When: Early to Middle Cambrian (~540 to 500 million years ago)
Where: Found in what is now British Columbia and China
What: Hallucigenia is another odd fossil first known from the Burgess Shale formation of Canada. This largest individuals only reach 1.2 inches (~3cm) long, but there has been a lot of scientific debate centered around this tiny species. Before we get into the debate over its phyogenetic position, first we need to talk about which way is up! Or anterior for that matter. The first reconstructions of Hallucigenia had it walking on the stiff looking spiny projections, with the more flexible tentacles used to bring food to its mouth, which was reconstructed as being on a large bulbous projection. The modern interpretation is reversed in almost every way; it walks on the tentacle feet, the spines are on the dorsal surface for protection, and its head is on the opposite end. The modern reconstruction does not even have a large bulbous projection, as it is now thought the appearance of this blob in fossils is the inner organs of Hallucigenia being squeezed out though its posterior as it was flattened either at or after death. This strange form walked along the ocean floor, eating tiny food particles.
So now we /might/ know how this animal really looked… but what is it related to? Common suggestions have been: velvet worms (Onychophore), an extremely basal Arthoropoda, or as a member of a phylum now extinct. There is no firm consensus even today.
The Royal Ontario Museum recently put up a spectacular website on the Burgess Shale that you should check out if you would like to learn more about Hallucigenia and its contemporaries.