Pincer-wielding 507m-year-old fossil sheds light on evolution of crabs – The Guardian

A fossilised ancient creature boasting huge pincers resembling can-openers, a hinged two-piece shell and more than 50 pairs of legs has been discovered, shedding light on the evolutionary past of a huge and diverse group of animals.

Researchers say the creature, thought to have lived about 507 million years ago during the Cambrian period, offers insights into the early body plan of mandibulates a group that encompasses creatures including millipedes, crabs and ants. The group takes its name from the presence of mouth parts known as mandibles, which the animals use to help hold or eat food.

Because it is such a big group, the question is why was it so successful, why did it manage to diversify so much?, said Cedric Aria, co-author of the study from the Nanjing Institute for Geology and Palaeontology, in China. We really lacked an insight into the characters, the traits, that really were fundamental to that diversification.

The sturdy-looking creature, adds Aria, was about 10cm long and would have been found walking on the seafloor, perhaps occasionally swimming, and probably fed on soft-bodied animals that were adept at escaping or hiding.

The prey, says Aria, would have been caught by the animal using its two large pincers. When I first started to study this animal I really thought that they looked like one of those old can openers, he said.

The prey, he adds, would then have been passed to the animals many legs under the body which have spine-like features at their base. The spines might have helped to crush the prey and the remains of that prey would have been brought back to the front where the mandibles would have cut the flesh into small pieces and so that facilitated digestion, he said. The mandibles would have been a revolutionary tool to process food.

Previously discovered fossils of similar creatures with two-part shells had lacked details around the head, including evidence of mandibles. As a result, such fossilised animals had been proposed to be early forms of a category of creatures known as true arthropods. This category includes both mandibulates and other invertebrates that have an exoskeleton and segmented body and appendages, including spiders and the extinct marine creatures called trilobites.

But the new finding, published in the journal Nature, squashes the idea. Rather than occurring at the base of the true arthropod family tree, the new discovery suggests that these creatures with two-part shells actually appeared later in the family tree and are in fact early mandibulates.

It might reflect the body plan of the ancestor of that super mega-group, said Aria of the new find, adding that the presence of legs with a segmented, spiked base in the creature was an important feature. Those segmented bases of the limbs actually explain the diversity of the limbs in mandibulates and they explain the origin of the mandibles themselves, said Aria.

Unearthed in recent years at a site near Marble Canyon in the Canadian Rockies of British Columbia, the newly discovered fossilised creature has been dubbed Tokummia katalepsis a nod to the Tokumm Creek that is surrounded by the Marble Canyon and the Greek word for grasping.

Graham Budd, professor of palaeobiology at Uppsala University in Sweden who was not involved in the study, cautiously welcomed the new discovery. If it is true, [this research shows] that a large number of quite important fossils from the Cambrian are actually all close relatives of the modern day crustaceans and insects, he said. This is very significant because for the first time it allows us to really understand the origins of this really important group of organisms.

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Pincer-wielding 507m-year-old fossil sheds light on evolution of crabs - The Guardian