Early Cambrian Protomelission is a scleritomous eumetazoan but not a bryozoan or dasyclad alga

Abstract The origin of Bryozoa has long been an intriguing mystery, largely due to the dearth of unambiguous bryozoan fossils within Cambrian strata1-5. Recently reported earliest bryozoan Protomelission 2, an early Cambrian phosphatized microfossil from Australia and China, was subsequently contested and proposed as calcified dasyclad algae1. Our re-analysis of raw micro-CT data from Protomelission specimens, however, unveils previously unrevealed structures, indicating Protomelission to be neither bryozoan nor alga, but almost a complete scleritome of Cambroclavus, an early Cambrian microfossil typically found as disarticulated sclerites. The long-awaited scleritomes suggest that enigmatic cambroclaves likely represent an extinct group of epitheliozoan-grade eumetazoan, with spiny sclerites covering their epidermis for defense. The prevalence of bizarre animals like cambroclaves, lacking modern analogues, implies a greater disparity of animal body plans during the Cambrian Explosion than previously understood. Our findings underscore the absence of definitive early Cambrian bryozoan fossils, further highlighting the dilemma of the origin of Bryozoa.