Callichimaera perplexa - Printable Version

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Callichimaera perplexa - Taipan - 04-25-2019

Callichimaera perplexa

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Temporal range: Late Cretaceous (Cenomanian to Turonian)

Scientific classification
Kingdom:  Animalia
Phylum:  Euarthropoda
Subphylum:  Crustacea
Class:  Malacostraca
Order:  Decapoda
Suborder:  Pleocyemata
Infraorder:  Brachyura
Superfamily:  †Callichimaeroidea
Family:  †Callichimaeridae
Genus:  †Callichimaera
Species:  Callichimaera perplexa

A crab belonging to the new family Callichimaeridae. The type species is C. perplexa.

Meet Callichimaera perplexa, the platypus of crabs

by Yale University
The crab family just got a bunch of new cousins—including a 95-million-year-old chimera species that will force scientists to rethink the definition of a crab.

An international team of researchers led by Yale paleontologist Javier Luque announced the discovery of hundreds of exceptionally well-preserved specimens from Colombia and the United States that date back to the mid-Cretaceous period of 90-95 million years ago. The cache includes hundreds of tiny comma shrimp fossils, several true shrimp, and an entirely new branch of the evolutionary tree for crabs.
The most intriguing discovery, according to the researchers, 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. The name derives from a chimera, a mythological creature that has body features from more than one animal. Callichimaera's full name translates into "perplexing beautiful chimera."
Luque noted that Callichimaera's "unusual and cute" appearance, including its small size—about the size of a quarter—large compound eyes with no sockets, bent claws, leg-like mouth parts, exposed tail, and long body are features typical of pelagic crab larvae. This suggests that several of the larval traits seen in this "perplexing chimera" might have been retained and amplified in miniaturized 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 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."

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Callichimaera perplexa: The oldest swimming crab from the dinosaur era. Credit: Daniel Ocampo R., Vencejo Films

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Artistic reconstruction of Callichimaera perplexa, the strangest crab that has ever lived. Credit: Elissa Martin, Yale Peabody Museum of Natural History

A study about the discovery appears in the April 24 online edition of the journal Science Advances.
"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.

Journal Reference:
J. Luque el al., "Exceptional preservation of mid-Cretaceous marine arthropods and the evolution of novel forms via heterochrony," Science Advances (2019). DOI: 10.1126/sciadv.aav3875 ,
Journal information: Science Advances 

Evolutionary origins of novel forms are often obscure because early and transitional fossils tend to be rare, poorly preserved, or lack proper phylogenetic contexts. We describe a new, exceptionally preserved enigmatic crab from the mid-Cretaceous of Colombia and the United States, whose completeness illuminates the early disparity of the group and the origins of novel forms. Its large and unprotected compound eyes, small fusiform body, and leg-like mouthparts suggest larval trait retention into adulthood via heterochronic development (pedomorphosis), while its large oar-like legs represent the earliest known adaptations in crabs for active swimming. Our phylogenetic analyses, including representatives of all major lineages of fossil and extant crabs, challenge conventional views of their evolution by revealing multiple convergent losses of a typical “crab-like” body plan since the Early Cretaceous. These parallel morphological transformations may be associated with repeated invasions of novel environments, including the pelagic/necto-benthic zone in this pedomorphic chimera crab.