An article published in the journal “Science Advances” reports the discovery of a marine arthropod dating back to about 95 million years ago, during the Mid-Cretaceous period. A team of researchers led by paleontologist Javier Luque of the American Yale University announced the discovery in Colombia and in the USA of hundreds of exceptionally preserved specimens of various species that include the one so far unknown that they named Callichimaera perplexa. It’s a crab that represents the oldest swimming arthropod known after the extinction of sea scorpions, which occurred about 250 million years ago. Its strange features could lead to rethinking the definition of crab.
Crabs form the infraorder scientifically called Brachyura within the arthropod phylum. They typically have features such as a large carapace, two strong claws and small eyes. The oldest fossils identified as crabs date back to the Jurassic period but in the subsequent one, the Cretaceous, lived a relative with really curious features. The top image (Courtesy Javier Luque et. Al. / Science Advances) shows some fossils of Callichimaera perplexa from different points of view including some of their anatomical details.
The new species named Callichimaera perplexa was very small given that the specimens have a carapace between 4 and 10 millimeters wide but above all they have characteristics like eyes much larger than those typical of crabs and others that are typical of the larvae of the species Pleuroncodes planipes, which is a lobster and therefore part of the infraorder Anomura.
The process called heterochrony consists precisely in maintaining and amplifying certain larval traits in adult individuals following changes in the timing and speed of development. In these cases, a species can evolve developing physical characteristics that are very different from the original ones. In Callichimaera perplexa’s case, the result is unique and strange, different from the typical crabs to the point that Javier Luque called it the platypus of crabs.
Callichimaera perplexa’s paddle-like legs made it skilled in swimming, a characteristic that existed in arthropods, to be precise in sea scorpions, many millions of years before. It’s a characteristic that evolved a number of times among arthropods. There were some evolutionary convergences even though some anatomical parts had different origins.
A precise positioning of Callichimaera perplexa in crabs family tree is difficult: it has some characteristics that convinced the researchers that it wasn’t a lobster and was defined a chimera for the mix of characteristics seen in different living and fossil groups of crabs and lobsters but their set was never found in a single group.
According to Javier Luque, Callichimaera perplexa’s strange characteristics should lead to rethinking the definition of crab. It was certainly a curious little animal that revealed the existence of a possible separate branch in crabs family tree showing that they were already considerably diversified in the Cretaceous period.
