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Yale: Weird new crab species forces rethink of definition of crab

Artistic reconstruction of Callichimaera perplexa: The strangest crab that has ever lived
Artist’s concept of Callichimaera perplexa/Elissa Martin, Yale Peabody Museum of Natural History

And maybe of how animals evolve over time:

The crab family just got a bunch of new cousins, including a 95-million-year-old species that will force scientists to rethink the definition of a crab — and perhaps the disparate ways animals evolve over time…

“Callichimaera perplexa is so unique and strange that it can be considered the platypus of the crab world,” said Luque. “It hints at how novel forms evolve and become so disparate through time. Usually we think of crabs as big animals with broad carapaces, strong claws, small eyes in long eyestalks, and a small tail tucked under the body. Well, Callichimaera defies all of these ‘crabby’ features and forces a re-think of our definition of what makes a crab a crab.” Jim Shelton, “Meet Callichimaera perplexa, the platypus of crabs” at YaleNews

What makes a crab a crab? This almost sounds like philosophy class.

Anyway, National Geographic describes it as “crazy” Researchers found a fossil site in Colombia from 90 million years ago featuring very well-preserved bodies including the“platypus” of crabs:

Today’s adult true crabs typically have wide, oval-shaped bodies, and their eyes are mounted on stalks. Crabs that burrow or swim, such as blue crabs, might have flattened, oar-like pairs of hind legs. What’s more, true crabs’ tails are tucked underneath the body.

Callichimaera breaks all of these rules. Its body is far narrower and fuselage-shaped than the standard crab body plan, and its Ping Pong ball-like eyes aren’t on stalks, a trait more often seen in larval crabs today than in full-grown adults. In addition, its tiny tail isn’t tucked underneath the body, and it has two pairs of enlarged, paddle-like limbs—but they’re on the animal’s front, not its rear. Its shell also bears a pattern unlike anything seen on another crab, living or dead.Michael Greshko, “How did crabs evolve ‘crabbiness’? It’s complicated.” at National Geographic

Relax. We didn’t know there were rules for crabs. But this one was compared to a “flying dolphin.”

Maybe we will find a lot more such oddities as we explore more. The overall group (shrimps, crabs, lobsters) is said to date from 350 to 370 million years ago.

See also: Cambrian shrimp’s heart more complex than modern one

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