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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 the family Nephtyidae. Members of this family still exist today and are sometimes referred to as cat worms. Didontogaster has a fairly basic body plan. It’s most defining characteristic is a pair of conical shaped jaws that are usually well preserved. Due to the position of the jaws within the body, early collectors nicknamed Didontogaster as a Tummy Tooth Worm. Didontogaster was able to extend it’s jaws outward to feed (see below). It was likely a burrower as they are occasionally found on multiple bedding planes within the concretion. Preserved gut contents show they fed on ostracodes along with plant material. If outstretched, this first specimen would measure around 8 centimeters which is about as big as they get. Most specimens range between 4-5 centimeters.