Fossil throws crustacean evolution into disarray

Images by Daniel Ocampo R., Vencejo Films, & Javier Luque, Yale University; animation and 3D reconstruction by Alex Duque

The crab family just got bigger, and scientists suggest we may even have to rethink how we define it.

Among hundreds of exceptionally well-preserved specimens from the mid-Cretaceous period – 90 to 95 million years ago – recently uncovered in Colombia and the US was Callichimaera perplexa, a previously unknown species that combines traits from several extinct and living crab species, and from different crustacean life stages.

And that suggests that the body plan of the “true crab” was lost and re-evolved many times over, according to palaeontologist Javier Luque from Yale University in the US, who led the international research team.

“Callichimaera perplexa is so unique and strange that it can be considered the platypus of the crab world,” he says.

“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.”

Its small size, large compound eyes with no sockets, bent claws, leg-like mouth parts, exposed tail, and long body are features typical of pelagic crab larvae, Luquehe says, suggesting that several of its larval traits might have been retained and amplified in miniaturised adults via changes in the timing and rates of development.

This is a process called heterochrony, which may lead to the evolution of novel body plans.

Callichimaera perplexa translates as “perplexing beautiful chimera”, and Luque notes that it is cute as well as unusual.

Details of the new discoveries are published in a paper in the journal Science Advances.