[grumbles] -[Narrator] Millions of years ago, the land was dominated by dinosaurs.

But it's long been thought that this was the boundary of their kingdom.

[water gurgling] Underwater, plesiosaurs and ichthyosaurs reigned supreme.

Often mistaken for dinosaurs, these were actually marine reptiles.

No trace of a waterborne dinosaur has ever been found.

Until now.

New evidence is telling a different story about what went on beneath the waves.

Of what would be the world's first aquatic dinosaur.

And that story starts in the desert.

[drums pounding] While digging in the Moroccan Sahara, paleontologist Nizar Ibrahim and his team unearthed something amazing.

The gigantic bones of a Spinosaurus.

An 8 ton, 39-foot-long apex predator.

-[Nizar Ibrahim] So, a Spinosaurus is several meters longer than T. Rex, so it is the largest predatory dinosaur known.

-[Narrator] It was the most complete Spinosaurus skeleton ever found.

For more than 100 years, Spinosaurus has been known as a ferocious terrestrial predator.

But as he was reassembling it, Nizar noticed something strange.

-[Nizar] This dinosaur has water loving written all over it.

We realized that this was really a river monster.

-[Narrator] It's a potentially game changing hypothesis.

This would make it the first ever aquatic dinosaur, overturning more than a century of scientific thinking.

But no new theory is without its skeptics.

Nizar knows he's going to have to prove it.

So back at his Portsmouth University lab, he went to work.

Spinosaurus' most striking feature was a long paddle-like tail.

-[Nizar] When we first looked at those tail bones we thought well, "That looks like a propulsive structure."

-[Narrator] But lots of dinosaurs have powerful tails.

The question was whether this one could propel 8 tons of dinosaur.

Could Spinosaurus really swim and hunt underwater?

[water gurgles] To find out, he teamed up with engineers from Harvard University.

They designed a special tank to test Spinosaurus' prowess in the pool.

-We built a robotic Spinosaurus tail in a tank to compare the performance of a Spinosaurus tail fin to the tails of other predatory dinosaurs.

-[Narrator] Spinosaurus' tail was almost three times better at pushing through the water than any other dinosaur tested.

It was comparable to the powerful tail of a crocodile.

To Nizar, this was the first evidence that Spinosaurus could easily hunt fish underwater.

But the tail wasn't the only clue that this was a specialist fisher.

Using the bones, they created an accurate model of Spinosaurus' skull.

And it had a very unusual jaw that Nizar thinks was perfect for catching fish.

-That the opening here can actually be widened to basically deal with the large, big, chunky fish that Spinosaurus would have fed on.

-[Narrator] Even its unusual teeth looked like fishing tools.

-These are the conical teeth of a fish eater.

-[Narrator] Meat eaters tend to have thinner, flatter teeth for tearing their food apart.

Conical teeth are better for spearing and hanging on to slippery fish.

Thousands of these teeth have been discovered in the beds of the ancient rivers where Spinosaurus lived.

To Nizar, it's a big clue it spent most of its time in the water.

-[Nizar] Dinosaurs will go through a lot of teeth throughout their lifetime.

Spinosaurus will shed teeth, and you know, new replacement teeth grow.

But the only way you can get this abundance of Spinosaurus teeth is really by having an animal that spends almost all of its time inside the river.

-[Narrator] But there's another piece to this puzzle.

If Spinosaurus is truly the first aquatic dinosaur, even its brain will be adapted to life in the water.

And Nizar can figure out its structure thanks to an important skull bone, the brain case.

-[Nizar] The brain case is essentially the back of the skull, the part that encloses the brain.

If we look at the brain case, we can reconstruct the actual shape of the brain that was inside.

-[Narrator] Using the x-rays of a CT scanner, Nizar can look inside these fragments and recreate a detailed 3D image of the brain.

And it's already given him one big clue.

-[Nizar] But they look pretty good, right?

-[Man] Yeah, yeah, yeah.

These look great.

-[Narrator] It looks like major nerves ran from the brain to openings at the far end of the upper jaw.

-[Nizar] Crocs and alligators have openings at the tip of their snout.

And these openings house pressure receptors.

-[Narrator] These sensors make the snouts super sensitive to changes of pressure underwater, such as the movement of fish.

[water gurgling] In the murky waters that Spinosaurus called home, this would have allowed it to catch prey based on the slightest vibrations.

A key adaptation if you spend most of your time hunting in water.

-And it's quite likely that Spinosaurus would have taken full advantage of, of this.

-[Narrator] The evidence is mounting up.

But Nizar knows to upend a century's worth of conventional wisdom, he'll need to build a watertight case that Spinosaurus left behind a life on land for the water.

Because if he's right, it could change much of what we thought we knew about this dinosaur.

[gentle music]