Some of the most unusual creatures scuttling across the sea floor 520 million years ago were the “great-appendage” arthropods, which had scissorlike projections sprouting from their heads. Looking only at their general crustaceanlike body plan (inset), paleontologists have long debated where these animals fit within the arthropod family tree, a diverse group that includes insects, spiders, millipedes, and long-extinct trilobites. Now, researchers have gained clues about the creatures’ closest evolutionary kin by blasting fossils of one group of the animals with high-energy x-rays, which caused various elements in the fossils to fluoresce. They also took CT scans. The most useful images, the researchers found, were those produced by fluorescing iron (depicted in magenta, main image) and the CT scans (green). Together, these images denote the creature’s optic nerves (its eyes are the four dark circles at the top of the main image), brain, and the nerve tissue serving eight of its 11 body segments. That arrangement is most like the one seen in a group of modern-day arthropods known as chelicerates , the researchers report today in Nature. Living members of that group include spiders, scorpions, and horseshoe crabs. The finding clears up the picture of the arthropod family tree, which is particularly important because some of these creatures’ features were so unlike those of their presumed kin. If used more broadly, the technique used to analyze these fossils could help paleontologists gain insights into evolutionary relationships among other enigmatic, long-gone species, including many of the unusual animals strolling the sea floor during the same time period.