An anatomical absurdity
When paleontologist Charles Doolittle Walcott first collected fossils of a strange, spiky worm from the Burgess Shale in 1911, he classified it as a species of polychaete worm called Canadia sparsa. For decades, it remained an obscure fossil among many. That changed in 1977, when Simon Conway Morris re-examined the specimens and realized they were something else entirely. The creature was so strange he named it Hallucigenia, for its "bizarre and dream-like appearance."
Conway Morris's initial reconstruction presented an animal that walked on seven pairs of rigid, stilt-like spines. A row of seven fleshy tentacles waved along its back, which he proposed passed food forward toward a poorly defined head. This version of Hallucigenia, which lived about 508 million years ago, was an anatomical puzzle that seemed to defy biological principles. How could it walk on inflexible stilts? The interpretation was so perplexing that some scientists even proposed Hallucigenia was not a complete animal but rather the appendage of a larger, unknown creature.
This first attempt at reconstruction was, as later work would prove, completely wrong. The animal was interpreted as being upside down and backwards.
Flipped, turned, and finally seen
The first major correction came in 1991. Paleontologists Lars Ramsköld and Hou Xianguang, after studying similar fossils from China, proposed a radical idea: flip Hallucigenia over. They argued that the structures Conway Morris identified as "tentacles" were actually paired, claw-tipped legs for walking, known as lobopods. The stiff "stilts" were, in fact, protective spines that ran along the animal's back. Further preparation of the original fossils confirmed this, revealing the second row of legs that had been buried in the rock matrix. The animal now made biological sense, resembling other "worms with legs," or lobopodians, which are ancient relatives of today's velvet worms (Onychophora).
Still, one mystery remained: which end was the head? A dark, balloon-like stain at one end of many fossils was presumed to be the head, but its lack of features was puzzling. Many suspected it was simply an artifact of decomposition, where fluids seeped out after death. The final answer came in 2015 from a study by Martin Smith and Jean-Bernard Caron. Using electron microscopy on over 100 specimens, they found that the opposite end contained a distinct head. This head possessed a pair of simple eyes and a mouth with a ring of teeth. Its throat was lined with fine, needle-like teeth, likely used as a ratchet to pull food into its gut. After more than a century of study, the creature, ranging from 10 to 50 millimeters in length, was finally complete.