This week’s paper comes just one year after the same team published in Nature a description of another 520-million-year-old fossil with a complex brain. The fossil, Fuxianhuia protensa, had the primitive body plan expected of something dating to the Cambrian period, some 233 million years before the Triassic period when dinosaurs appeared. But it had a brain much more complex than the one that scientists had expected to find in something so old – a brain, in fact, like that of a mandibulate, the branch of the arthropod tree that includes modern shrimp and insects.
This meant that animals with mandibulate-like brains split from the arthropods to form a separate tree branch at least 520 million years ago. But what about the other, major group of arthropods, the chelicerates, which include modern spiders, ticks, horseshoe crabs, and scorpions? Might the forerunners to this big group of modern crawlers have also existed some 520 million years ago?
There was some evidence that they did. For years, scientists had been unsure where in the evolutionary tree to place an extinct group of marine animals known as the megacheirans, or “mega claw,” after the pair of scissor-like appendages at their heads. Scientists had suggested that these animals could be the ancestors to the chelicerates, since the elbow joint in the appendages at their head resembled the joint in the biting mouthparts of modern spiders and scorpions.
Still, that morphological evidence was not sufficient to prove that megacheirans were the chelicerates’ ancestors. That would require more substantial proof – it would require, in fact, a find as extraordinary as the Fuxianhuia protensa fossil: the scientists needed a megacheiran with a preserved brain.
As chance would have it, the fossil the team was hoping was found, plumbed from the Chengjiang formation in southwest China.
“To our extraordinary surprise, this fossil turned up,” says Dr. Strausfeld.