5 - Hallucigenia
No. 5 on our countdown dates back more than 500 million years to a time known as the Cambrian Period when most of the major groups of animals first appeared. The Cambrian Period was also a time of evolutionary experimentation in which countless bizarre body plans were tried out and then left behind in the primordial soup. One particularly odd set of fossils come from an area of the Canadian Rockies known as the Burgess Shale, including a strange little arthropod named Hallucigenia.
Hallucigenia gets its name from its body plan, which appears to have been designed by someone under the influence of psychedelic drugs. It has an elongated body with a row of tentacles on one side and a string of stiltlike spines on the other. At first, researchers had no idea which way to orient the animal — on its tentacles or on its spines — but chose the spines since the tentacles appeared to have mouthparts on the end of them. However, recent evidence suggests that Hallucigenia's tentacles were actually paired appendages used for walking and that the spines were most likely for protection from predators. Based on this discovery, Hallucigenia was finally turned right side up.