Sidneyia is an extinct arthropod known from fossils found in the Cambrian-age Burgess Shale formation of British Columbia. 144 specimens of Sidneyia are known from the Greater Phyllopod bed, where they comprise 0.27% of the community.

 Sidneyia ranged from 2 to 5 inches (51 to 130 mm) in length and is one of the largest arthropods found at the site. It is thought to have been a benthic carnivore that walked along the sea floor in search of hard-shelled prey. Gut contents have revealed that Sidneyia fed on hyolithids (molluscs) and other small crustaceans and trilobites.

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