Reconstructing extinct organisms from their fossils is not always straightforward. For example, Hallucigenia‘s pairs of spines, and unpaired tentacle-like appendages led paleontologists of the 1970s to guess that the spines pointed down and were used for walking. After all, how could the animal have walked on a single row of tentacles? This reconstruction was unusual but also seemed plausible since scientists had discovered many other bizarre Cambrian animals.

A reconstruction revolution

In the early 1990s, however, paleontologists discovered a Cambrian fossil much like Hallucigenia, but with tube-like legs and spines on its back. They realized that this animal might have been closely related to Hallucigenia. Perhaps, Hallucigenia‘s spines also ran along its back and its “tentacles” were simply one half of a series of paired legs, with the other legs crushed underneath the fossilized animal. Sure enough, when Hallucigenia fossils were dissected, traces of the other set of legs were found.

The new discovery turned Hallucigenia on its head! Right side up, Hallucigenia reveals itself as one member of a modern animal group, the onychophorans.