Sanctacaris Arthropoda Chelicerata Related to today's, horseshoe crab, Sanctacaris was only first described in 1981. It possessed a large flat tail, suggesting it was a good swimmer, a group of six appendages in each side of its body, and a very streamlined head. Sidneyia Arthropoda Unassigned Sidneyia was a large 13 centimetres (5.1 in) long predator of the Burgess Shale, and ate trilobites, ostracods, and hyolithids.