There is a stratum of Onsongia Sesonensis fossil on a mountain slope nearly 1km south of Hyangdang-ri, Onsong County, North Hamgyong Province.
Discovered in Juche 64 (1975), the stratum is 4 metres wide and 30 metres long.
The fossil is about 27 cm long and 15.5 cm high.
It has a round head and thin teeth, and its body is covered with scales.
It has 38 to 42 vertebral segments with 11 to 12 dorsal and caudal fin rays and 8 to 9 ventral fin rays.
The fossil is under preservation as a natural monument for its significance in studying the evolution of vertebrate animals, fishes in particular.