In the early 1980s, palaeontologist Harry Whittington was preparing a Burgess Shale fossil when he solved the mystery of Anomalocaris’s identity. Much to his surprise, Whittington uncovered two Anomalocaris “shrimps” attached to the head region of a large body, which also had the jellyfish Peytoia as its mouth apparatus. Collecting at the Burgess Shale by the Royal Ontario Museum in the early 1990s led to the discovery of several complete specimens, which allowed for the reconstruction of Anomalocaris canadensis with greater accuracy. Anomalocaris is now classified as a kind of primitive arthropod, a group that contains modern insects, arachnids, and crustaceans.
Why are only fragments of Anomalocaris canadensis fossils so often found? It is possible that as Anomalocaris decayed, body parts fell off and may have been fossilized in different locations. Mouth parts and claws were made of chitin (the same material composing the exoskeletons of insects) which made them more susceptible for preservation. It is equally (or perhaps even more) likely that individual body components were separated during regular moulting, and only the more resilient exoskeletal elements of the feeding apparatus (claws and oral ring) were preferentially preserved. Less resilient parts were probably rapidly degraded. The claws and complete oral ring would also behave differently in currents, and end up being deposited in different places. Claws, consisting of a single more-or-less continuous sheath, would survive much more commonly than complete oral rings, which would separate fairly quickly into smaller elements.