Here's some bizarre headcanon for you: Anorith is the result of convergent evolution, and is not actually the direct counterpart to the real-life Anomalocaris.
Firstly, there are a few pieces of information, including the most concrete data on when it was around, that point to it being much more recent than the Cambrian-only Anomalocaris. The end of Anomalocaris' range is about 50M years before Japan started forming, which would prevent there from being naturally-occurring claw fossils in Hoenn and Sinnoh if we assume those regions are the same age as their real-life counterpart. Meanwhile, Sword's dex entry states Anorith were around circa. 100 M years ago, which falls about 400M short of the Cambrian period. Interestingly, there's also no overlap between the time ranges of Anomalocaris and the Crinoids that were the inspiration for Lilleep, but the latter has not currently gone extinct and so could be contemporary with any later organisms that evolved into being Anomalocaris-shaped.
Secondly, and on the topic of Dex entries, both the Emerald and HGSS ones refer to evolving the wings over time in response to living in the ocean. However, this really doesn't make any sense in the context of a Cambrian-era animal. Every complex lifeform lived in the ocean at that point, and that style of limb structure shows up a lot more often than actual walking legs. This info does, however, make perfect sense in the context of a Cretaceous-era terrestrial arthropod that ended up moving into the oceans.
