[chitters] [thrilling music] -[Narrator] Utahraptor is the largest species of raptor that ever lived.

[rasping screeches] [intense music] But until relatively recently, no-one knew this fearsome predator even existed.

[heavy footsteps] [sharp, piercing call] [gentle music] In the early 1990s, paleontologist Jim Kirkland was working in the Utah desert.

When his team discovered a huge claw belonging to an unknown dinosaur.

-[Jim Kirkland] Well when I first saw this claw, you know I got real excited, we need to find more of this animal.

This is something new and it is huge.

-[Narrator] Jim thought it might belong to a new species of raptor.

[blows] But to be sure, he needed more evidence.

Then in 2005, he got the lucky break he needed.

-[Jim] A geology student reported some dinosaur bones coming out of the base of a cliff.

And we went out and excavated, we found it was such a dense accumulation of bones you couldn’t put an icepick in without hitting something.

[clanking] -[Narrator] So many bones so close together meant the team had to take them all out in one huge slab.

-[Jim] We ended up having a nine-ton block filled with skeletons.

-[Narrator] After nearly a decade of excavation, they finally got it back to the lab.

-[Jim] Lift.

And fold.

-[Narrator] And could begin to uncover what was inside.

-[Jim] There’s, uh, more therapod material than anything I’ve ever seen.

Yeah, this thing, you know, at least 95% meat-eating dinosaur bones.

-[Narrator] The first bones to emerge all belonged to small dinosaurs.

-[Jim] Okay.

-[Narrator] But deeper into the rock, the team began to find much larger bones.

-[Jim] We pulled a big tibia out of there with the ankle in place, we have part of an adult tail here, more of the base of the tail right there.

Have a big adult animal.

And those bones are stout and massive.

-[Narrator] Jim had finally found the dinosaur his mysterious claw belonged to.

-[Jim] So this thing is built like Arnold Schwarzenegger, it’s anchoring massive muscles with long blades on their feet and hooked raptorial claws on their hands.

[rasping screeches] -[Narrator] Utahraptor, the size of a grizzly bear.

But with most raptors the size of a turkey, how did Utahraptor get so big?

Jim thinks the answer could be linked to a little known extinction event at the end of the Jurassic.

-We’re putting together a new story that shows a major extinction in Earth history that hadn’t really been recognized before.

We lose three major groups of large predatory dinosaurs, the kings of their creation, they all go extinct.

-[Narrator] For reasons no one fully understands, [snarls] big carnivores like Torvosaurus and Allosaurus completely disappear from this part of North America.

-Other animals do survive elsewhere but in this region we have nothing.

It’s almost a total wipeout.

-[Narrator] But the extinction of one species creates opportunities for the survivors.

With all the large predators gone, there was a gap at the top of the food chain.

-My baby.

-[Narrator] And over millions of years, Utahraptor evolved to take over.

-[Jim] There’s this niche open, you know, where no-one is filling it, these things just got real big to take advantage of the hole in the landscape.

-[Narrator] Transforming into a new and terrifying top predator.

-[Jim] The Utahraptor liked to stalk in the brush where the prey had no clue they’re there till they were in striking distance.

And as soon as they did it, it would flex open its arms, you know, and leap screaming at its prey, it would happen so quick when you look at a lion attack or leopard attack, bam they’re on it.

-[Utahraptor screeches] -[Gastonia grunts] And that’s how Utahraptor would be too.

-[Narrator] A fortuitous find in the desert and an evolutionary lucky break gave us the largest raptor that’s ever lived.

[heavy footsteps]