As far as feathered animals go, Pachystruthio dmanisensis was a monster. With an estimated mass of about 450 kilograms (nearly half a tonne), it would make a 150-kilogram adult ostrich - the world's largest living bird - look like a canary.

Bigger birds have existed, but it's not so much its size that makes this flightless avian stand out, but the unexpected location its remains were found on the northern coast of the Black Sea.

When roadwork broke open a cave on the Crimean Peninsula in the summer of 2018, it proved to be a bonanza for palaeontologists.

Among the pickings that included bones from mammoths, sabre tooth cats, hyenas, horses, and even a small wolf, there was a rather odd femur that just didn't belong.

Russian Academy of Sciences palaeontologist Nikita Zelenkov initially assumed the fossilised leg bone with its impressive heft had to come from a Malagasy elephant bird.

"However, the structure of the bone unexpectedly told a different story," says Zelenkov.

Animals of unusual size – whether massive beasts like moas and elephant birds, or diminutive humans and elephants – are often the result of the kinds of ecological forces found on islands.

In fact, a little over half a century ago, a young biologist named Bristol Foster came up with a rule describing the changes in size certain species experience as they are confined to the resources of a small space.

Without clear signs of large birds evolving on Europe's mainland, palaeontologists have simply assumed Foster's rule stood strong, keeping all European birds to a boringly average size.

This new discovery challenges that assumption, making it the first clear sign that a giant flightless bird once made a home on ancient European soil.

The femur itself is approximately the size expected of an elephant bird, but with a slightly more slender look suggesting it was a runner.

Further estimations based on the bone's proportions put its height somewhere around 3.5 metres (11.5 feet), meaning we might picture P. dmanisensis as either a tall, slim elephant bird or a rather stocky ostrich.

"We don't have enough data yet to say whether it was most closely related to ostriches or to other birds, but we estimate it weighed about 450 kilograms," says Zelenkov.

"This formidable weight is nearly double the largest moa, three times the largest living bird, the common ostrich, and nearly as much as an adult polar bear."

The crown for largest member of class Aves goes to an extinct species of elephant bird called Vorombe titan, which once roamed the African island of Madagascar before dying out roughly a thousand years ago.

At a whopping 860 kilograms (1,895 pounds), it would have double the estimated mass of P. dmanisensis.

Going on the mix of animals found in the cave, researchers estimate they would have been laid to rest somewhere between 1.5 and 2 million years ago.

Homo erectus bones found to the east of the Black Sea have been dated to roughly the same period, making it more than likely relatives of our ancestors not only shared the bird's territory, but might have even hunted it.

With humans on its tail, not to mention predators including sabre-tooth cats, wolves, and hyenas, it's not surprising that P. dmanisensis evolved into a sprinter.

Finding a giant bird on the European mainland helps us better understand not just the kind of fauna humans might have encountered as they migrated across the landscape, but the environment itself.

Foster's rule might well explain the size of some large, flightless birds, but to explain why emus and ostriches stand so tall, researchers turn instead to the Jarman–Bell Principle.

When the going gets tough, animals turn to tough foods. And when it comes to squeezing all you can from a bunch of low-nutrition, fibrous meals, bigger bodies offer bigger advantages.

Applied to P. dmanisensis, we might imagine a drying landscape on the edge of the open steppes, where ancient humans and long-toothed predators seek a quick and easy meal in a fleet-footed, oversized chicken.

Whether or not humans had a hand in its extinction, it's too hard to tell at this stage. Hopefully these won't be the last bones we'll ever find of this massive bird.

This research was published in the Journal of Vertebrate Paleontology.