The newly-discovered fossil of a giant extinct sea creature could reveal secrets of the early evolution of arthropods.
The lobster relative, dubbed Aegirocassis benmoulae, is believed to have swam the seas around 480 million years ago, and boasted "modified legs, gills on its back, and a filter system for feeding," Yale University reported.
"Aegirocassis is a truly remarkable looking creature," said Yale University paleontologist Derek Briggs, co-author of a Nature paper, describing the animal. "We were excited to discover that it shows features that have not been observed in older Cambrian anomalocaridids - not one but two sets of swimming flaps along the trunk, representing a stage in the evolution of the two-branched limb, characteristic of modern arthropods such as shrimps."
Arthropods have been one of the most diversity-rich animal groups on Earth since they first appeared about 530 million years ago. The group includes modern animals such as "horseshoe crabs, scorpions, spiders, lobsters, butterflies, ants, and beetles." Arthropods are characterized by their hard exoskeleton that is molted during growth, as well as bodies and legs that are made up of multiple segments.
Researchers have been working to understand how modern arthropods' "double-branched limbs" evolved. They thought the answers lied in a long-extinct group called anomalocaridids, which had lateral flaps used for swimming. Until now, the researchers thought the ancient animals had only one set of flaps per trunk segment and that their walking legs had disappeared completely.
The discovery of Aegirocassis benmoulae suggests anomalocaridids had two separate flaps per segment, and those located on the lower portion of the body were actually walking limbs that had been modified for swimming. This means the new fossils represent an evolutionary stage that occurred before the fusion of the upper and lower branches into the double-branched limb seen in modern arthropods.
"It was while cleaning the fossil that I noticed the second, dorsal set of flaps," said first author Peter Van Roy, an associate research scientist at Yale and leader of the research. "It's fair to say I was in shock at the discovery, and its implications. It once and for all resolves the debate on where anomalocaridids belong in the arthropod tree, and clears up one of the most problematic aspects of their anatomy."
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