The Monster Shrimp From Hell

Actually, this thing couldn't hurt you. If couldn't even if it still existed. But it LOOKS like something from a James Cameron movie out to filet swimming humans alive.  Some 520 million years ago, Tamisiocaris borealis was one of the biggest animals on the planet at 70cm in length, and it cut a deadly swath - through plankton, at least. One of the odder results (and that's saying something) of the Cambrian explosion, the new predator was found in Greenland in a formation that, half a billion years ago, was located in the tropics. There was a bloom in the population of shrimplike creatures at the time, so these hard-shelled, appendage-waving creatures grew bigger. The group T. borealis belonged to, the anomalocarids, include species up to a possible 2m long and are most commonly classed as "stem arthropods" - that is, they were on the line leading to the arthropods, which today number over a million species at the very least and include everything from insects to lobsters.  (That's right: the roach you're stepping on is related to the lobster you're about to eat.  Yum.)
Evolution on Earth made so many experiments that we will never catalog them all.  Some fossils will always be too rare, too small, or too fragile, and of course many tiny invertebrates and microbes left no records at all.  But what we can know, and what we do know, is endlessly expanding and endlessly fascinating.