[Narrator] Evolution can sometimes appear like an arms race.

Each improvement by attack or defence then needs to be matched by the enemy.

This arms race took place over more than 20 million years, between the ferocious Utahraptor and its would-be prey, Gastonia.

-[Jim Kirkland] Here we had raptors and armoured dinosaurs here for 22 million years.

So, it was an arms race, both of offensive weapons and defensive.

-[Narrator] Over 135 million years ago, Utahraptor had taken advantage of an earlier extinction event.

In the absence of other major predators, Utahraptor ballooned in size, twice as long and up to 10 times heavier than the iconic Velociraptor.

making Utahraptors the largest raptors ever to walk the earth.

And suddenly, much bigger herbivores were on the menu, including Gastonia.

But Gastonia was far from defenseless.

-This is the spiniest dinosaur that probably ever lived.

-[Narrator] Palaeontologist Jim Kirkland has found hundreds of Gastonia bones across the Utah desert, all of which have revealed that this animal was armoured like a tank.

-Spines on their back, spines on their sides, spines on their tail.

And this animal, these elements seemed to be the biggest, you know, the spiniest of each one of these things.

-[Narrator] Gastonia's spikes would have made it a formidable target for any hungry Utahraptor.

And if that wasn't enough, these blades could overlap, slicing through attackers like a pair of scissors.

-You know, think of someone who try to protect themselves, you know, with a chainsaw blade on a handle.

The way they would swing their tail and the way these things would crisscross, you know, these things would chop into a predatory animal because their tail is so powerful.

You know, a slash into a predatory animal the neck, the head, you know, cause extreme damage and they would back off.

-[Narrator] To take down such a formidable opponent, Utahraptor had to up its game.

And luckily for them, they had both a deadly weapon and cunning tactic to shift the odds back in their favor.

If they were able to flip Gastonia over, they'd render them defenseless, exposing their softer underbelly, which Utahraptor would swiftly rip to shreds with its 10-inch claws.

Predatory tactics like this would have required complex cooperation between different raptors.

And luckily for them, Utahraptors were highly intelligent, which we can tell by looking at their brain case.

-[Jim] Look at the size of this.

It's about the size of a parrot's brain.

And you figure how intelligent a parrot is.

This was a very smart dinosaur, no doubt.

-[Narrator] Advantage, Utahraptor.

But Gastonia wasn't going to take this lying down.

And the benefit of ganging up was not lost on them either.

Most armoured dinosaurs seem to have lived solitary lives, but Gastonia developed the skull of a distinctly social animal, which indicates that they grouped together for protection.

-[Jim] You know, when you put the head in right position, just tilt it down a bit, the eyes can look right at you for it.

You know, look them in the eye.

Plant eating animals rarely do that.

-[Narrator] And for a dinosaur that wouldn't have been able to run away, this could have been essential in tipping the balance back in their favor.

But just when it looked like this arms race was heading for a stalemate, Gastonia had one more trick up its sleeve.

Because Jim thinks that not only did they group together, they fought together.

Like an elite military unit, they turned inwards to face each other so they could coordinate their movements.

-They could have had inflatable nasal sacs, chambers and things in there so that then they could vocalise and probably very distinctly and be able to, you know, look at each other and probably make, you know, signals.

-[Narrator] Leaving their attackers facing an impenetrable wall of their greatest weapon.

-[Jim] These animals, you know, when they're forming a defensive structure, they'd be looking forward into the ring and swinging their tails.

The defense, if these things did go into a group, you couldn't get through them.

I don't see any way you could do it.

They're almost impenetrable.

They're probably just about immune to Utahraptor.

-[Narrator] While Utahraptor would undoubtedly still have attacked Gastonia, facing not just the spikiest dinosaur known to history, but a whole brigade of them, it suggests that this time the arms race was won by the defense.

[guttural grunting]