(host) Around 10 million years ago off the coast of what's now New Zealand, a spiny, leggy creature with claws cruised the Pacific Ocean looking for food.

This creature was an ancient ancestor of modern-day king crabs, known to scientists as Paralomis debodeorum.

It had a round, spiky carapace and a body about the size of a baseball, but not much else is known about it.

This fossil is the only one of its kind ever found.

And based on how modern king crabs live, scientists think that Paralomis probably lived in very deep, cold water.

But no matter how Paralomis spent its days, scientists agree that if you saw one today, you'd recognize it as a crab.

But it wasn't a crab, at least not a true crab.

Paralomis and its modern counterparts belong to an infraorder known as anomura, a huge group of crustaceans that includes everything from the porcelain crab to the...coconut crab.

Am I the only one who's really horrified by coconut-- just--coconut crabs?

Give me a minute.

Anyway, none of those animals that we call crabs are actually crabs.

They all evolved from crustaceans that were longer and had tails, with a body plan more like a shrimp or a lobster.

And then for some reason, these animals evolved into things that looked like crabs independently over and over again.

They turned into false crabs, fake, imposters, I tell you.

So why does this keep happening?

What is it about the crab's form that makes it so evolutionarily successful, so successful that non-crabs are apparently jealous of it?

Well, it might sound odd to say this about an animal with a shell, but the answer seems to be that it's all about flexibility.

Crab-like creatures date back to the late Devonian period, about 365 million years ago.

And they started with the first decapod crustaceans, like paleopalaemon.

Decapods are named for their ten feet, and their order includes things like shrimp, crayfish, lobsters, and crabs-- both true and false-- and paleopalaemon is the oldest lobster-like decapod ever found and also one of the oldest decapods period.

It probably lived on the seafloor like a modern lobster does and may have had specialized feeding appendages for browsing around on soft seafloor mud, but then around 260 million years ago, decapods split into two groups: the anomurans--or those false, fake, wannabe crabs-- and the brachyurans-- or true crabs, the crabs that aren't lying to you.

The oldest undisputed true crab is Eoprosopon, a crustacean that lived 185 million years ago in the early Jurassic period.

And for a while, scientists argued over whether it actually belonged in the crab family, because it has some un-crab-like traits, like a slightly more elongated abdomen and really prominent antennae.

But after reevaluating the only known specimen with new imaging techniques, experts decided that Eoprosopon was a very early member of the brachyuran.

Now, on the other side of the decapod family tree, there's Platykotta, just a little older than Eoprosopon at about 200 million years old from the late Triassic period.

It's thought to be the oldest false crab, and it looked more like a lobster.

But over the course of evolutionary time, both the anomurans and the brachyurans came closer and closer to what we think of as the crab shape that we know today.

In both groups, time and time again, crustaceans started out with the elongated body of a lobster, only to evolve into the rounder, flatter shape of a crab.

And scientists have been trying to figure out why this happens for more than 100 years.

One of the first naturalists to recognize this was an English zoologist named Lancelot Alexander Borradaile, and in 1916, he named the process carcinization, which means to become crab-like, and he also described how he thought the process happened.

Basically, the long tail of a lobster, called a pleon, grows shorter over time and gets tucked under the body.

And at the same time, the narrow front part of the lobster, the carapace, grows wider and flatter until it eventually winds up looking like what we'd call a crab.

Okay, but so why would these animals repeatedly go from being long and narrow to flat and round?

Well, back in the 1980s, a study analyzed all of the fossil crustaceans throughout the whole Mesozoic era and discovered that there was an explosion of crab-shaped crustaceans during this span of time.

Another more recent study found that this phenomenon really took off during the Cretaceous period, which experts sometimes call the Cretaceous crab revolution or, if you prefer, the Mesozoic decapod revolution.

[laughs] If you happen to have a band that you started that needs a name, both of those are available.

Nearly 80% of the major groups of true crabs that we know today originated in this period, and throughout the Mesozoic era, there was a long-term shift in diversity toward more crab-shaped species, specifically toward true crabs and away from long-bodied ones.

The crab-like creatures also seem to have exploited many more different kinds of habitats than their more lobster-y relatives.

One possible explanation for this is that their shape allowed for greater mobility.

The rounder, flatter shape of crabs lets crabs walk, run, swim, and burrow more efficiently.

There are even crabs that can climb trees, which is its own special kind of nightmare fuel.

By contrast, animals that are shaped more like lobsters or shrimp are often limited to just swimming, burrowing, or scuttling on the ground.

Other researchers say the transformation to a crab shape was a way for organisms to better evade predators.

By losing the pleon, that long tail, they had one less delicious appendage their predators might grab on to.

so since the crab shape allowed crustaceans to go more places, do more things, and evade more predators, that shape was selected for over time.

And lots of elongated crustaceans came to be shaped like crabs, even if they weren't crabs, But of course, sometimes certain adaptations, no matter how helpful they might seem, just don't stick.

And just to keep things interesting, there have been many instances in which crustaceans lost their crab-like shape.

This is called-- you guessed it-- decarcinization.

This has happened among true crabs, like in a fossil crab called Callichimaera perpelxa.

Its name means perplexing, beautiful chimera, and it lived between 95 million and 90 million years ago during that Cretaceous crab revolution thing.

And it looked kind of like a crab but not totally.

Its large eyes weren't on stalks, it had front limbs shaped kind of like oars, and its body shape wasn't quite the same as what we normally think of as crabby.

Look at those big adorable eyes.

It's like the baby Yoda of crabs.

All told, the phenomenon of carcinization is one of the more fascinating examples of convergent evolution.

There are a lot of reasons a crustacean might evolve into the shape of a crab, even if it's not actually a crab.

It allows for a lot of versatility in locomotion and lifestyle.

Today crabs can clamber around the deep sea, scuttle over beaches, burrow into the sand, and propel themselves through the water.

They live in fresh water, salt water, and on land.

They're pretty much everywhere, true crabs and false alike.

At the end of the day, it's all about how well an organism can survive in its environment.

So maybe we can't blame the false crabs for wanting to be crabs.

Just please keep those coconut crabs the heck away from me forever.

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