A pair of ancient fossilized brains have shed new light on the evolutionary transition from soft to hard bodies among the predecessors of modern arthropods, the group of creatures that includes insects, arachnids and crustaceans.

The two brains, each some 500 million years old, belong to a soft-bodied trilobite and an odd submarine-like creature with the scientific name Odaraia alata. Despite the differences in appearance, researchers found the their brains and the nerves that connect their eye-like features were similarly developed -- suggesting a common ancestor.

The unique eye-like features are called anterior sclerite. Somewhere along the way they were discarded, deemed unnecessary by the processes of evolution. But they mark a little-understood transition period in the evolution of arthropods -- a time during the Cambrian explosion when hard-bodied, jointed arthropods burst onto the scene, inheriting a world previously dominated by worms and jellyfish.

"Heads have become more complex over time," Dr. Javier Ortega-Hernandez, an earth scientist and postdoctoral researcher at the University of Cambridge, in England, said in a press release. "But what we're seeing here is an answer to the question of how arthropods changed their bodies from soft to hard. It gives us an improved understanding of the origins and complex evolutionary history of this highly successful group."

The fossils belong to collections at the Royal Ontario Museum in Toronto and the Smithsonian Institution in the U.S. capital. They were both recovered from the Burgess Shale deposits in Western Canada.

The study was published in the journal Current Biology.