How arthropods got their legs

Fossils show flaps that fused to make modern limbs

ancient arthropod

FLAPPING AROUND  A newly discovered ancient arthropod had two rows of flaps that later melded into limbs like those sported by lobsters, horseshoe crabs, and other relatives alive today.

Marianne Collins/ArtofFact

Newly discovered fossils fill a gap in scientists’ knowledge of how arthropods evolved their legs. The extinct creatures’ limblike flaps represent a stage before the two parts fused into the limbs seen in many arthropod species now living.

The fossils, which belong to a group of very early arthropods called anomalocaridids, were found from 2009 to 2014 in Morocco. The earliest anomalocaridids lived 530 million years ago, during the Cambrian Period. Paleontologists thought that the group had gone extinct by 505 million years ago, but the fossils of the new species, named Aegirocassis benmoulae, are 480 million years old.

A. benmoulae has two rows of flaps running down its body. It would have used the top row to steer and breathe, and the bottom to propel itself through the water, says Peter Van Roy of Yale University, a coauthor of the new research published

ANCIENT ARTHROPOD Aegirocassis benmoulae used the flaps on its body (two visible on this fossil) to swim and breathe. Peter Van Roy/Yale University
online March 11 in Nat ure .

Many present-day arthropods, such as crustaceans, have combined these two flaps, forming legs. “You can then connect muscles between those different segments,” says Van Roy. These modifications give arthropods more control over their movement.

“In anomalocaridids these structures are still separate on the body,” he says. “This shows that anomalocaridids predate … the fusion of these two structures.”

Though earlier anomalocaridids actively chased down their prey, A. benmoulae was a filter feeder. At about 2 meters long, it was also the largest animal alive in the Ordovician Period that he knows of, says Van Roy. To support such large filter feeders, plankton in the Ordovician oceans were probably plentiful and diverse.

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