The strangest crab that has ever lived

See Callichimaera perplexa in 3D: Watch this video to see an animated, three-dimensional recreation of the puzzling mid-Cretaceous crab that, scientists say, is forcing a rethink of what a crab is.

A recent discovery of hundreds of fossils has given the crab family a new cousin, a 95-million-year-old extinct species that, scientists say, will force us to rethink the definition of a crab.

An international team of researchers discovered the exceptionally well-preserved specimens, dating back to the mid-Cretaceous period of 90-95 million years ago, in rock formations in Colombia and the United States.

The most intriguing discovery, according to the researchers, is Callichimaera perplexa. They say the creature, about the size of a quarter with paddle-like legs, is the earliest example of a swimming arthropod since the extinction of sea scorpions more than 250 million years ago. The name derives from the chimera, a mythological creature that has body features from more than one animal. Callichimaera’s full name translates to “perplexing beautiful chimera.”

Weird many-legged sea creature with big eyes swimming undersea.
Artistic reconstruction of Callichimaera perplexa. Image via Elissa Martin, Yale Peabody Museum of Natural History.

Yale University paleontologist Javier Luque led the study, which was published April 24, 2019, in the journal Science Advances. Luque said that the crab’s “unusual and cute” appearance — large compound eyes with no sockets, bent claws, leg-like mouth parts, exposed tail, and long body — are features typical of crab larvae from the open sea. He said this suggests that some ancient crabs retained a few of their larval traits into adulthood, amplified them, and developed a new body architecture. This is an evolutionary process called heterochrony. Luque said:

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

Many crabs on black background.
Endless forms most beautiful: the diversity of body forms among crabs, including the enigmatic Callichimaera perplexa (center). Image via Arthur Anker & Javier Luque, Yale University.

Luque added:

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.

Smiling scientist holding rock with small fossil crab visible on flat side.
Javier Luque poses with Callichimaera perplexa. Image via Daniel Ocampo R., Vencejo Films.

Bottom line: Scientists say a new fossil discovery, Callichimaera perplexa, is the strangest crab that ever lived.

Source: Exceptional preservation of mid-Cretaceous marine arthropods and the evolution of novel forms via heterochrony

Via Yale University

May 7, 2019

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