Clues contained in ancient brain point to the origin of heads in early animals

A new study from the University of Cambridge has identified one of the oldest fossil brains ever discovered — more than 500 million years old — and used it to help determine how heads first evolved in early animals. The results, published in the journal Current Biology, identify a key point in the evolutionary transition from soft to hard bodies in early ancestors of arthropods, the group that contains modern insects, crustaceans and spiders.

The study looked at two types of arthropod ancestors — a soft-bodied trilobite and a bizarre creature resembling a submarine. It 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 recognise 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 Javier Ortega-Hernández, a postdoctoral researcher from Cambridge’s Department of Earth Sciences, who authored the study. “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.” Source: Clues contained in ancient brain point to the origin of heads in early animals

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