If you’d been around 520 million years ago and been living at the bottom of the ocean, you would not have wanted to stumble across the extinct creature scientists just identified from fossil records as belonging to the group known as “great appendage arthropods.” It was surely an ugly beast—shaped something like a spider, something like a lobster, with great tusk-like forelimbs. That said, it was also only about an inch long. Still, if you’d indeed in fact been living in its world, you might not have been much bigger, and that would have made it a fearsome sight.
Today, the little animal is mostly a matter of scientific interest, and in this week’s Nature, investigators took a run at determining just who its contemporary kin are. Does it belong in the branch of the arthropod tree currently occupied by the crustaceans, including crabs and lobsters? Or does it fit with the chelicerae, the group that includes scorpions and spiders? Working to reconstruct the nervous system of the animal, dead for half a billion years, from traces left in rock, the investigators have concluded that the wiring of its nervous system links it to the modern chelicerae.
The idea that fossils can yield up information about the nervous system at all is in some ways the most intriguing news out of the study. The great majority of what gets preserved in the fossilizing process is the hard, substantial stuff, like bones and shells. It’s only in the rare fossil bed, where the chemical conditions are just right, that soft tissues like skin can be seen. The nervous system, however, is a bigger challenge since it’s deep inside the body. To get a look at that, the investigators—from institutions in Japan, China, the U.K, and the U.S.—used CT scanning and what’s known as X-ray fluorescence microscopy, the latter of which could work at all only because the nerves, now long gone, collected iron-rich particles as they fossilized.
The scientists first conducted their CT scanning, producing a 3D image of the arthropod’s nervous system. Then, they ran their X-ray fluorescence. Overlaying these two images and eliminating any areas that didn’t overlap allowed them to be sure that what they were looking at was indeed a clear picture of nervous system structure alone. Using this map, they were able to pick out the nerves that led to the arthropod’s eyes and those that led to its “great appendages,” as well as to its rudimentary brain. The way the nerves were hooked up, with a central brain, is very different from the wiring plan in the crustacean’s nervous system, which has a series of ganglion clusters, like mini-brains, spread throughout the body. The neural schematic also supported the idea that those great forelimbs were the forebears of the mouthparts that modern spiders use to grasp their prey.
Earlier this year, some of the same scientists were part of a group that published a paper exploring the fossilized brain of another early arthropod, called Fuxianhuia protensa, that helped shed light on crustacean evolution. That specimen and this one were both found in China, but the researchers suggest that their technique could be used on fossils from the famous Burgess Shale in Canada, which come from roughly the same time period. The animals that walked and scrabbled and crept and oozed across the ocean floors hundreds of millions of years ago may be long-gone and decidedly primitive. But the artifacts they left, on both sides of the world, may teach us more about the origins of the species that live alongside us so many epochs later.