For years, scientists thought that a dark, balloon-shaped mark on fossils of a spiny wormlike creature were its head. Instead, it’s preserved “decay liquids,” squeezed out of the creature’s guts during the fossilization process. This isn’t the first time the 508-million-year-old worm, known as Hallucigenia (seen walking in the video reconstruction above), has given scientists the squirm-around. When the enigmatic animal was described in the 1970s, some of its flexible legs were still hidden by layers of rock on the fossil, leading scientists to conclude that the worm walked on its pairs of stiff spines like stilts. Now, using an electron microscope and new fossils discovered in the Burgess Shale in Canada, researchers have finally been able to make heads and tails of the creature, they report today in Nature. In addition to figuring out which end was which, the scientists also identified the tiny worm’s eyes, its mouth, and its foregut—stretching from the back of the mouth down its thin neck—complete with plates and circular teeth. Those structures are similar to ones in roundworms, in small marine invertebrates called mud dragons, and in the common ancestor of arthropods, which include today’s insects, arachnids, and crustaceans. Previously, genetic studies have been used to group the seemingly disparate animals together in a superphylum called Ecdysozoa. These new findings provide some of the first morphological evidence for the grouping, the researchers say.
(Video credit: Lars Fields)