A restudy of Utahcaris orion (Euarthropoda) from the Spence Shale (Middle Cambrian, Utah, USA)

Utahcaris orion Conway Morris & Robison, 1988, from the lower middle Cambrian (Series 3, Stage 5) Spence Shale Member situated in Utah, USA, is redescribed based on a restudy of the original material. Newly recognized features, including ventral lateral eyes, trunk appendages, and a bulbous cephalic shield with subtriangular extension, reinforce similarities with Sanctacaris uncata from the middle Cambrian Burgess Shale. Both are assigned to the new family, Sanctacarididae. Sanctacaridids represent the oldest chelicerates. Their ecology and that of their nearest non-chelicerate relatives indicate that Chelicerata were plesiomorphically predatory.