Didontogaster is a polychaete worm (sometimes referred to as bristleworms). The first formal descriptions of Mazon Creek polychaetes did not occur until 1979. At the time they were described, they were the earliest known polychaetes in the fossil record. Since this time, polychaete fossils have been found reaching back as early as possibly the Ediacaran. It is the most common worm found in the Essex portion of the Mazon Creek deposit. It has been classified as belonging to th