The fossil was long misunderstood. At first, the mouth was thought to be a jellyfish of some sort, while the grasping claws (below) were thought to be some kind of shrimp. The tail was thought to be a third creature. It wasn't until 1985 that researchers realized that the separate bits were part of one of the first predators on planet Earth. And what a terrifying predator it was!
Most of the Burgess Shale creatures were small, usually a few inches or less in length. The Anomalocaris was close to three feet long. Imagine if you were being chased and consumed by a ravenous monster the size of an Apatosaurus!
Comparison diagram from Wikipedia
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