Scientists Discover ‘Strangest Crab That Ever Lived’

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Researchers have discovered new branches of the crab family tree, including a 95-million-year-old species that’s being called the “strangest crab that has ever lived” and is helping shed light on the evolution of “crabbiness.”

A team of international researchers, led by Yale University paleontologist Javier Luque, uncovered the hundreds of exceptionally well-preserved specimens in rock formations in Colombia and the United States that date back to the mid-Cretaceous period of 90-95 million years ago.

The discovery, described in the journal Science Advances, includes hundreds of tiny comma shrimp fossils, with their telltale comma-esque curve; several carideans, which are the widely found “true” shrimp; and an entirely new branch of the evolutionary tree for crabs.

But the most intriguing find is Callichimaera perplexa, the earliest example of a swimming arthropod with paddle-like legs since the extinction of sea scorpions more than 250 million years ago. About the size of a quarter, the Callichimaera has “unusual and cute” features — large compound eyes with no sockets, bent claws, leg-like mouth parts, exposed tail, and long body —  typical of crab larvae from the open sea.

This suggests that some ancient crabs may have retained a few of their larval traits into adulthood, amplified them, and developed a new body architecture. This is an evolutionary process called “heterochrony.”

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.”

Javier Luque poses with Callichimaera perplexa — a 95-million-year-old species that is shedding light on crustacean evolution. (Photo Credit: Daniel Ocampo R., Vencejo Films)

Luque also noted the significance of making the discovery in a tropical region of the world, where there are fewer researchers looking for fossils in the tropics, and where he amount of ground cover and thick vegetation make access to well-exposed rocks more challenging.

“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.