In 2002, researchers studying fossils from  Transylvania dating to around 70 million years ago reported an unusual discovery.

They were analyzing a few big bone  fragments - bits of a skull and humerus - which had first been  described a decade earlier.

At the time, they were thought to belong  to a huge theropod dinosaur - possibly some kind of tyrannosaur, on  account of their enormous size.

But upon their re-analysis, the  researchers realized that these bones weren’t those of a theropod at  all, or any other kind of dinosaur.

Instead, they belonged to a colossal pterosaur  -- easily one of the largest ever discovered.

They named it Hatzegopteryx  [HOTS-egg-OP-ter-icks].

Its huge size wasn’t the only  remarkable thing about it.

Unlike other pterosaurs that  had a generally slender build, the bones of this specimen seemed uniquely  stocky, robust, and built for strength.

And its initial mistaken identity as a giant  theropod is pretty understandable because today, it’s thought that Hatzegopteryx was basically a  pterosaur playing the role of a giant theropod.

As it turns out, its bizarre proportions were  likely the result of a particular ecological situation -- one that turned a  pterosaur into a giant apex predator and the dinosaurs it hunted into dwarfs.

In the Late Cretaceous Period, Europe was a  fragmented archipelago - a collection of islands, some large, some small, that had become  isolated due to rising sea levels.

And while Europe’s fossil record  from this time is often patchy, the Hateg basin region of  Romania where Hatzegopteryx was discovered has provided some key insights  into the area’s weird animal life.

Back then, the basin was one of the archipelago’s islands, Haţeg Island, which was a little smaller than Ireland in size and 200 to 300 kilometers away from the nearest landmass.

And as far back as a hundred years ago,  paleontologists excavating in the basin realized that there was something strange about its dinosaurs: they seemed….miniaturized.

It was proposed that Hateg’s dinosaurs  shrank in response to the limited resources available on the island in  a process called insular dwarfism.

Now, you might’ve heard of this before.

It’s happened many times over the history of life, when large animals found themselves  isolated on islands and rapidly shrank in response.

And this resulted in islands  of things like dwarf mammoths and hippos.

But these examples are relatively recent -within the last few hundred thousand years or so.

Hateg Island was the first example of this process  affecting dinosaurs in the much deeper past.

And the fossils we’ve found from  the region - as well as the fossils we haven’t found - paint a picture of  an ecosystem that was just the weirdest.

One that created a unique opportunity for  a pterosaur to become the apex predator.

Among Hateg’s strange fauna were tiny sauropods.

Now, sauropods are generally known for being  some of the biggest land animals of all time, sometimes growing more than 30 meters long.

But the sauropods from Hateg  Island shrank to only around 6 meters long - making them some of the  smallest species of sauropods ever found.

And as small as these sauropods were, they’re still the biggest dinosaurs  we know of from the Island... Because insular dwarfism seems to  have affected many of the Island’s other herbivorous dinosaurs too, like the  around-4-meter-long dwarf hadrosaur called Telmatosaurus.

It was around  half the size of your average mainland hadrosaur.

And you’d think that these little  dinos would have been easy prey for large- or even medium-sized carnivorous dinosaurs.

After all, we know many predatory theropods were  at the top of the food chain across much of the rest of the planet at the time, like T. rex  in North America and Carnotaurus in Argentina.

Yet, we’ve found no evidence to date  of any big theropods from Hateg.

And this suggests that the ecological  niche of apex predator was empty, waiting to be occupied by something large,  mobile, and powerful enough to fill it.

Enter Hatzegopteryx: a pterosaur with  an estimated wingspan of up to 12 meters and a skull nearly 3 meters long.

One pterosaur expert even described it as “a  giant mix of a shoebill stork, a ground hornbill, and the Terminator.” The idea that Hatzegopteryx was basically a  pterosaur turned T. rex was first proposed in 2017 by researchers studying  one of its neck vertebrae.

The bone was particularly short, wide, thick and  robust compared to other known giant pterosaurs.

And this led them to think that, while  Hatzegopteryx was truly massive, its neck would have actually  been proportionally quite short, but thick and heavily muscled  - trading length for strength.

By modeling the biomechanics  of Hatzegopteryx’s neck, they concluded that it might be the  strongest of any known pterosaur.

This would allow it to withstand  the kind of stresses experienced by a terrestrial predator that  used its enormous head to bite, bash, and stab prey that was too big to  swallow whole - like dwarf dinosaurs.

And while a huge, chunky pterosaur might seem like  it should be pretty clumsy on the ground, it’s been suggested in recent years that pterosaurs  were actually capable and efficient walkers.

Analysis of fossil trackways and  skeletal features of other giant pterosaurs suggest they weren’t catching  food in flight.

Instead many would have been terrestrial generalist foragers that  ate small animals and scavenged on carrion.

But Hatzegopteryx is thought to have taken  this lifestyle up another notch - taking down proportionally much larger prey  than any other known pterosaur, thanks to its uniquely robust build.

Based on its size, Hatzegopteryx would have  probably been big and powerful enough to prey on all of the island’s known dinosaurs, other  than maybe fully-grown dwarf sauropods.

Now, you might be thinking: if Hatzegopteryx  was a terrestrial apex predator and had such a massive, stocky build, could it still fly?

Well, there are reasons to  think, yeah, it probably could!

Based on the few bone fragments we have  from its wings, they seem to have still been functional.

The proportions of its humerus  match those of a normal flying pterosaur.

And despite being so robust, it actually  would have been surprisingly light.

The internal structure of its bones have a  series of weight-saving adaptations, like tiny and densely packed hollow  structures called alveoli that kinda look like polystyrene - rigid but light.

In fact, its weight has been  estimated at only around 250 kg!

That's an animal the size of a giraffe,  with the wingspan of a small plane, weighing only roughly the  same as a large black bear.

So it seems that Hatzegopteryx landed in an  evolutionary sweet spot - able to grow big and powerful enough to hunt dwarf dinosaurs, while  still probably staying lightweight enough to fly.

And flight might have been a key factor that  allowed Hatzegopteryx to get and stay so gigantic, while many of the Island’s other animals shrank.

If it could fly, then its food resources wouldn’t  have been limited to only one island, the way they were for the dwarf dinosaurs.

Hatzegopteryx  could’ve ranged across the European archipelago, avoiding insular dwarfism.

And this also makes Hatzegopteryx not  just a contender for the most imposing pterosaur of all time, but also one of  the largest things that ever flew.

The largest flying animal around today,  the wandering albatross, has a wingspan up to 3.7 meters -- Hatzegopteryx’s  was a little over 3 times that.

And throughout the entire fossil record  it’s rivaled by only two other pterosaurs, one of which was Quetzalcoatlus  from North America, which was also enormous, but much more slender.

Despite rising to the top of the food  chain and ruling Hateg Island for millions of years - and probably the  rest of the European archipelago, too - Hatzegopteryx’s reign came to a  catastrophic end 66 million years ago.

Like the dwarf dinosaurs it preyed upon, it didn’t survive the mass extinction  at the end of the Cretaceous.

But the story of its rise shows  us that being at the right place at the right time can be a major  driving force in evolution -- one that could turn a pterosaur into the  Hateg Island version of a T. rex.