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 who is really horrified by coconut….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!
Impostors 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 Palaeopalaemon.
Decapods are named for their ten feet, and their order includes things like shrimp, crayfish,
lobsters, and crabs -- both true and false.
And Palaeopalaemon 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 in soft seafloor mud.
But then, around 260 million years ago, decapods split into two groups -- the Anomurans, or
those false, fake wanna-be crabs, and the Brachyurans, or the 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 re-evaluating the only known specimen with new imaging techniques, experts decided
that Eoprosopon was a very early member of the Brachyura.
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, a crustacean 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 a hundred
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.
OK, so, but whyyyy 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.
If you happen to have a band that you've started that needs a name
Both of those are available!
Nearly 80 percent of the major groups of true crabs that we know of 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 seemed 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 climb tree, 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 that predators
might grab onto.
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 times when 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 perplexa
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 kinda looked 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.
I mean look at this things 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 in the sand,
and propel themselves through the water.
They live in freshwater, 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 for false crabs for wanting to be crabs.
Just please keep those coconut crabs the heck away from me, forever.
Big thanks to this month’s clawsome Eontologists: Sean Dennis, Jake Hart,
Annie and Eric Higgins, Jon Davison Ng, and Patrick Seifert.
You can become an Eonite by supporting us at patreon.com/eons.
And remember - Eonites get perks like submitting a joke for us to read!
Which I’m going to do right now... this one is from Stephen O'Leary
He says "The Amniotic Egg was egg-sackly what we needed."
Oh man
And as always thank you for joining me in the Konstantin
Haase studio.
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