Fossil Discovery Has Scientists Questioning: What Makes a Crab a Crab?

The newly described C. perplexa seems to have retained larval features into adulthood

Artistic reconstruction of Callichimaera perplexa, the "strangest crab that has ever lived." Elissa Martin, Yale Peabody Museum of Natural History

In 2005, Yale paleontologist Javier Luque was mapping the geology of the Colombian town of Pesca when he stumbled upon the well preserved fossil of a tiny, ancient crab. The creature was like nothing he had ever seen before.

“Usually we think of crabs as big animals with broad carapaces [hard outer shells], strong claws, small eyes in long eyestalks, and a small tail tucked under the body,” Luque says. But the strange specimen boasted a very different appearance. No bigger than a quarter, it had a lobster-like shell, claws resembling those of modern frog crabs, and several traits that suggested it had retained larval features into adulthood: an exposed tail, a spindle-shaped body, leg-like mouthparts and large, bulging eyes with no sockets.

“Imagine you had a flying dolphin,” Luque tells Michael Greshko of National Geographic. “[T]hat’s what we have.”

It took years of research to confirm that the curious crab was, in fact, a crab, reports CBC News. Now, writing in the journal Sciences Advances, Luque and his team of fellow scientists have described the bizarre crustacean, which they say represents “a unique lineage of ancient true crabs.” The team has dubbed the animal Callichimaera perplexa, or “perplexing beautiful chimera”—a reference to the hybrid monster of ancient Greek mythology.

C. perplexa paddled through the seas between 90 and 95 million years ago, during the Cretaceous period. Its oar-like legs represent “the earliest known adaptations in crabs for active swimming,” according to the study authors, and its small but mighty claws suggest that C. perplexa hunted even tinier crustaceans. With its bulbous eyes, C. perplexa may have been nocturnal.

Since his initial discovery of the fossil, Luque was able to collect around 70 specimens that had been found not only in Colombia, but also in Wyoming and Morocco, according to Live Science’s Laura Geggel. The wide gepgraphic range of fossil specimens in turn indicates that the crab was an adaptable little creature.

Scientists are excited by C. perplexa not only because it is new and fascinatingly weird, but also because it helps to fill in important gaps in crab’s evolutionary history. Crabs are certainly diverse and multitudinous: more than 7,000 extant species have been described and another 3,000 are known from fossils. But even still, there is much to be learned about the past and present links between them.

According to the study authors, what we think of as a “typical” crabby body has been gained and lost several times among both true crabs and “false” crabs, a group that includes hermit crabs and king crabs, writes Discover’s Charles Choi. C. perplexa shows just how complicated that evolutionary trajectory may have been; the creature seems to have adapted to its environment by developing a pastiche of advanced traits—like swimming legs and strong claws—and larval ones.

“It hints at how novel forms evolve and become so disparate through time,” Luque says. “[C. perplexa] defies all of these ‘crabby’ features and forces a re-think of our definition of what makes a crab a crab.”

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