FEMALE NARRATOR: 153 million years ago in Montana, the giant dinosaur known as diplodocus wandered through a seasonal wetland looking for its lunch.

Like most other sauropod dinosaurs, diplodocus was pretty big.

It weighed almost as much as four elephants and was about as long as three school buses.

And a full 25% of that length was its neck, far longer than any mammal's neck would or could ever be.

If it set back on its haunches to rear up, like some paleontologists have hypothesized that it could have, then diplodocus might have reached a full 11 meters up into the trees.

So you might understand why some paleontologists were surprised after studying the teeth of a diplodocus.

Based on the scratch marks found on the teeth, it turned out that the primary diet of diplodocus wasn't the leaves of trees, it was ground cover, like ferns and horse tails.

And it's not just diplodocus that ate this way.

Other sauropods that lived in the same area, a region known today as the Morrison Formation, had unexpected diets, too, like the 25-metric-ton 23-meter-long apatosaurus, which also ate mostly ground-level plants, or the 15-meter-long camarasaurus, which ate tougher shrub-sized plants and short trees, despite being around 4 meters tall at the shoulder, which is almost two and a half Callies tall.

Each of these dinosaurs had a neck that was much longer than the animal was tall, way longer than it needed to be for the dinosaur's mouth to reach the ground.

But if so many sauropods were eating ground-level plants, why did they all have such long necks?

It turns out that it did give them a huge advantage when it came to food, but not in the way you think, and this benefit would allow them to become the biggest terrestrial animals of all time.

[INTRO MUSIC] Being big comes with a fair number of advantages.

It makes you harder to attack and eat, for one, and larger animals tend to lose heat more slowly, which means they can spend less energy regulating their body temperature.

Bigger bodies can also store more energy for later, which helps them make it through tough times.

They can also move farther and faster than most small bodies can, so they can survive in environments where food sources are farther apart.

And if bigger is better, then sauropods were the best, as they became the largest animals that ever walked on land.

Some, like Argentinosaurus from the Cretaceous of-- wait for it-- Argentina, got to be 35 meters long and could have weighed as much as 70 metric tons.

But while they would end up evolving to be absolutely enormous, the sauropod dinosaurs had some pretty humble beginnings.

Sauropods are part of a larger group of dinosaurs called the sauropodomorphs, or sauropod-like dinosaurs.

Scientists are still arguing about what the precise distinction is between the two, but the most common definition is that once sauropodomorphs had to stand on all four legs and stopped having grasping fingers, they were officially sauropods.

Still, all of the similarities between sauropods and sauropodomorphs means that it's a little hard to tell exactly when sauropods themselves evolved.

The first possible sauropod tracks show up towards the end of the Triassic period, around 210 million years ago, in North America.

But the earliest known definitely probably sauropod bones appear about 5 million years later in Thailand.

And based on those bones, and the partial skeleton of another, slightly younger species, paleontologists think that the first sauropods probably looked something like the early Jurassic dinosaur from Zimbabwe, known as vulcanodon.

Vulcanodon was not as big as its later relatives, but it was a lot bigger than you.

Estimates vary, but suggest it was around 9 or so meters long and around 10 metric tons.

And it looked a lot like the sauropodomorphs that it coexisted with, having a somewhat shorter neck and generally smaller body size than later sauropods.

But vulcanodon had one subtle, but important, new feature.

Unlike other sauropodomorphs, its front legs were about as long as its back legs, and it probably spent all of its time on all fours.

That shift to four-leggedness was an important glimpse of what was to come.

Being big requires a lot of stable support, and once they got that stability they needed, they pretty quickly started to increase in size.

By the middle Jurassic, sauropods like patagosaurus were almost twice the length of vulcanodon.

But that was just the beginning.

By the end of the Jurassic, sauropods had become truly massive.

For example, giraffatitan was a 26-meter-long monster from Tanzania and is estimated to have weighed as much as 78 tons.

And as sauropods got bigger and heftier, their necks did too.

The 10-meter-long neck of giraffatitan had nothing on the 15-meter-long neck of supersaurus, the longest-necked sauropod found so far.

Now, paleontologists think that these long necks weren't just some weird byproduct of their large body size.

Instead, they actually made it possible for sauropods to become as massive as they did.

The best accepted hypothesis for why long necks evolved in the first place is that having a long neck expanded a dinosaur's feeding envelope.

Essentially, a feeding envelope is the space within which an animal can reach food without having to move around too much.

After all, moving consumes energy, so the less you have to move in order to get your food, the better.

Maximizing your feeding efficiency is a key step for getting big, so long necks and big bodies are pretty closely linked in sauropod dinosaurs.

There were, of course, always exceptions.

But on the whole, the bigger the body, the longer the neck.

Long necks were such a successful adaptation that, in the late Jurassic, sauropods were represented by 19 different Genera worldwide, and 10 of those can be found in the Morrison Formation in Montana, where diplodocus and many others lived.

But while long necks are a good clue that a sauropod was eating a lot, they aren't actually a good indication of what exactly it was eating.

Sure, there were some sauropods that fed on the treetops.

Brachiosaurus, for example, had wear on its teeth that matched well with a tree-munching lifestyle.

But the ones that ate at ground level, like diplodocus, had equally long necks and big bodies.

And while that might seem a little strange, it makes more sense when you understand the environment that diplodocus lived in.

It didn't rain much in the Morris Formation.

Instead, most water flowed in from nearby mountains, channeled by seasonal streams that often flooded their banks.

Without rain, most of this big basin was home to an open, dry environment and very few trees.

Scientists compare it to a savanna, but with cycads and short ferns instead of grasses, which hadn't evolved yet.

But the low lying areas formed small lakes and vast wetlands, and these were ringed with trees.

So all of these different environments meant that the Morrison could support those 10 different Genera of sauropods because they were eating different things and living in slightly different places.

For example, the high browsing brachiosaurus and shrub-eating camarasaurus likely spent most of their time eating trees that grew near the lakes and streams.

But diplodocus was eating plants that grew in the wetlands.

And if you're a large dinosaur, wetlands aren't a very safe place.

Getting stuck in the mud and having to fight your way back out would have been exhausting and potentially deadly.

In fact, some muddy rocks of the Morrison seem to preserve sauropods that died in just this way.

But diplodocus' long neck made it easy to reach deep into these wetlands and eat ferns and other plants without ever getting its toes muddy.

As a result, diplodocus was able to access food that other large herbivores, like stegosaurus, couldn't, and carve out a space for itself where it didn't compete with other herbivores.

So long sauropod necks for the win.

They allowed some of these giant dinos to have a huge feeding envelope, even if, for some of them, that envelope was horizontal rather than vertical.

And next are one area where the current ruling class of animals, the mammals, will never be better than the dinosaurs.

The giraffe, of course, has the longest neck of any living mammal, but it's only about a third as long as a diplodocus neck, and there's one big constraint that no giraffe in the past or the future can overcome.

Almost all mammals, including you and giraffes, have only seven neck vertebrae.

Instead of having more bones in their necks, giraffes just have longer ones.

But sauropods and other dinosaurs didn't have that limitation.

They could have up to 19 vertebra in their necks.

That's why today, the surviving dinosaurs, also known as birds, are the ones carrying on that long-necked tradition.

Birds have even more uses for their necks than sauropods did.

They use them for specialized feeding in wetlands, for preening their hind feathers, and for helping their head stay level as they walk and fly.

Some birds, like parrots, even use their necks as an additional limb to help them climb trees.

And if we're just counting the number of vertebra, today's dinosaurs have the extinct ones totally beat.

Some birds have up to 25 vertebra in their necks.

But luckily for us, those long-necked birds haven't evolved to contend with sauropods for the title of biggest dinosaur.

A regular-sized goose is bad enough, but one the size of a supersaurus?

No, thank you.

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