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. Â
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 -Â 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. Ok now! Do you want more Eons content? Then
be sure to follow Eons on social media! You can find us on Instagram, Twitter, and Facebook.
And you can join me on Instagram at westerndigs.  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. Become an Eonite by supporting us at patreon.com/eons.
And remember - Eonites get perks like submitting a joke for us to read! 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? And as always thank you for joining me today
in the Konstantin Haase studio. If you like what we do here, subscribe at youtube.com/eons!
That we know of.
I love Eons