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! If you’d like to join them in supporting this channel, head over to patreon.com/eons and make your pledge for some neat n nerdy rewards. Now, if you’re interested in the past, and how things got to be where they are, you HAVE to watch The Origin of Everything a show that uses history to answer the big questions that are hidden in the stuff of everyday life, from tattoos, to middle names, to conspiracy theories. Check out the link in the description! And lastly, what do you want to learn about? Leave me a comment, and don’t forget to go to youtube.com/eons and subscribe.
