Width: 20 mm. Jaw of ammonoids, an organ corresponds to the beak of squid. There are two types of ammonoid jaws are known, anaptychus and aptychus. Anaptychus is univalved type, and aptychus is bivalved type composed of a pair of plates. This specimen is anaptychus. A set of jaw is composed of the upper plate and the lower plates. It seems this specimen is the lower plates. Recovered from siltstone of the early-middle Tithonian (Late Jurassic, 150 Ma) Kuzuryu Subgroup in the Tedori Group. The sedimentary environment of the Kuzuryu Subgroup is shallow marine. This outcrop has been thought as a part of the Bathonian-Callovian (Middle Jurassic, 165 Ma) Kaizara Formation, but recovered ammonoid fossils (Parapallasiceras sp.) suggests that it belongs to the Kurotodo Formation.