Dr Javier Ortega-Hernández of the University of Cambridge, UK, has discovered one of the oldest fossil brains ever found, and used it to help identify a key point in the evolutionary transition from soft to hard bodies in early ancestors of arthropods.

Dr Ortega-Hernández looked at two types of arthropod ancestors from the middle Cambrian Burgess Shale in Canada – a soft-bodied trilobite called Helmetia expansa and Odaraia alata, a bizarre creature resembling a submarine.

He found that a hard plate, called the anterior sclerite, and eye-like features at the front of their bodies were connected through nerve traces originating from the front part of the brain, which corresponds with how vision is controlled in modern arthropods.

The new results also allowed new comparisons with anomalocaridids, a group of large swimming predators of the period, and found key similarities between the anterior sclerite and a plate on the top of the anomalocaridid head, suggesting that they had a common origin.

Although it is widely agreed that anomalocaridids are early arthropod ancestors, their bodies are actually quite different.

Thanks to the preserved brains in these fossils, it is now possible to recognize the anterior sclerite as a bridge between the head of anomalocaridids and that of more familiar jointed arthropods.

“The anterior sclerite has been lost in modern arthropods, as it most likely fused with other parts of the head during the evolutionary history of the group,” said Dr Ortega-Hernández, who authored the paper in the journal Current Biology.

“What we’re seeing in these fossils is one of the major transitional steps between soft-bodied worm-like creatures and arthropods with hard exoskeletons and jointed limbs – this is a period of crucial transformation.”

The paleontologist observed that bright spots at the front of the bodies of Helmetia expansa and Odaraia alata, which are in fact simple photoreceptors, are embedded into the anterior sclerite.

The photoreceptors are connected to the front part of the fossilized brain, very much like the arrangement in modern arthropods.

In all likelihood these ancient brains processed information like in today’s arthropods, and were crucial for interacting with the environment, detecting food, and escaping from predators.

“Heads have become more complex over time. 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,” Dr Ortega-Hernández said.

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Javier Ortega-Hernández et al. Homology of Head Sclerites in Burgess Shale Euarthropods. Current Biology, published online May 07, 2015; doi: 10.1016/j.cub.2015.04.034