In the 1970s, when researchers first examined the bizarre half-billion-year-old fossils of a worm-like creature with a double row of spines on its back and several pairs of legs that end with claws, they couldn’t make heads or tails of it. But now, this enigmatic marine worm named Hallucigenia has finally showed its face (in the fossil record). The findings are published in Nature this week.
The tubular body of Hallucigenia sparsa ranges from 10 to 50 millimeters in length. It’s a member of the wildly diverse group of animals called ecdysozoans that are united by one thing: They molt. This includes the familiar bugs, spiders, millipedes and crustaceans, as well as roundworms, velvet worms, and tardigrades (the water bears). Just last year, a close examination of their claws revealed that Hallucigenia is an ancestor of modern-day velvet worms. These are tropical legged worms that fire jets of slime at their prey.
However, features of its head were still unknown until now. University of Cambridge’s Martin Smith and Jean-Bernard Caron from the University of Toronto used electron microscopy to examine more than 165 Hallucigenia specimens unearthed from the 508-million-year-old sediment of the Burgess Shale in Canada’s Yoho National Park back in the 1990s. Some of these fossils came complete with a pair of simple eyes that sat above a ring of teeth.
Hallucigenia sparsa from the Burgess Shale. The fossil is 15mm long. Jean-Bernard Caron
“A large balloon-like orb at one end of the specimen was originally thought to be the head, but we can now demonstrate that this actually wasn’t part of the body at all, but a dark stain representing decay fluids or gut contents that oozed out as the animal was flattened during burial,” Smith says in a statement. Once they figured this out, they dug away the sediment covering the head. “When we put the fossils in the electron microscope, we were initially hoping that we might find eyes,” Caron adds, “and were astonished when we also found the teeth smiling back at us!”
Hallucigenia’s head was small and elongated and sat at the end of a thin neck. The ring of teeth surrounding its mouth likely helped to generate suction to draw food into its throat, which was also lined with plates and circular teeth. This foregut armor probably kept the food from slipping out as it sucked. There were also three pairs of tentacles along its neck.
“We previously thought that neither velvet worms nor their ancestors had teeth,” Caron says. “But Hallucigenia tells us that actually, velvet worm ancestors had them, and living forms just lost their teeth over time.” This mouth resembles those of roundworms as well as the common ancestor of arthropods. Previous molecular work had already grouped arthropods and roundworms into Ecdysozoa, and this is finally being confirmed by morphological evidence.