramidia, home of some weird and wonderful dinosaurs that scientists
are still discovering.
And in between, there was water teeming with life.
Fossils of marine fauna like crinoids are common from this time.
Paleontologists have also found the hard, internal body parts of gigantic squid — some
of which may have been more than 7.5 meters long.
Clams covered parts of the sea floor, including species more than a meter across.
And other species banded together in huge masses, forming reefs.
Now, where there’s a sea, there are usually predators.
And the Western Interior Seaway had more than its share.
Fossils across the Midwest show us, for example, that more than twenty different species of
sharks once swam in this sea.
One of the most common was the 5 meter long Squalicorax.
It was probably an able predator, but its teeth and tooth-marks have also been found
on the remains of animals, like mosasaurs, that seem to have decayed.
So, some experts think that Squalicorax was also a resourceful scavenger, earning it the
nickname “the crow shark.”
Slightly bigger than that was Cretoxyrhina.
It was equipped with hundreds of teeth so sharp that it’s been called “the Ginsu
shark,” after the infomercial kitchen knives.
And maybe the scariest-looking fish in these waters wasn’t even a shark.
It was Xiphactinus a large carnivore with an upturned snout that bristled with long,
cylindrical teeth.
But of course, there was more than just fish in this sea.
Huge, predatory reptiles were out in force too.
Mosasaurs, for example, are icons of the Cretaceous seaway, their fossils having been found from
Canada to Mexico
Tylosaurus was one of the biggest, at 15 meters long, but others were more modest in size,
like a species of Clidastes that was only about a third as big.
And you can’t talk about awesome marine reptiles without giving a shout-out to the
plesiosaurs.
The Western Interior Seaway was home to more than half a dozen genera.
Styxosaurus was a 12-meter-long leviathan whose enormous neck was about half of its
total body length, while other plesiosaurs had shorter necks with longer snouts.
Sharks, mosasaurs, plesiosaurs, and big, toothy fish -- that’s a lot of predators.
And normally, an environment becomes strained when too many animals vie for the same resources.
Typically, some predators would have to go extinct, as they’re outcompeted by others
that do the same thing they do, only better.
So how did all of these big killers manage to live in the same waters?
Well, one possibility is that they all lived at different times in the Cretaceous.
After all, fossils accumulated over almost 30 million years in the seaway.
So, just because two organisms fossilized in the same place doesn’t mean that they
were contemporaries.
But a study in 2010 reviewed all of the deposits in the Seaway where some of these large predators,
like Xiphactinus, Tylosaurus, and a number of big sharks, were found.
And the results confirmed that many of them did live at the same time, but their geographic
ranges didn’t change to accommodate each other, as some experts had expected.
Instead, all of these carnivores also lived in the same places.
So it seems that the predators of the Western Interior Seaway observed a time-honored rule:
If you can’t beat ‘em, ...um... work around ‘em....
In other words, they underwent the phenomenon known as niche partitioning, in which competing
organisms evolve to live and work around each other.
And niche partitioning can work in lots of different ways.
Sometimes, predators evolve slightly different methods of hunting in order to reduce competition.
Or they might begin to eat slightly different kinds of foods.
Or they might adapt to very specific physical niches within the same habitat.
In the Western Interior Seaway, we can see how these predators found ways to co-exist
by looking for clues in their anatomy.
For example, some sharks, like Ptychodus, had flat, knobby teeth that were perfect for
crushing shells.
This suggests that they probably found their food on the ocean floor.
But other sharks, like Cretoxyrhina and Squalicorax, were built for hunting higher up in the water
column.
They both had mostly circular vertebrae, forming the kind of backbone we see in many living
sharks that have streamlined bodies for life in the open sea.
And yet, even if the habitats of these two sharks overlapped, their feeding methods may
have differed.
Cretoxyrhina had really big eye sockets, but the hollows of its skull that housed its smelling
organs were smaller than those of the crow shark.
So maybe one shark was more reliant on eyesight and the other preferred to follow its nose.
This would make a lot of sense if, as some experts think, Cretoxyrhina chased down big
fish and marine reptiles, while Squalicorax used its sense of smell to scavenge.
Another driving force of niche partitioning is body size.
Bigger predators tend to pursue bigger prey, leaving the smaller prey for smaller predators.
But even though there were a lot of big carnivores in the Western Interior Seaway, there might
have been just enough variety in size to allow them to partition their prey.
And a really useful clue here is fossilized stomach contents.
It doesn’t happen very often, but there have been instances in where Seaway predators
have been found with their last meals still intact.
Such as the massive mosasaur Tylosaurus where many specimens have been found with the remains
of other reptiles and large fish in its guts.
In one case, a nine-meter Tylosaurus apparently swallowed a juvenile marine reptile that was
2.5 meters long.
Meanwhile, some of the sea monster’s smaller contemporaries seemed to have gone after smaller
prey.
When scientists are lucky enough to find specimens of the mid-sized mosasaur Platecarpus the
belly area is often filled with fish bones.
So maybe Platecarpus had a specialized, fish-based diet and the massive Tylosaurus was more of
a generalist.
Now mind you, bigger predators don’t always go after bigger prey.
Long-necked plesiosaurs were some of the biggest reptiles in the Western Interior Seaway.
But their stomachs are often found to be filled with the smallest fish, because their tiny
heads and teeth kept them from eating anything more than about a half a meter long.
And eating tiny prey would have kept them from competing with a lot of the other predators.
So differences in body size, anatomy, and habitat helped reduce competition between
some of the seaway’s top predators.
But we know they did interact.
In fact, the evidence tells us that sometimes, they turned on each other.
Paleontologists have found teeth from Cretoxyrhina embedded in the vertebrae and flippers of
some unlucky marine reptiles--including an adult mosasaur.
Meanwhile, scars that perfectly match the distinctive teeth of Squalicorax have been
discovered on the bones of mosasaurs and other sharks--in addition to plesiosaurs, sea turtles,
fish, pterosaurs, and even the occasional dinosaur that must’ve washed out to sea!
Mosasaurs have likewise been found with the remains of sharks, plesiosaurs, and other
mosasaurs in their guts.
But by far the weirdest gastronomic story to emerge from the Western Interior Seaway
involved that toothy fish Xiphactinus.
On display at the Sternberg Museum of Natural History in Hays, Kansas is the skeleton of
a 4.2-meter-long Xiphactinus that apparently died after swallowing another two-meter fish
whole!
Nobody knows for sure, but that big meal may have ruptured an organ or two, killing the
Xiphactinus in the process.
Clearly its eyes were bigger than its stomach.
Or it's entire digestive system
So, there were a lot of ways to get eaten in the Western Interior Seaway.
But, while all of that hunting was going on, the Cretaceous sea itself was changing.
At its maximum, around 90 million years ago, the seaway covered everything from central
Utah to western Minnesota and from Mexico to Canada’s Northwest Territory.
But beginning around 80 million years ago, the heartland of North America entered another
period of geologic uplift.
The precise causes of this aren’t totally clear, but it seems likely that as the Farallon
Plate descended into the earth’s mantle, part of it detached, and relaxed its pull
on the continental crust.
The result was that the region we now call the Great Plains rose back up.
And as it did, the Seaway receded.
And yet, it’s still with us.
Fossil-bearing limestones left behind by the inland sea are common in many parts of the
Great Plains today.
And for hundreds of years, these rocks have been a popular building material.
So now, if you take a close look at the old limestone buildings in central Kansas, like
courthouses and churches, you might just see Cretaceous seashells in the walls.
Buildings like these are accidental monuments to the bygone sea that once divided North
America.
And out on the plains, farmers earn their livelihoods over the bodies of aquatic giants
— beasts that found ways to thrive, even in spite of each other.
Thanks for joining me today, and thanks to the Sternberg Museum of Natural History in
Hays, Kansas, for their help with this episode.
And of course, I want to personally thank our Eontologists,
Jake Hart, Jon Ivy, John Davison Ng and STEVE!
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