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Sometimes it seems like the past favored large animals, but it turns out that each one on this list has a different reason for its size.

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Sharks bigger than a city bus. Snakes twice as long as anyone alive has ever seen.

Go to a natural history museum, and it’s populated with giants so huge, everything alive today seems small by comparison. Unless, of course, you’re a blue whale. Those hold the title of being the largest animal to ever exist.

Still, whales aside, what made it possible for certain species to grow so large in the past, when their modern equivalents are so modest? What changed between then and now, and, more importantly, why can’t I hug a sloth the size of an elephant? Well, there’s no “one size fits all” reason for ancient animals to have been bigger.

Instead, evolution works with what it’s got -- and it’s had different conditions to work with throughout history. Here are seven of the biggest versions of today’s animals -- and how they got to be giants. Long before giant land vertebrates, arthropods had their chance to reach enormous sizes.

They ruled during the Carboniferous and Permian periods, or from about 360 to 251 million years ago. And if you aren’t a fan of bugs, you’ll be happy you weren’t around back then. One of these Permian arthropods was Meganeuropsis permiana, likely the largest insect to ever be capable of flight.

Although it technically belonged to a group known as griffinflies, it closely resembled a dragonfly, and it had a wingspan up to 75 centimeters. That’s larger than some hawks, and more than ten times that of a modern dragonfly. Meganeuropsis probably lived the same way dragonflies do today, scooping prey out of the air at the water’s edge -- although it was likely a lot less agile since it was so huge.

What many scientists think made this insect so large is a commodity that was probably far more common in the Permian: oxygen. Back in the Carboniferous, the amount of oxygen in the atmosphere skyrocketed thanks to massive plant growth. It went from around 20% -- which is close to what it is today -- to at least 30%, and stayed high well into the Permian.

That really helped out these critters. See, insects rely on tubes called tracheae to allow diffusion of oxygen into their bodies. And this process is limited by the insect’s size.

If it gets too large, a healthy amount of oxygen won’t be able to make it all the way down the tracheae. But if there’s more oxygen available, it can diffuse further. And since there was more oxygen in the Permian, it, according to one popular hypothesis, allowed.

Meganeuropsis to grow much bigger than modern insects. Of course, like a lot of things in natural history, it’s still not totally settled. Another idea states that so much oxygen might have actually been poisonous to Meganeuropsis’ larvae.

So they could have quickly grown to large sizes to decrease their surface area to volume ratio. That would have allowed their bodies to more easily handle all that extra oxygen. Flash forward a few million years, and Cretaceous North America was a playground for giant dinosaurs.

But between 83 and 72 million years ago, before the evolution of T. rex, the largest predator around may have actually been… an early alligator. Deinosuchus, whose name appropriately means “terrible crocodile”, was a type of crocodile that’s a cousin of modern alligators. Its fossil remains are too fragmentary for us to be sure exactly how big it got, but estimates hover around eight to ten meters or more.

That’s almost twice as long as a pickup truck. Compare that to today’s American alligator, which usually hovers around three to four meters. Like modern alligators, this creature probably used its massive jaws to ambush prey from beneath the water.

But unlike modern alligators, which reach more or less full size within ten years or so, Deinosuchus kept growing up to the age of thirty-five. We know that from counting growth rings in the bony plates in its skin, almost like counting tree rings. Without more complete remains, we can’t be totally sure why it kept growing for so long.

But scientists think it could have been to compete with dinosaurs or even to eat them. After all, teeth marks in dinosaur bones -- including a duck-billed hadrosaur and a smallish type of tyrannosaur -- are proof of confrontations with Deinosuchus. Just because it bit dinosaurs, though, doesn’t mean it hunted them regularly.

Fossilized poop, officially known as coprolites, provides better evidence of what something ate. And Deinosuchus coprolites contain bits of shell from large turtles. So that could be another reason for it to grow to such sizes -- in order to crunch through those thick shells.

Deinosuchus likely required a large hunting area, so they probably tended to have smaller populations than some other animals. And ultimately, that could be one reason why they were vulnerable to extinction. Besides having enormous alligator-like animals, the Cretaceous also featured giant sea turtles that put their modern cousins to shame.

Archelon lived about 74 million years ago, and was more than 4 meters from nose to tail. Meanwhile, a modern leatherback turtle -- today’s largest sea turtle -- is usually closer to 2 meters. Like leatherbacks, Archelon also had a much thinner shell than other turtles, and it was supported by a relatively light framework of bone.

This may have been to help it maintain buoyancy despite its huge size. Right now, scientists know very little about Archelon. Like, it could’ve been so large to make it hard for predators -- like mosasaurs -- to eat it, but we aren’t sure.

More fossil remains might help, though, since we haven’t dug up many of them in recent decades. We’re also still putting together the pieces about Archelon’s diet, but one idea says that it was too large to chase after quickly-swimming prey. So it might’ve just hung out and snacked on whatever was around.

We do know it had a ferociously powerful bite, though, but it may have still favored soft prey like squid or some plants. But one thing’s for sure: I really wouldn’t want to mess with one. This list wouldn’t be complete without mentioning Megalodon, the largest shark that ever lived.

It was alive during the Miocene and Pliocene epochs, or from 15.9 to just 2.6 million years ago -- and no, Discovery Channel, they aren’t somehow still alive. Megalodon was once thought to be so closely related to the modern great white shark that it was placed in the same genus. Most scientists no longer think that should be the case, but its exact taxonomic placement is extremely unclear.

It doesn’t help that Megalodon is mostly just known from teeth and a vertebra here and there. Shark skeletons are made of cartilage, which doesn’t fossilize as well as bone, though it still can. Still, we do know that it was probably about 18 meters in length -- three times the length of a modern great white.

It was so big, it could -- and did -- eat whales. Specializing in big prey is a good reason to grow to big sizes. But like any good predatory shark, Megalodon was an opportunist that probably ate whatever it could sink its teeth into -- like fish, squid, and turtles.

Usually, the abundance and diversity of large prey is cited as the reason this shark grew so big. But recently, another explanation has been proposed for Megalodon’s fast-swimming, predatory, super-giant lifestyle: It could have been sort of warm-blooded. Some fish, including great white sharks, have evolved a way to conserve body heat.

Specialized blood vessels warm certain parts of the body, like the brain and eyes, by running blood vessels carrying hot blood past colder ones -- a bit like a radiator. It’s called regional endothermy. If Megalodon was closely related to white sharks, it might make sense for it to have this ability as well.

Warm-blooded creatures are generally more active than cold-blooded ones. So endothermy may have aided Megalodon in pursuing its huge prey, and given it a competitive edge in an ocean full of giants. The Paleocene epoch, which happened right after the extinction of the dinosaurs 66 million years ago, was a bit lacking in large vertebrates.

But despite the lack of huge dinosaurs running around, it did have a snake at least thirteen meters long. It was called Titanoboa, and was from Paleocene Colombia. Green anacondas, the largest modern snake by body weight, also hail from South America.

But they only reach 6 meters in length on average. Meanwhile, Titanoboa, even according to more conservative estimates, would have been double that length. They make anacondas look cute and tiny by comparison.

So what does a thirteen to fifteen meter snake eat? Anything it wants to. If it lived like a modern anaconda, as researchers believe, it probably wasn’t picky.

It’s been found alongside fossils of turtles, crocodilians, and lungfish, and its discoverers believe these crocodiles may have been its favorite meal. Yeah. A snake that eats crocodiles.

Besides being enormous, Titanoboa’s huge size also has been interpreted as a clue about the climate of Paleocene South America. See, snakes are poikilothermic, meaning their body temperatures fluctuates with the temperature of their environment. And the maximum body size of poikilothermic vertebrates is known to be limited by temperature.

Based on its size, researchers think the jungles Titanoboa lived in must have been a toasty annual average temperature of at least 27° Celsius -- definitely warmer than today. Which means the Paleocene hothouse probably made monster snakes possible. Before we leave South America, there’s another meat-eating giant we should mention: the largest flightless bird.

In most places, following the extinction of the dinosaurs, mammals evolved to be top predators. But in South America, for whatever reason, most mammals were vegetarians. That means, 60 million years ago, there was an open slot for a big predator at the top of the food chain.

And it was filled by terror birds. These animals only went extinct around 2.5 million years ago, and had time to evolve a variety of forms -- at least 17 species. Some were long-legged, swift runners.

Others were more stocky, and built like ambush predators. All of them had the hooked beak that’s a signature of modern predatory birds, and the biggest were easily taller than a modern ostrich, at perhaps 3 meters. For South American prey animals, it’s like the dinosaurs never left.

Terror birds are popularly shown dining on horses, although it’s worth mentioning that horses were smaller for most of their history. And the exact feeding style and favored prey of terror birds is hotly debated. Their closest relatives alive today are 80 centimeter tall birds called seriemas, which feed by slamming their prey against the ground to break their bones.

It’s hypothesized that terror birds might have followed a similar strategy. Since there were so many different kinds, though, they probably also had more than one way of getting a meal. Sloths are cute.

This is internet fact. So scale one up to the size of an elephant and it’s just that much cuter, right? Megatherium was the biggest of the giant ground sloths, and it lived during the Pliocene and.

Pleistocene epochs, 5 million years ago to almost, but not quite, the present day. They’re called ground sloths because, perhaps not surprisingly, they couldn’t actually live in trees like their modern counterparts. A three toed sloth is well under a meter in length, but Megatherium might have measured six meters from nose to tail.

But it was still a vegetarian, and probably exploited its size to reach into the treetops and pull down vegetation that smaller competitors couldn’t reach. They could have been so big because there was plenty of land area available to support their populations -- although not everyone agrees. But it does seem clear that the dinosaurs left niches wide open for large herbivores to fill.

Overhunting by humans is often blamed for driving the giant sloths to extinction. But climate change at the end of the last glacial period, as well as other complex factors, may have been involved as well. Different animal groups have evolved extraordinary sizes for different reasons, whether it’s because of temperature or food or physiology.

The evolutionary circumstances are always different, and it’s not like there’s a bias toward bigness in the past. After all, the blue whale is still around today. The big stuff does catch our attention, though.

There’s a lot we can learn from these fossil giants about the what our planet used to be like -- and the kinds of creatures that roamed it. And as a bonus, these fossils also look pretty cool in a museum. Thanks for watching this episode of SciShow!

If you want to learn more about natural history and the story of our world, you can check out one of our sister channels, Eons, over at [♪OUTRO].