A remarkably well-preserved fossil of a 480-million-year-old sea monster is helping researchers understand the evolution of arthropods. The creature, an anomalocaridid, has not one but two sets of legs on each of its body segments, showing that it’s an ancestor of modern-day arthropods, which include arachnids, insects and crustaceans.
Despite its size, A. benmoulae was a gentle giant, said John Paterson, an associate professor of paleontology at the University of New Englandin Australia, who was not involved in the study.
“Its feeding appendages [were] built for filtering plankton, not grasping prey,” he said. “This is in contrast to older [anomalocaridid] species, some of which are interpreted to be the apex predators of their time.”
This intricate filter-feeding net allowed A. benmoulae to feed on plankton and other organisms floating in the ancient ocean near modern-day Morocco.
The animals have a peculiar anatomy, researchers said.
“I like to describe anomalocaridids as being like a mutation experiment gone wrong,” Paterson told Live Science. The animal looks like a cross between a mantis shrimp, a cuttlefish and a baleen whale, he said.
“The head is very whalelike in being rather large and elongate, and with long, bristlelike appendages underneath,” he said. “The body is segmented — as in most arthropods, such as shrimp and lobsters — and the series of flaps along the sides are reminiscent of the fins of modern cephalopods, like cuttlefish, that aid in swimming.”
This image shows a dorsal view of the fossil alongside a drawing of the specimen. Note the double set of lateral flaps — a discovery that researchers overlooked in previous anomalocaridid fossils.
“Until now, paleontologists have thought that these ‘primitive’ arthropods had one set of flaps on each side, which could not be easily reconciled with our understanding of the anatomy of true arthropods,” Paterson said.
(Photograph by Peter Van Roy, Yale University; drawing by Allison C. Daley, University of Oxford)