What has the mouth of a shrimp, the claws of a frog crab, and the carapace of a lobster?
We’ll give you a hint: the animal is definitely extinct.
If you guessed Callichimaera perplexa then you are correct. If you didn’t, well, it’s as weird as it sounds.
The discovery of “exceptionally well-preserved” 95-million-year-old fossils of a chimera species in the Andes Mountains of Colombia is forcing researchers to rethink the definition of what a crab is and where exactly this new species sits on the crustacean tree of life.
"Callichimaera perplexa is so unique and strange that it can be considered the platypus of the crab world," said palaeontologist Javier Luque in a statement. "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."
Over 70 specimens of this one particular type of crab were preserved in soft clay along with hundreds of other prehistoric crustaceans, such as shrimp and lobster. Their exceptional preservation allows us to see in detail what these mid-Cretaceous crustaceans looked like.
"We found dozens of animals, from tiny baby specimens to mature individuals in which we found reproductive organs – a smoking gun that proves these were adult organisms and not larvae. We can even see individual facets on the large compound eyes of these creatures," said Luque. "It's an incredible amount of detail, and we've been able to reconstruct them like they were living yesterday."
Callichimaera exhibits features of many marine arthropods in one; it's only the size of a quarter with large compound eyes void of sockets and has tiny bent claws, a mouth that resembles legs, an exposed tail, and an abnormally long body. Altogether, it looks more like a pelagic crab larva than an adult crab. Its paddle legs and large eyes indicate the critter spent most of its life swimming through shallow seas rather than crawling like the crabs of today. The larval traits may have been retained and amplified in miniature adults via a process called heterochrony, which results in the evolution of new and unique body plans.
So unique, in fact, that the crustacean has earned itself a new branch on the crustacean tree of life.
"It is very exciting that today we keep finding completely new branches in the tree of life from a distant past, especially from regions like the tropics, which despite being hotspots of diversity today, are places we know the least about in terms of their past diversity," Luque said.
Callichimaera – whose name translates to “perplexing beautiful chimera” in an ode to the Greek mythological lion-headed, goat-bodied, snake-tailed creature – was described in Science Advances.