Part of why we’re so fascinated with extinct
dinosaurs, I think, is that some of them were just so big, it’s just hard to imagine animals walking around on the surface of the planet that size Speaking as a mammal that’s about two meters
tall, I gotta say it’s hard to grasp what it would be like, to be in the presence of
a creature as tall as a five-story building. And yet, they existed! From the Jurassic to the Cretaceous Periods,
when our ancestors were the size of voles and shrews, dinosaurs like Supersaurus, Sauroposeidon,
and Argentinosaurus were shaking the Earth. So, how did they get to be so big? And why did we mammals never even come close
to dinosaurs in size? Well, to our credit, the most massive animal
ever is, of course, a mammal: the blue whale. It can get up to 30 meters long and weigh
as much as 145 metric tons -- more than twice as heavy as the most massive known dinosaur. But, to be fair, the rules of biomechanics
are different in the water. Buoyancy and blubber can do amazing things,
allowing sea creatures to grow to sizes that would be impossible on land. However, when it comes to the largest terrestrial
animals, mammals were never any competition for the non-avian dinosaurs. There’s always some debate about what the
biggest dinosaur was. But the current record holder for the largest
specimen belongs to the newly-named titanosaur Patagotitan. Experts estimate that this Cretaceous herbivore
stretched over 36 and a half meters, and weighed upward of 64 metric tons. By contrast, the biggest mammal that ever
walked on land was the hornless rhinoceros known as Paraceratherium. It weighed a mere 15 tons, and stood about
5 meters high at the shoulder, roaming Eurasia from Romania to China, at the end of the Oligocene
Epoch, long after the non-avian dinosaurs disappeared. So, how can two such successful and prolific
types of animals end up having such different size constraints? Part of it might have to do with how they
reproduced. Paraceratherium was a placental mammal, like
us, meaning that it gestated its young inside its body. And if this ancient rhino was anything like
the large mammals alive today, that would’ve taken a very long time. Big mammals like giraffes, rhinos, and elephants
usually have only one offspring at a time, and gestation can last longer than some other
kinds of animals live. Elephants, for example, carry their babies
for more than two years! Now consider the dinosaurs. They didn’t really have to carry their babies
at all. Because, all dinosaurs laid eggs. Even the biggest of the giant dinos hatched
from an egg no bigger than a soccer ball. What does this have to do with size? Well, bigger mammal species give birth to
bigger young, which requires a huge amount of time and energy to gestate. Dinosaurs totally bypassed that problem. Instead of having bigger babies, the largest
dinosaurs laid comparatively small eggs, which held equally small hatchlings. Reproducing this way - with babies hatching
and growing outside the mother’s body - removed the size limits that gestation places on mammals. Dinosaurs also had another evolutionary advantage. Their skeletons had a special feature that
mammals lacked: a sophisticated system of air sacs. These sacs were basically pockets of soft
tissue that were connected to the lungs. Think of them as biological balloons. Some of these sacs sat in the body cavity,
next to bones, usually in the neck, back, and hips. But others ran inside the bones themselves. These air sacs helped shape the dinosaur’s
skeleton, and allowed the bones of the biggest dinosaurs to remain light, without sacrificing
strength. How do we know that extinct dinosaurs had
these sacs? Partly, because non-extinct dinosaurs have
them too! Birds have a similar system of sacs that help
draw air into their tiny lungs, while also making their skeletons remarkably light. And if you compare the respiratory system
of birds to those of the giant dinos, you’ll see that the resemblance is pretty striking. In sauropods, for example, the vertebrae of
the neck and back have the same pockets and divots we find in birds today, where these
air sacs were attached. And, when paleontologists scan fossils of
some dinosaur bones, like vertebrae, they often find hollow spaces inside the bones
where the air sacs used to sit. Now, keep in mind that bones with spaces created
by air sacs are different from the hollow bones that you see in the legs of birds and
other theropod dinosaurs. And, not all of the extinct dinosaurs had
these handy air bags. Only the type of dinos known as saurischians
had them. In the traditional dinosaur family tree, this
group encompasses the theropods -- that is, the two-legged dinosaurs that include today’s
birds -- as well as the sauropods, the quadrupeds that include the giant titanosaurs. All of the rest of the dinosaurs -- like the
horned, armored, and duckbilled dinosaurs -- are known as ornithischians, and they didn’t
have these features, so they weren’t as light on their feet. And of course, we mammals don’t have anything
like them, either. We retained a skeletal system of dense, heavy
bones that puts a limit on how big we can get before our bones crack under our own weight. But the thing is, these adaptations are just
what allowed dinosaurs to get so big. They don’t tell us why these giants all
got so enormous in the first place! And that’s a totally different evolutionary
question, with lots of possible answers. Maybe living large was a way to stay safe
from predators. Or maybe their size allowed dinosaurs to cover
more ground, or reach higher leafy branches, in search of food. Or maybe there’s something paleontologists
haven’t thought of yet. And then there’s another question to consider:
Was the ability to grow so large really an advantage? After all, the sauropods are gone. So are most of their relatives. Of the entire dinosaur family, only birds
-- a single group of saurischian theropods -- survives, and they range in size from the
hummingbird to the ostrich. So, today, even if it’s hard for us to picture
a dinosaur as huge as Patagotitan, we can at least understand how animals like it were
physically possible. Perhaps we’re lucky that we never reached
such great heights. Evolutionarily speaking, it seems that bigger
is not always better. So what do you want to know about the story of
life on Earth? Let us know in the comments. And don’t forget to go to youtube.com/eons
and subscribe! Now do yourself a favor and check out some
of our sister channels from PBS Digital Studios. After all, we all have a lot to learn!
Those air sacs were attached to their respiratory system to cool their insides. Because, at their mass, their normal heat production would otherwise push their internal temperature above that of cooking meat.
Much like a bird 🤔
They ate their vegetables.
square-cube law BTFO
Air suspension! Cool!
Geeze, all the recent revelations make dinosaurs weirder and weirder...
Yeah. I'm not fat, that isn't a gut, I just have "Air sacks". Am I endangered?