- [Narrator] Sea monsters are considered

to be mythical creatures at the center of tall tales.

(lighting crackling)

But science tells a story of real-life monsters

lurking in Earth's prehistoric seas,

monsters called plesiosaurs.

Plesiosaurs were ancient marine reptiles

that first appeared about 215 million years ago.

They belonged in the order Plesiosauria,

which is divided into two groups:

the long-necked plesiosaurs and the short-necked pliosaurs.

Plesiosaurs were massive animals.

They grew up to 43 feet long,

with large bodies and small, triangular heads

with smooth sharp teeth.

Their most signature feature was their long necks.

Spanning half the length of their body,

plesiosaur necks had up to 76 distinct vertebrae,

over 10 times more than that found in humans.

Older depictions portray these necks

as willowy and snake-like,

but recent discoveries have shown

that they were, in fact, relatively stiff.

This allowed the plesiosaurs to stay streamlined

while swimming or while hunting fish, squid, and clams.

While the Plesiosaurs evolved long necks and small heads,

their pliosaur cousins went the opposite direction.

Their necks were short and their heads were large,

measuring up to 10 feet long.

But much like plesiosaurs, pliosaurs were massive,

growing up to 50 feet long

and weighing almost 100,000 pounds.

Despite their large size, pliosaurs were excellent swimmers,

capable of reaching speeds near 10 kilometers per hour.

Their speed, along with their great size,

allowed pliosaurs to become ferocious predators,

devouring large marine animals and even other Plesiosauria.

Despite their differences,

pliosaurs and plesiosaurs shared a few key features

that were characteristic of the genus Plesiosauria.

They used all four of their flippers to swim

in a form of underwater flight,

similar to the movement of a dragonfly.

And while they spent their entire lives in the sea,

plesiosaurs had no gills.

Rather, they were air-breathers,

coming up to the surface for oxygen before every dive.

By about 66 million years ago,

these powerful predators died out;

but today, they continue to be a point of fascination,

inspiring legends as grand as the monsters themselves.