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.