Greek and Roman mythology is full of stories about exotic islands that were home to weird
and fearsome monsters -- like one-eyed ogres, seductive sirens, and man-eating giants.
And I’m here to tell you: Back in the late Miocene epoch, there was in fact an island--or
maybe a group of islands-- in the Mediterranean Sea that was populated with fantastic giant
beasts.
But, these giants were actually … pretty small.
Some of them you might even consider cute.
There were enormous hamsters.
And big, fat waterbirds that could neither fly nor swim.
And animal like hedgehogs that were as big as house cats.
These organisms were only giants compared to their ancestors, which had lived on the
European mainland.
But once they became isolated on these Mediterranean islands, some of these little critters attained
massive body sizes.
That’s because geographic isolation is a powerful force in evolution.
Sometimes, when a population of animals gets cut off from many of its normal predators
or competitors, it gives rise to new species of unusual sizes.
Sometimes, they turn out to be miniaturized versions of their ancestors.
Other times, giants!
The tale of these Mediterranean island beasts of the Miocene is no myth.
It’s a lesson in the very strange, but very real, powers of natural selection.
In 1969, a group of Dutch paleontologists were working in Gargano, a region on the eastern
coast of Italy, when they discovered deep fissures, or cracks, in the limestone quarries
and road cuts there.
And these cracks turned out to be full of fossils!
For millions of years, it seemed, animals had been stumbling into these fissures, like
natural traps, starting about 8 million years ago.
The result was an amazingly rich fossil deposit, a snapshot of life in this place in the Late
Miocene Epoch.
But a few things about the Gargano fossils turned out to be really … odd.
For one thing, scientists were confused about the fossils they didn’t find.
Elsewhere in Europe, deposits from the late Miocene tend to contain a lot of large mammals
-- like extinct elephants, cats, and perissodactyls, the group of hoofed mammals that includes
horses.
But at Gargano, none of those animals were anywhere to be found.
Instead, researchers uncovered an eclectic and bizarre bestiary, including fossils of
smaller animals that seemed ... overgrown.
There were extra-large owls, alarmingly big hamsters, and supersized hedgehogs.
This strange assortment of animals came to be known as the Mikrotia fauna.
And for decades, the Gargano Peninsula --along with a handful of nearby towns -- were the
only places on the planet where these creatures were known.
Then in the 1990s, fossils of some of those same, peculiar animals turned up at Miocene
sites near Scontrone, a town in central Italy.
So, what exactly were these animals?
Why were the Miocene animals of Gargano and Scontrone so different from their counterparts
in the rest of Europe?
For paleontologists, the most likely explanation was that these areas must’ve been cut off
-- geographically separated -- from the rest of Europe.
Specifically, they were probably islands -- or maybe parts of the same island.
This would explain not only why these animals appear just at those sites; it would also
account for the strange body sizes of animals that were ordinarily quite small.
I’ve talked before about how, in island environments, large-bodied animals often get
smaller over time.
In the case of Columbian mammoths on the Channel Islands of California, for example, the absence
of large predators, combined with limited food, led natural selection to favor smaller
body sizes.
So, after thousands of years, some of the giant Columbian mammoths had developed into
what scientists consider a new, separate and smaller species: the Pygmy mammoth!
This phenomenon is known as insular dwarfism.
But it’s only part of the story about how island environments can tinker with animals’
body plans.
The flip-side is what happened to some of the animals of Gargano and Scotrone: Insular
Gigantism.
Both processes are part of what’s known as Foster’s Rule, identified by biologist
J. Bristol Foster in 1964.
Essentially, Foster’s Rule says that in isolated environments, large animals frequently
get smaller -- and tiny animals often become larger.
And this is what happened to many animals in the Mikrotia fauna.
And it happened because their habitat allowed it to happen.
Much like the modern Channel Islands, the regions of Gargano and Scontrone in the deep
past were higher in elevation than the surrounding areas.
So as sea levels rose and fell over millions of years due to changes in climate, these
places became isolated from the rest of Europe, and then reconnected, many times.
And it may have been during one of these periods of low sea levels when some ancestors of the
island giants first made their way to Gargano and Scontrone.
Researchers think that, about 30 million years ago, in the Oligocene Epoch, a global cooling
trend lowered sea levels in the Mediterranean and elsewhere, forming a land bridge between
the mainland and these two spots.
But then, about 15 million years ago, the climate warmed again and sea levels rose,
turning those high spots into islands and severing their connection to the mainland.
So, maybe the ancestors of the Mikrotia fauna simply walked across the land bridge before
it became submerged.
But other scientists have proposed that most of them arrived much more recently -- just
around 8 million years ago -- when the island or islands were properly isolated.
In that case, it’s thought that the animals might have rafted on piles of floating debris--like
logs, branches, and palm leaves -- much in the same way that the ancestors of South America’s
rodents are thought to have migrated from Africa!
Either way, once they’d reached their new island habitats, many of the animals started
to follow different evolutionary paths.
Some stayed small, and some may have actually shrunk.
But others responded to their new environment by getting downright huge.
Just look at Deinogalerix, a shrew-like member of the hedgehog family that’s found only
in these Mikrotia fossil deposits.
Early species of this genus were about 30 to 45 centimeters long, making them only slightly
bigger than a modern European hedgehog.
But over time, bigger and badder Deinogalerix started to evolve alongside them.
The largest species--found only in the most recent deposits--was some 60-centimeters long!
And even though it was a member of the hedgehog family, Deinogalerix had teeth that were more
like those of cats and dogs.
Using its long, pointy incisors and crushing cheek teeth, the largest of this genus probably
behaved like a badger or raccoon, hunting much bigger prey than its ancestors ever could.
And it did, simply because it could!
Back on the mainland, the ancestors of Deinogalerix needed to stay small in order to avoid all
the large predators.
But in this island environment, there were very few large, terrestrial carnivores.
So without the selective pressure to remain small, Deinogalerix was free to expand in
size and take advantage of new niches.
Its ancestors probably specialized in eating insects, but this animal could’ve also hunted
other, larger prey, like vertebrates.
In the process, it emerged as one of the area’s biggest land-dwelling hunters.
This was Foster’s Rule in action!
But a big old hedgehog-like thing still had some competitors--and natural enemies.
Because, these islands were also home to some unusually large birds of prey.
Take the giant owl known as Tyto gigantea, which appeared on Gargano in the Late Miocene.
We don’t know for sure which species it evolved from, but we do know that there was
nothing near its size on the mainland.
Based on its fossils, scientists think Tyto gigantea may have had a wingspan of two meters
or more -- twice the size of a living barn owl.
Likewise, the islands were also home to a bird of prey known as Garganoaetus, which
included two species.
The larger one probably rivaled today’s golden eagle in size, and it’s only found
in more recent deposits.
So this suggests that Garganoaetus, like the giant owl, developed a bigger body size over
time.
But here’s a question: On an island with few other predators, why would these birds
of prey get so large?
Wouldn’t that just mean they’d need more food?
Well, some researchers think that on these islands of giants, both predators and prey
were engaged in a sort of evolutionary arms race.
Research has shown that the modern barn owl can’t swallow any rodents whose heads are
longer than 17 millimeters.
But by being bigger, these birds would’ve had an easier time hunting--and gulping down--the
local rodents, which were also getting quite hefty.
For example, there’s the burrowing rodent Mikrotia, the Gargano native that gave this
whole group its name.
One species of Mikrotia had a skull that was twice the size of a modern rat’s head, suggesting
the rest of the animal was pretty big too.
There was also a giant species of dormouse -- which are usually tiny and adorable -- but
this one weighed about a kilogram….Which still sounds pretty cute to me!
And then there was Gargano’s strange, giant hamster Hattomys gargantua.
It was three times heavier than today’s common hamster and much bigger than the any
species that lived on the mainland at the time.
Mind you, not all of these island animals expanded in size over time.
Small and normal-sized birds and rodents coexisted with giant species.
But some rodents definitely got bigger, either because they simply could or because it let
them expand into new dietary niches.
And as they grew, the birds of prey also evolved larger sizes in order to hunt and eat them.
This same phenomenon occurred among other kinds of birds, too.
Take the case of Garganornis, first reported in 2013.
Garganornis belonged to the same family of birds as ducks and geese.
But it weighed between 15 and 22 kilograms, up to twice as much as a large Canada goose.
Unlike modern geese and ducks, its toes and lower leg bones were really short, suggesting
that it spent most of its time walking on land and not paddling in the water.
And judging by the bones in its wings, it probably couldn’t fly, either.
So, researchers think that Garganornis adapted to take advantage of its island’s lack of
big, herbivorous mammals.
By growing bigger and staying on land, it may have been able to broaden its niche and
forage on a wider range of terrestrial plants, while its size could’ve helped it fend off
some of those birds of prey.
And size adaptations seemed to have benefitted some larger mammals on the island, too.
One herbivore known exclusively from Mikrotia deposits was the truly strange Hoplitomeryx,
a genus of hoofed mammals with saber-like fangs and five horns on their heads.
Five!
And there were lots of different species of this so-called “prongdeer” that varied
a lot in size.
By one estimate, the smallest only weighed around 5 to 6 kilograms, while the biggest
may have tipped the scales at 113 kilograms.
And species of different sizes seem to have lived at the same time, which kind of makes
sense.
DIfferent sizes would make different food sources available to each species.
This would’ve not only kept the “prongdeer” from competing with each other, it could also
have prevented from from overgrazing their little island.
But there was a change on the horizon that none of the weird and beautiful Mikrotia fauna
could adapt their way out of.
About 5.3 million years ago, a massive amount of Atlantic water passed through the Strait
of Gibraltar--which had once been lifted above sea level.
This flooded the Mediterranean.
Rising water levels dramatically shrank the little giants’ island habitat, which is
probably what drove them to extinction.
Just a few million years later, in the early Pleistocene, some areas of central and southern
Italy went through a period of tectonic uplift.
That reunited Gargano and Scontrone with the rest of the mainland, where they remain today.
So, competition, food availability, predator/prey relationships, and niche expansion: any or
all of these things can, very gradually, make either pygmies or giants out of animals that
are isolated on islands.
It’s not mythology!
It’s natural history!
And we have their fascinating fossil remains to remind us of the odd little giants that
once roamed those ancient Mediterranean islands.
Thanks for watching!
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