Hakobuchi Formation

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]

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
Region	East Asia
Country	Japan
Extent	Hokkaido

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

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