Scientists uncover ancient platypus crab

Artistic reconstruction of Callichimaera perplexa
Artistic reconstruction of Callichimaera perplexa. (Credit: Elissa Martin/Yale Peabody Museum of Natural History).  

An international team of scientists under Yale paleontologist Javier Luque has discovered a 95 million year-old “platypus crab” that may throw into question how you define a crab. Found in rock formations in Colombia and the United States dating back to the mid-Cretaceous period 90 to 95 million years ago, the tiny swimming crab with the scientific name of Callichimaera perplexa shows larval features that produce new insights on evolution.

With a name that translates as “perplexing beautiful chimera,” the platypus crab is the size of a US quarter coin and was found as part of a fossil collection of crustaceans that the team says include hundreds of comma shrimp fossils, carideans or “true” shrimps, and what is referred to as an entirely new branch of the evolutionary tree for crabs.

Callichimaera‘s strange, wide-eyed appearance is due to the process of heterochrony, which is a way that animals evolve by changing the timing of how they develop into adults. Examples of this include how some humans developed larger skulls by continuing to grow the relatively large heads that infants have, or how giraffes have long necks through allowing the cervical vertebrae to continue to elongate.

Javier Luque poses with a Callichimaera perplexa fossil.  (Credit: Daniel Ocampo R/ Vencejo Films).

In the case of Callichimaera, it carried over the large compound eyes, lack of eye sockets, bent claws, leg-like mouth parts, exposed tail, and long body into adulthood. This not only made it adorable in a crabby sort of way, but by retaining the larval water-treading ability, it was also the first known swimming arthropod with paddle-like legs since sea scorpions went extinct 250 million years ago during the Permian–Triassic extinction event.

More important, a crab like this disrupts current ideas of how we define a crab.

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

The research was published in Science Advances. The video below shows the structure of the platypus crab.

Source: Yale University

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