Why Do Animals Keep Evolving Into Turtles?

You are evolving into a turtle. Well, not for a while at least. For some reason, scientists have been noticing a strange effect in the animal kingdom over the last few millions of years. Tons of completely unrelated animals are evolving into turtles. You have ancient reptiles that went into the water and became a turtle.

 You have dinosaurs that grew armor so thick they became dino turtles. Even mammals, completely unrelated to the turtle lineage, have become turtles. So, what is actually happening? Why do completely different animals keep evolving into turtles? And are we going to become turtles? When most people think of a turtle shell, they usually imagine it as some kind of armor that just sits on top of the animal.

 We’ve all seen the cartoons where a turtle gets scared and literally jumps entirely out of its shell, leaving it behind on the ground like a hermit crab. A turtle shell is definitely not a removable house. The shell is their skeleton. To actually become a turtle, an animal has to go through a level of mutation that’s frankly just horrifying.

Their ribs don’t just curve around their chest like a normal animal. Instead, the ribs physically expand outwards, flatten into wide plates, and then permanently fuse together. But, it gets worse. In a normal animal, the shoulder blades sit on the outside of the rib cage. This is what allows you to actually move your arms around.

 But, because a turtle’s ribs expand [music] so far outward to create the dome, their ribs actually grow over their shoulder blades. Their shoulders are trapped inside their own rib cage, which is exactly as uncomfortable as it sounds. This is a permanent deformity of the entire skeletal system. You’re permanently locking your spine and your ribs into a solid bone prison that can never bend or twist again.

 So, if getting a shell requires you to literally mutate your entire skeleton inside out, why would any other animal actively try to copy this? Well, to see how badly nature wanted to replicate this skeletal deformity, we have to go all the way back to the Triassic period, roughly 240 million years ago. During this time, the oceans were filling up with some of the first major marine reptiles.

 And the problem with being a marine reptile in the Triassic is that the ocean was already completely packed with highly aggressive [music] predators that wanted to eat you. If you were just a soft reptile trying to swim around, you were basically just free protein. You were essentially just waiting for a giant sea monster to swim by and go, “Oh, nice, a snack.

” So, a group of reptiles called placodonts found a solution. If you looked at a placodont, you’d immediately assume it was a turtle. They had wide, flat bodies completely encased in hard bone shell. They had little paddle-like legs for swimming, and they even had hard beaks that looked like, well, this, which were perfect for snapping up food from the ocean floor.

 They looked and acted exactly like a modern sea turtle. But they weren’t turtles. They weren’t even closely related to turtles. Placodonts were an entirely different branch of reptiles that just coincidentally figured out that turning your body into a giant bone plate was a really good way to not get eaten by sea monsters.

 But this raises a pretty obvious question. If this shape is so overpowered for surviving, why isn’t everything just a turtle? The reason every animal doesn’t fuse its ribs into a giant shield is because the turtle body plan comes with a crippling tax. When you turn your torso into a solid block of bone, you lose basically everything.

 You can’t twist your body to look behind you. You’re essentially locking yourself inside a heavy bone prison that you have to drag around everywhere. And honestly, the lack of speed isn’t even the worst part. The real nightmare is breathing. If you take a deep breath right now, you can feel your rib cage expanding to let your lungs fill with air.

 This is how almost all mammals and reptiles breathe, but a turtle’s ribs are permanently fused together. Their chest literally cannot expand. So, to simply get oxygen into their bodies, turtles had to evolve an incredibly strange workaround. They have specialized abdominal muscles that act like a biological sling, physically pulling their organs down to create empty space in their chest so their lungs can inflate.

 It takes actual muscular effort just to take a single breath. And for some turtles that spend long periods under water, even that isn’t enough. Some species actually have to absorb oxygen through the water through highly vascularized tissues inside their cloaca. Yes, they literally have to breathe through their butts. Nature basically said, “You get to be invisible, but you have to breathe out your rear end.

” Evolution is majestic, isn’t it? So, for slow-moving reptiles that don’t need a lot of oxygen anyway, making this trade-off makes sense. But, what happens when an animal that actually needs to move fast tries to do this? Reptiles have a very low metabolism. They can afford to sit entirely still for days at a time, but mammals are warm-blooded.

 We have to constantly burn energy just to keep our body temperature up, which means we need a constant supply of oxygen and food. Because of this, a mammal completely giving up its mobility to become a slow, heavy box should be biologically impossible. It goes against everything that makes a mammal successful, but during the ice age, a mammal actually did it. It was called the glyptodon.

This was a prehistoric relative of the modern armadillo, but instead of just having a few flexible bands of armor on its back, it took things to the extreme. The glyptodon grew to the size of a Volkswagen Beetle, and its entire torso was covered in a solid dome of bone made from thousands of fused plates.

 It had an armored tail and a bony cap on its head. If you saw one in the wild, you’d assume someone just took a tortoise and scaled it up to the size of a car, and then glued some fur onto its face. And honestly, that’s exactly what it looks like. This warm-blooded mammal completely abandoned the speed and flexibility of its ancestors just to cosplay as a giant turtle.

 And the fact that a mammal was willing to accept the metabolic penalty of carrying around a giant bone dome shows just how overpowered this shape actually is. But realistically, how does a giant, slow-moving mammal actually survive in an era filled with some of the deadliest predators in Earth’s history? Let’s imagine how this would actually play out in the real world.

 You have a glyptodont minding its own business, eating some grass. Suddenly, a Smilodon, the famous saber-toothed cat, drops out of the bush and goes in for the kill. The cat has teeth that look like Yeah, absolute nightmare fuel. Meaning they were perfectly designed to slice through thick muscle and sever arteries. But when the cat actually lunges, nothing happens.

 The glyptodont just tucks its legs in, drops its massive dome to the dirt, and sits there. The saber-toothed cat bites down, and its teeth just scrape against solid bone. You can’t bleed out a rock. The cat wastes a massive amount of energy trying to find a weak point, eventually gives up, and walks away. The defense is flawless. Except, it’s not.

 The shell is incredible at stopping teeth, but it has one major structural flaw. Being a giant dome means you’re incredibly top-heavy. If a predator was smart enough, or just strong enough, it wouldn’t try to bite through the dome. It would just shove the glyptodont from the side. And once that car-sized animal rolls over onto its back, it’s completely over.

 The belly is entirely unarmored. And because their limbs are so short and restricted by the shell, they physically cannot flip themselves back over. The turtle defense is insanely effective, but it comes with a fatal weak point. So, how do you fix that? If hiding in a box leaves you vulnerable to getting flipped over, the logical solution is to make sure nothing can ever get close enough to flip you in the first place.

 And millions of years before the glyptodon existed, the dinosaur took the basic turtle blueprint and completely weaponized it. The most famous example of this is the ankylosaurus. Just like the turtle and the glyptodon, it had a wide body covered in thick bone plates. The ankylosaurus took this a step further.

 It covered its entire dome in razor-sharp bone spikes. If a predator tried to push it over, it would literally impale itself, which is a pretty solid way to say don’t touch me. And just to make sure the message was perfectly clear, the ankylosaurus added a massive club at the end of its tail. This heavy lump of bone was powered by massive tail muscles.

 If a T-Rex actually managed to get close enough to try and flip the ankylosaurus, a single swing from that tail club would instantly shatter the predator’s legs. Dinosaurs prove that evolving into a turtle doesn’t just mean you have to hide and hope the predator gets bored. If you take the turtle base and add weapons to it, you basically become untouchable.

 So, we have marine reptiles, ice age mammals, and giant dinosaurs all independently deciding to turn into a box. What exactly is triggering this? If you look at the placodont, the glyptodon, the ankylosaurus, and the modern turtle, you start to notice a very clear pattern. None of these animals are apex predators.

 None of them are chasing down fast-moving prey. They all rely on incredibly low energy diets. Placodonts ate slow-moving clams on the ocean floor. Glyptodons ate grass. Ankylosaurus ate low-flying ferns. And modern tortoises eat leaves and cacti. When the thing you’re hunting is a literal leaf, you don’t exactly need to be a track [music] star.

 But the problem is you still live in an environment surrounded by things that do eat meat. You’re a slow-moving target in a world full of fast-moving killers. So, evolution just takes a completely different route. If you don’t need speed to catch your food, you shouldn’t waste any energy on building long legs or flexible spines.

 You should take all of these evolutionary stat points and dump them entirely into pure defense. Turning into a turtle is what happens when a species completely gives up on trying to run away. It’s the ultimate commitment to just standing there. So, that explains why all these other animals turn into turtles. But, that brings us to the actual turtles themselves.

Because the way the real turtle got its shell was basically a complete accident. Given everything we’ve just talked about, it’s incredibly easy to assume that the ancestors of modern turtles slowly grew their shells over millions of years, specifically to protect themselves from predators. It’s the most logical conclusion, but evolution doesn’t always follow logic.

 A few years ago, paleontologists discovered a fossil of an ancient reptile called Eunotosaurus. This creature lived around 260 million years ago, and it’s widely considered to be the early ancestor of the turtle. When researchers looked at its skeleton, they noticed that its ribs were extremely wide and thick, just like the early stages of a shell.

 But, they also noticed something else. Eunotosaurus had incredibly thick arm bones with huge muscle attachments. It was actually trying to build a shovel. Eunotosaurus lived in a harsh environment, and the best way to survive was to dig deep burrows into the hard dirt. But, to dig efficiently, you need a strong anchor for your arm muscles to pull against.

 If your ribs are thin and flexible, your chest will just cave in under the strain of digging. So, Eunotosaurus widened its ribs to create a rigid base for its massive digging muscles. The widened ribs had absolutely nothing to do with stopping teeth. It was purely just to give them a stronger base to dig with. What is now the most famous defense mechanism in the animal kingdom originally started out as just a really aggressive way to dig a hole.

 But obviously, you don’t go from having slightly wide ribs to being a fully enclosed bone box overnight. There’s a middle step, and it’s highly uncomfortable. We like to think of evolution as a smooth process. An animal needs armor, so it slowly grows armor until it’s perfectly protected. But the reality is that evolution is incredibly messy, and it usually leaves animals in a state of terrifying vulnerability for millions of years.

 To see this, we have to take a look at another ancient turtle ancestor called Odontochelys. This creature lived about 220 million years ago, and it had a very specific problem. It spent most of its time swimming in shallow coastal waters. If you look at the fossil of Odontochelys, you’ll see that it actually had a fully formed shell, but only on its stomach.

 The bottom half of the turtle, called the plastron, was perfectly solid bone, but its back was completely exposed. It had normal ribs and soft skin on top. This seems like a terrible design. Why would you armor your belly and leave your back completely open to the world? Well, in the water, the biggest threats usually don’t come from above.

 Predators like massive marine reptiles and ancient sharks hunt by swimming deep below their prey and looking up, waiting to strike the soft underbelly. Odontochelys armored its stomach because that’s where the teeth were coming from. But this means that for millions of years, [music] the ancestors of turtles were just swimming around as awkward, half-armored targets.

 They didn’t magically spawn with a full defensive dome. They had to survive countless generations with a glaring weak point on their backs before the top half of the shell finally fused together. So, after millions of years of awkward half steps, the turtle finally got its full shell, and clearly it worked.

 So, why aren’t there any T-Rex size turtles walking around today? If the turtle body plan is the ultimate defense and getting bigger usually makes you even harder to kill, you’d expect modern land tortoises to be the size of elephants, but they aren’t. The largest living land turtles are the Galapagos tortoises, and while they’re definitely huge, they max out around 900 lb.

 They never reach the massive car-size scale of the ancient Glyptodons or Ankylosaurus. And the reason for this is entirely due to gravity. A turtle shell isn’t made of lightweight cartilage. It’s dense, heavy bone. And the larger a turtle gets, the exponentially heavier that bone becomes. On land, you have to physically carry that weight every single time you take a step.

 If a land turtle gets too big, the sheer mass of its own shell becomes a death trap. If a multi-ton land turtle were to accidentally trip over a rock and fall down a slight incline, the impact of thousands of pounds of bone slamming into the ground means, well, you’re probably not walking away from that. And even if it survived the fall, the sheer weight of the shell processing down on its lungs would slowly crush its internal organs before it could ever flip itself back over.

 Or really, before it could do anything. While getting bigger is usually an advantage in the animal kingdom, combining a massive body with a heavy dome bone on land creates a hard limit to how big you can actually get. You literally just get too heavy to exist. So, if gravity is the only thing stopping turtles from becoming massive, what happens if you just remove gravity entirely? If you want to break the physical limits of the turtle body plan, you have to leave the land completely and go back into the water.

 In the ocean, buoyancy effectively cancels out the crushing weight of gravity. You don’t need pillar-like legs to hold up your shell, and you don’t need to worry about your own weight crushing your lungs. And this is exactly how we got Archelon. Archelon was a prehistoric marine reptile that lived in the late Cretaceous period and it’s the largest turtle to ever exist on Earth.

 It measured over 15 ft long and weighed close to 5,000 lb. Because it lived in the ocean, Archelon didn’t have to worry about the gravity limit. It could expand its shell to ridiculous proportions using powerful flippers to glide through the water. It shared this ocean with some of the most terrifying predators in history like the Mosasaur, which looked like this.

 And it survived purely because its shell was so massive that almost nothing could get its jaws around it. When you remove the weight limit, the turtle shape can finally reach its absolute maximum potential. But even when you remove the weight limit and max out your size, the turtle blueprint still has one unavoidable flaw. The entire point of evolving into a turtle is to make yourself immune to the things trying to eat you.

 And for the most part, it works flawlessly. But the fatal flaw turtle blueprint is actually the planet itself. When you fuse your ribs together and turn your body into a rigid box, you’re making a permanent commitment to a very specific lifestyle. You’re slow. You require a very specific type of environment to survive.

 If the climate suddenly changes, you cannot easily adapt. If a drought wipes out your local vegetation, you cannot just sprint hundreds of miles to a new location like a herd of antelope can. You’re too slow and migrating takes massive amounts of energy that your rigid body isn’t designed to expend. This is why heavily armored giants like the Glyptodonts and the Ankylosaurus are all extinct.

 Their armor made them basically immune to teeth, but it made them incredibly vulnerable to extinction events. When the ice age ended or the asteroid hit, their rigid bodies couldn’t adapt to the rapidly changing food sources and temperatures. A shell is fantastic at protecting you from the animals on the planet, but it’s completely useless at protecting you from the planet itself.

 And surprisingly, the animals that understand this flaw better than anyone else are the turtles themselves. After millions of years of evolution, mutating their ribs, fusing their spines, and perfecting the ultimate bone box, you’d think turtles would be doubling down on their shells. Instead, the most successful marine turtles alive today are actually doing the opposite.

 Take the leatherback sea turtle, the largest living turtle on the earth today. If you look at its back, you won’t find a hard, solid dome of bone. Instead, its shell is made of a flexible layer of leathery skin and tough connective tissue. Or look at the soft-shell turtles that live in rivers and lakes.

 They have completely flattened out, abandoning the heavy bone plates for a rubbery pancake-shape that lets them bury themselves in the mud at lightning speed. These turtles realized that carrying around a heavy bone box in the water is actually a really bad idea. Bone is heavy, and it slows you down. If you want to dive thousands of feet deep to hunt jellyfish, or if you want to swim fast enough to escape a modern great white shark, you need to be lighter and more hydrodynamic.

 The very turtles that define this entire body plan are actively trying to get rid of their own signature trait. They’re slowly trying to unturtle themselves. But even if a leatherback gets rid of the heavy bone, it still can’t ever truly go back to being a normal animal. Because once you go down this path, you’re stuck.

 Turning into a turtle is undeniably an incredible survival strategy. It’s why marine reptiles, dinosaurs, ice age mammals, and actual turtles all independently stumbled upon the exact same blueprint. When the world is full of things trying to kill you, turning yourself into a heavily armored box is a guaranteed way to stay alive. But it’s also an evolutionary trap.

 Once it flattens its ribs, traps its shoulder blades, and fuses its spine into a solid dome, it can never undo it. A crab can evolve to be a slightly different shape of a crab. A mammal can evolve to run faster or climb trees, or even return to the ocean and become a whale, but a turtle can only ever be a turtle.

 It can never evolve wings and take to the sky. He has permanently locked its skeleton into this exact shape forever. Nature keeps building the box because the box works, but once you step inside the box evolution closes the lid, and you’re never getting out. But anyways, I’m personally just glad humans never tried to evolve into a box.