In an arid desert basin in Mongolia, around 30 million years ago, a very large mammal
was on the lookout for food.
Its preferred snack was the most succulent leaves at the tops of the highest tree branches,
which it could reach with its exceedingly long neck.
We call this animal Paraceratherium, and it was one of the largest land mammals to ever
walk the Earth.
Today the biggest terrestrial mammal is the African elephant, but Paraceratherium was
no elephant.
It was actually a kind of rhinoceros, though you probably couldn’t tell by looking at
it.
But back in its day, rhinos came in all shapes and sizes. It arose from rhino ancestors
that were a lot smaller, but Paraceratherium would take a different evolutionary path - one
that would offer both the advantages and disadvantages of large size.
Believe it or not, it actually became so big that it probably got close to what scientists
think might be the actual upper limit for a land mammal.
In other words, Paraceratherium might’ve gotten literally as big as a terrestrial mammal
can get!
So what happened to it?
Welp, the short answer is: It ran into some problems that were even bigger than it was.
The first fossils of Paraceratherium were found in 1846 in what is now Pakistan, and
more fragments were soon found at sites across Asia.
But it wasn’t until 1922, when more remains were discovered in Mongolia, that scientists
began to really understand this animal.
From the anatomy of its feet, they could tell that it was a perissodactyl, or an “odd-toed
ungulate,” that stood on three toes.
Perissodactyls today include animals like horses, tapirs and rhinos.
But how did they figure out that this big, weird looking thing was a rhino?
Well, just like a walrus can still be a walrus even if it doesn’t have tusks -- which was
news to me -- it turns out that it’s not the horn that makes a rhino a rhino.
Instead, it’s the shape of its teeth. Specifically, its back teeth. Rhinos have very distinctively
shaped molars.
Their upper molars have a chewing surface that’s kind of shaped like the Greek letter
pi, while the lower molars have more of an L-shaped pattern to them.
And Paraceratherium had exactly these kinds of patterns on its molars.
From these early fossil discoveries, some reconstructions of Paraceratherium gave it
a more rhino-like appearance. It had squat, robust legs with a short neck that stuck out
parallel to the ground.
But as more of these animals were uncovered, it started to look less like a modern rhino
and more like some kind of cross between a rhino, a giraffe, and an elephant.
Like a giraffe, it had a really long neck - about 2 to 2.5 meters long - that it likely
held at an angle.
And, like modern elephants and even sauropod dinosaurs, it had thick, column-like legs
to support its heavy build.
But the proportions of those legs were very different.
In elephants and sauropods, as those animals got larger throughout their evolution, their
humerus and femur grew longer, while the other bones in their forelimbs, hindlimbs, and feet
became shorter, more compressed, or even fused together.
But in Paraceratherium, the limbs didn’t look like this. Its humerus and femur were
shorter, and its lower limb bones were longer, more like the legs of other ungulates.
And these proportions were similar to those
of its ancestors, which were better built for speed.
One of these earliest rhino-ancestors was Hyrachyus, a lightly built herbivore from
the Eocene epoch that looked kind of like a tapir.
And like Paraceratherium and modern rhinos, it had the characteristic pi- and L-shaped
molars.
From Hyrachyus would come three families of rhinos: the group that would lead to our modern
horned rhinos; a now-extinct amphibious, hippo-like group; and the group that would lead to Paraceratherium.
Early members of this group weren’t very big, but they were adapted for running, with
long legs like those we see in Paraceratherium.
Take Pappaceras from about 50 million years ago in China. It was around the size of a
Collie and used its long legs to escape predators. ...
But it wouldn’t be long before these running rhinos started to get bigger.
For example, Juxia from the Late Eocene epoch of China was about the size of a horse. It
also had a longer neck, probably to help it browse in places other mammals couldn’t
reach.
And by the start of the Oligocene epoch, about 35 million years ago, giants like Paraceratherium
measured a whopping 4 to 6 meters tall at the shoulder and around 7.5 meters long.
...
And with its long neck, it was likely the tallest land mammal that ever lived, with
a head height about 6 to 9 meters off the ground!
So how does a mammal that big, just, be that big?
Well some experts think Paraceratherium was probably close to the maximum weight that
a land mammal could reach -- it weighed between 10 to 15 metric tonnes, with the largest possibly
reaching up to 20 tonnes!
Interesting thing is, that weight limit might not exist because of biomechanics but because
of basic biology, specifically reproduction.
That’s because, typically, the larger a placental land mammal is, the longer its pregnancies
are.
For example, the African elephant, the largest placental land mammal today by mass, has a
gestation period of up to 2 years.
So imagine how long a female Paraceratherium -- which was probably twice as heavy -- would
have to carry her offspring!
That would require a lot of energy and nutrients for both mother and baby, which, like elephants,
needed to grow quickly at first to avoid predators.
So why did these animals get this massive in the first place?
Why go from the collie-sized Pappaceras to the enormous Paraceratherium?
Well, one reason that you’ve heard me mention before is just that being huge usually makes
it harder for predators to take you down.
Another reason is that larger animals are able to access foods that are out of reach
for smaller animals.
Based on the size, microwear patterns, and chemical analysis of their teeth, we know
Paraceratherium mainly ate leaves.
And with their long necks, they could reach plenty of leaves from the tops of trees, like
giraffes do today.
Researchers also think they had the same type of digestive tract as modern rhinos, which
means they probably didn’t get very much nutrition from their diet.
So, they likely needed to eat huge amounts of plant material to get enough nutrients,
which required a large gut to process all that food.
And fortunately, they could probably travel pretty far to sustain their giant bodies!
For example, African elephants, which need roughly 140 kg of food daily, are capable
of travelling around 32 km in a day, and have home ranges between 750-1500 square km.
And this is possible because their long legs give them a long stride, longer than any other
animal today.
Paraceratherium might’ve had an even longer stride, with an even larger range.
This would’ve been a big advantage, especially considering the changes to the climate that
were going on.
Around the start of the Oligocene Epoch, about 33 million years ago, Earth began to cool,
and glaciers in Antarctica started growing faster.
This cooling trend continued into the early Miocene Epoch and expanded areas of semi-arid
climate with more shrub lands, dunes, and savannas, gradually replacing the tall trees
that these animals preferred.
But these climate changes actually didn’t seem to have much of an effect on Paraceratherium,
at least not directly - they were probably already used to harsh environments.
But there was another unexpected factor that may have led to their extinction: the arrival
from Africa of gomphotheres - some ancient relatives of elephants.
Paraceratherium probably didn’t compete with these newcomers directly. Instead, the
gomphotheres helped accelerate the disappearance of the rhinos’ habitat.
We know that, as modern elephants graze, they can push down trees and greatly change the
environment around them, turning woodlands into grasslands.
Gomphotheres likely did the same thing, putting additional pressure on Paraceratherium as
they left even less food for them to browse on.
And this stress could also have made these ancient rhinos more vulnerable to disease
and droughts.
The start of the Miocene marked the end of these largest of the rhinos, with only their
smaller cousins left behind. And it set the stage for the rise of a different group to
take the crown.
Those elephant-relatives soon became the largest mammals on land, diversifying into niches
previously occupied by the giant rhinos.
And some of them reach enormous sizes, too.
In fact, some species, like Palaeoloxodon, probably got heavier than
Paraceratherium and maybe even taller at the shoulder.
But Paraceratherium still reigns supreme as the tallest mammal that ever was.
Extra large animals need a particular combination of climate, resources, and space to exist
- and these circumstances come and go as the planet warms and cools, and as new groups
diversify and migrate across continents.
But if modern rhinos, elephants, and even the whales that we know today are any indication,
it looks like getting big is a strategy that will never go out of style.
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Largest high fives ever, thank you Kallie, to this month’s Eontologists: Sean Dennis,
Jake Hart, Annie and Eric Higgins, Jon Davison Ng, and Patrick Seifert.
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This week's is from Stephen O'Leary and I have not read it before
So brace yourselves
"Evolution gave the Primates the Opposable-Thumbs-Up."
Why?
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