Hakobuchi Formation

Hakobuchi Formation
Hakobuchi Formation
Stratigraphic range: Early Maastrichtian PreꞒ Ꞓ O S D C P T J K Pg N
Type	Geological formation
Unit of	Yezo Group
Underlies	Ishikari Group, Poronai Group
Overlies	Haborogawa Formation, Kashima Formation
Thickness	Over 450 metres (1,480 ft)
Lithology
Primary	Mudstone, Siltstone, Sandstone
Other	Tuffite
Location
Location	42°48′N 142°08′E / 42.8°N 142.14°E / 42.8; 142.14
Region	East Asia
Country	Japan
Extent	Hokkaido
	Hakobuchi Formation (Japan)

Geological formation in Hokkaido, Japan

The Hakobuchi Formation is a geological formation in Hokkaido, Japan. It is the uppermost unit of the Yezo Group, being early Maastrichtian in age. It consists of bioturbated glauconitic sandstones, siltstones and conglomerates with coaly mudstone and minor tuffite. It was deposited in a continental shelf setting.^[1] It is noted for its fossil content with the invertebrates mainly consisting of bivalves and ammonites. With vertebrates including the mosasaurs Mosasaurus hobetsuensis and Phosphorosaurus ponpetelegans.^[2] As well the sea turtle Mesodermochelys^[3] and the hadrosaurid dinosaur Kamuysaurus.^[4]

Fossil content

Color key

Taxon	Reclassified taxon	Taxon falsely reported as present	Dubious taxon or junior synonym	Ichnotaxon	Ootaxon	Morphotaxon

Notes
Uncertain or tentative taxa are in small text; ~~crossed out~~ taxa are discredited.

Dinosaurs

Ornithopods

Ornithopods of the Hakobuchi Formation
Genus	Species	Location	Stratigraphic position	Material	Notes	Images
Kamuysaurus^[5]	K. japonicus				A edmontosaurin saurolophine

Mosasaurs

Mosasaurs of the Hakobuchi Formation
Genus	Species	Location	Stratigraphic position	Material	Notes	Images
Mosasaurus	M. hobetsuensis				A mosasaurine mosasaurid
Phosphorosaurus	P. ponpetelegans				A halisaurine mosasaurid

Turtles

Turtles of the Hakobuchi Formation
Genus	Species	Location	Stratigraphic position	Material	Notes	Images
Mesodermochelys	M. undulatus				A dermochelyid turtle

Fish

Cartilaginous Fish

Cartilaginous Fish of the Hakobuchi Formation
Genus	Species	Location	Stratigraphic position	Material	Notes	Images
Callorhinchus^[6]	C. orientalis				A callorhinchid chimaeran

Ray-finned Fish

Cartilaginous Fish of the Hakobuchi Formation
Genus	Species	Location	Stratigraphic position	Material	Notes	Images
Acipenseridae^[7]					A sturgeon

Molluscs

Bivalves

Bivalves of the Hakobuchi Formation
Genus	Species	Location	Stratigraphic position	Material	Notes	Images
Inoceramus	I. nipponicus				A inoceramid bivalve

Cephalopods

Cephalopods of the Hakobuchi Formation
Genus	Species	Location	Stratigraphic position	Material	Notes	Images
Pachydiscus	P. japonicus				A pachydiscid ammonite

Gastropods

Gastropods of the Hakobuchi Formation
Genus	Species	Location	Stratigraphic position	Material	Notes	Images

Paleoflora

Plants of the Hakobuchi Formation
Genus	Species	Location	Stratigraphic position	Material	Notes	Images
Mukawastrobus	M. satoi				A cypress
Nilssonia	N. yezoensis				A cycad

References

^ Takashima, Reishi; Kawabe, Fumihisa; Nishi, Hiroshi; Moriya, Kazuyoshi; Wani, Ryoji; Ando, Hisao (June 2004). "Geology and stratigraphy of forearc basin sediments in Hokkaido, Japan: Cretaceous environmental events on the north-west Pacific margin". Cretaceous Research. 25 (3): 365–390. doi:10.1016/j.cretres.2004.02.004. hdl:2115/17182.
^ Konishi, Takuya; Caldwell, Michael W.; Nishimura, Tomohiro; Sakurai, Kazuhiko; Tanoue, Kyo (2016-10-02). "A new halisaurine mosasaur (Squamata: Halisaurinae) from Japan: the first record in the western Pacific realm and the first documented insights into binocular vision in mosasaurs". Journal of Systematic Palaeontology. 14 (10): 809–839. doi:10.1080/14772019.2015.1113447. ISSN 1477-2019.
^ Hirayama R, Chitoku T. (1996). "Family Dermochelyidae (Superfamily Chelonioidea) from the Upper Cretaceous of North Japan". Transactions and proceedings of the Palaeontological Society of Japan. New series 184: 597-622. online, retrieved 28 July 2008
^ Sakurai, Kazuhiko; Sato, Tamaki; Chinzorig, Tsogtbaatar; Tanaka, Kohei; Fiorillo, Anthony R.; Chiba, Kentaro; Takasaki, Ryuji; Nishimura, Tomohiro; Kobayashi, Yoshitsugu (September 5, 2019). "A New Hadrosaurine (Dinosauria: Hadrosauridae) from the Marine Deposits of the Late Cretaceous Hakobuchi Formation, Yezo Group, Japan". Scientific Reports. 9 (1) 12389: 1–14. doi:10.1038/s41598-019-48607-1. PMC 6728324. PMID 31488887.
^ Yoshitsugu Kobayashi; Tomohiro Nishimura; Ryuji Takasaki; Kentaro Chiba; Anthony R. Fiorillo; Kohei Tanaka; Tsogtbaatar Chinzorig; Tamaki Sato; Kazuhiko Sakurai (2019). "A new hadrosaurine (Dinosauria: Hadrosauridae) from the marine deposits of the Late Cretaceous Hakobuchi Formation, Yezo Group, Japan". Scientific Reports. 9 (1) 12389. Bibcode:2019NatSR...912389K. doi:10.1038/s41598-019-48607-1. PMC 6728324. PMID 31488887.
^ Ota, A.; Nishimura, T.; Kobayashi, Y.; Moriki, K. (2025). "Callorhinchus orientalis sp. nov., a new callorhinchid from the Upper Cretaceous Hakobuchi Formation, Yezo Group, Hokkaido, Japan". Paleontological Research. 29 (1): 54–63. doi:10.2517/prpsj.240013.
^ Miyata, S.; Nakajima, Y.; Motoyama, I.; Nishimura, T.; Yabumoto, Y. (2025). "A first fossil sturgeon from the Upper Cretaceous (Maastrichtian) of Northwestern Pacific". Cretaceous Research. 179 106251. doi:10.1016/j.cretres.2025.106251.

[1] Takashima, Reishi; Kawabe, Fumihisa; Nishi, Hiroshi; Moriya, Kazuyoshi; Wani, Ryoji; Ando, Hisao (June 2004). "Geology and stratigraphy of forearc basin sediments in Hokkaido, Japan: Cretaceous environmental events on the north-west Pacific margin". Cretaceous Research. 25 (3): 365–390. doi:10.1016/j.cretres.2004.02.004. hdl:2115/17182.

[2] Konishi, Takuya; Caldwell, Michael W.; Nishimura, Tomohiro; Sakurai, Kazuhiko; Tanoue, Kyo (2016-10-02). "A new halisaurine mosasaur (Squamata: Halisaurinae) from Japan: the first record in the western Pacific realm and the first documented insights into binocular vision in mosasaurs". Journal of Systematic Palaeontology. 14 (10): 809–839. doi:10.1080/14772019.2015.1113447. ISSN 1477-2019.

[Hirayama_&_Chitoku,_1996-3] Hirayama R, Chitoku T. (1996). "Family Dermochelyidae (Superfamily Chelonioidea) from the Upper Cretaceous of North Japan". Transactions and proceedings of the Palaeontological Society of Japan. New series 184: 597-622. online, retrieved 28 July 2008

[Kazuhiko2019-4] Sakurai, Kazuhiko; Sato, Tamaki; Chinzorig, Tsogtbaatar; Tanaka, Kohei; Fiorillo, Anthony R.; Chiba, Kentaro; Takasaki, Ryuji; Nishimura, Tomohiro; Kobayashi, Yoshitsugu (September 5, 2019). "A New Hadrosaurine (Dinosauria: Hadrosauridae) from the Marine Deposits of the Late Cretaceous Hakobuchi Formation, Yezo Group, Japan". Scientific Reports. 9 (1) 12389: 1–14. doi:10.1038/s41598-019-48607-1. PMC 6728324. PMID 31488887.

[5] Yoshitsugu Kobayashi; Tomohiro Nishimura; Ryuji Takasaki; Kentaro Chiba; Anthony R. Fiorillo; Kohei Tanaka; Tsogtbaatar Chinzorig; Tamaki Sato; Kazuhiko Sakurai (2019). "A new hadrosaurine (Dinosauria: Hadrosauridae) from the marine deposits of the Late Cretaceous Hakobuchi Formation, Yezo Group, Japan". Scientific Reports. 9 (1) 12389. Bibcode:2019NatSR...912389K. doi:10.1038/s41598-019-48607-1. PMC 6728324. PMID 31488887.

[6] Ota, A.; Nishimura, T.; Kobayashi, Y.; Moriki, K. (2025). "Callorhinchus orientalis sp. nov., a new callorhinchid from the Upper Cretaceous Hakobuchi Formation, Yezo Group, Hokkaido, Japan". Paleontological Research. 29 (1): 54–63. doi:10.2517/prpsj.240013.

[7] Miyata, S.; Nakajima, Y.; Motoyama, I.; Nishimura, T.; Yabumoto, Y. (2025). "A first fossil sturgeon from the Upper Cretaceous (Maastrichtian) of Northwestern Pacific". Cretaceous Research. 179 106251. doi:10.1016/j.cretres.2025.106251.