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. If you like what we do here, go to youtube.com/eons
and subscribe!
I think the answer "because being crablike is better for crustaceans than not being crablike" leaves a lot to be desired, especially since the video mentions that during the same time period that this occurred, several crabs evolved to be less crablike.
Also doesn't explain why the crustaceans evolved to be not crablike in the first place, before they reversed course and became crablike.
Perhaps something changed in the environment that changed the optimal design?
advance to crab
Reject humanity, evolve into C R A B
Now I want a TierZoo video ranking crabs.
Zoidberg was ahead of the curve.
There is only one step
And it is crab
So someday the South Park prediction of crab people will come true?
Reminds me of my ex.
🦀🦀🦀