The animal kingdom got off to a slow start. Studies on DNA indicate that the first animals evolved more than 750 million years ago, but for well over 200 million years, they left a meager mark on the fossil record. As best as paleontologists can tell, the animal kingdom during that time consisted of little more than sponges and other creatures rooted to the ocean floor.

But then, about 520 million years ago during the Cambrian Period, animal evolution shifted into high gear. Fast-moving predators, scavengers and burrowers evolved. Many of the major living groups of animals left their first fossils during this so-called Cambrian explosion, including our own ancestors. But the Cambrian explosion also brought many bizarre species that have long puzzled paleontologists.

For almost 40 years, the poster child for the Cambrian explosion’s strangeness has been a hand-size armored worm with a name to suit its bizarre appearance: Hallucigenia.

But recently, Hallucigenia has lost much of its mystery. Scientists have worked out the creature’s anatomy, and they have figured out a lot about how Hallucigenia and its relatives thrived in the Cambrian oceans. And despite its odd appearance, Hallucigenia isn’t an incomprehensible zoological experiment. Paleontologists have been able to place it comfortably on the evolutionary branch that led to a group of invertebrates alive today called velvet worms.