Yezo Group

Not to be confused with Yeso Group.

The Yezo Group is a stratigraphic group in Hokkaido, Japan and Sakhalin, Russia which is primarily Late Cretaceous in age (Aptian to Earliest Paleocene). It is exposed as roughly north–south trending belt extending 1,500 kilometres through central Hokkaido from Urakawa to Cape Sōya and Sakhalin from the south coast to Alexandrovsk-Sakhalinsky District.^[1] It consists of marine forearc basin sediments, typically turbiditic and bioturbated mudstones and sandstones with subordinate conglomerate primarily deposited on the continental shelf and slope of the ancient Yezo subduction margin. It forms a continuous depositional sequence with the Sorachi Group, which overlies the Horokanai Ophiolite. The sequence gradually shallows upwards with the terminal Hakobuchi Formation representing a fluvial-inner shelf environment.^[2]

Yezo Group
Stratigraphic range: Aptian–Paleocene PreꞒ Ꞓ O S D C P T J K Pg N
Type	Stratigraphic group
Sub-units	Shuparogawa Formation, Maruyama Formation, Hikagenosawa Formation, Saku Formation, Mikasa Formation, Kashima Formation, Haborogawa Formation, Osoushinai Formation, Hakobuchi Formation
Underlies	Ishikari Group, Poronai Group, Neogene deposits
Overlies	Sorachi Group
Thickness	10,000 metres (32,810 ft)
Lithology
Primary	Mudstone, sandstone
Other	Tuffite, conglomerate
Location
Region	Hokkaido Sakhalin
Country	Japan, Russia

Numerous fossils are known from the unit, mostly ammonites and bivalves, but also marine vertebrates such as mosasaurs, plesiosaurs^[3] and marine turtles. Dinosaur remains are among the fossils that have been recovered from the group. These include a partial cervical vertebra and right hand from the therizinosaurid Paralitherizinosaurus, from Early Campanian Osoushinai Formation from northern Hokkaido.^[4][5] Nipponosaurus is known from an unnamed unit of the group from Southern Sakhalin, probably late Santonian or early Campanian in age.^[6] Kamuysaurus, which is known from the early Maastrichtian Hakobuchi Formation of southern Hokkaido, was also discovered in layers of this group.^[7]

Fossil content

Reptiles

Reptiles reported from the Yezo Group
Genus	Species	Presence	Material	Notes	Images
Anomalochelys	A. angulata	Hobetsu-cho, Hokkaido. (Cenomanian)[8]		A nanhsiungchelyid.
Chupkaornis	C. keraorum	Kashima Formation (Coniacian to Santonian).[9]	Cervical vertebrae, torso vertebrae, femur, and fibula.	A hesperornithiform.
Kamuysaurus	K. japonicus	Hakobuchi Formation.[7]	"A nearly complete skeleton with the skull and mandible (HMG-1219)".[7]	A hadrosaur.
Mesodermochelys	M. undulatus	Hakobuchi Formation[10] and upper part of the Osoushinai Formation.[11]	-NMV-3, a proximal half of right humerus (Osoushinai Formation).[11]	A dermochelyid turtle.
	M. sp.	Osoushinai Formation.[12]	A carapace with boreholes likely made by bivalves, and associated with many mollusc remains.[12]	A dermochelyid turtle.
Mosasaurus	M. hobetsuensis	Hakobuchi Formation.[13]		A mosasaurine mosasaur.
Nipponosaurus	N. sachalinensis	Sakhalin.[6][14]		A hadrosaur.
Nodosauridae	Indeterminate	Hikagenosawa Formation. (Cenomanian)[15]	Left rear half of the skull (MCM A522)
Paralitherizinosaurus	P. japonicus	Osoushinai Formation.[4]	Partial vertebra and partial right hand.[4]	A therizinosaur.
Phosphorosaurus	P. ponpetelegans	Hakobuchi Formation.[13]	A well-preserved skull with some associated vertebrae.	A halisaurine mosasaur with proportionately large eyes and binocular vision.
Pliosauridae gen. et. sp. indet.	Indeterminate	Cenomanian to Turonian localities in Hokkaido.[16]	4 indeterminate remains.[16]	Pliosaur remains, some with skull sizes comparable to Megacephalosaurus.
Taniwhasaurus	T. sp.	Kashima Formation.[17]	A fragmentary skull, not diagnostic to the species level.	A tylosaurine mosasaur also known from New Zealand, Antarctica, and South Africa; originally informally named 'Yezosaurus mikasaensis'.
Pterodactyloidea	Indeterminate		Fragmentary femur, phalanx, and cervical vertebra	Large sized form with an estimated wingspan of 6.8 metres (22 ft).[18]

Fish

Fish reported from the Yezo Group
Genus	Species	Presence	Material	Notes	Images
Apsopelix	A. miyazakii	Nakagawa Town[19] and Mikasa area,[20] Hokkaido (Turonian).	2 specimens.	A crossognathid.

Echinoderms

Echinoderms reported from the Yezo Group
Genus	Species	Presence	Material	Notes	Images
?Austinocrinus	?A. sp.	Unknown locality, possibly in south-central Hokkaido, thought to be Santonian-Campanian.[21]	"One nodal and two pluricolumnals, associated on a bedding plane, so probably from a single individual".[21]	A crinoid.
?Balanocrinus	?B. sp.	Upper Coniacian-Campanian portions of the Haborogawa Formation).[21]	One pluricolumnal.[21]	A crinoid.
Isocrinus	I. sp.	Haborogawa Formation (upper Turonian-Coniacian).[21]	One pluricolumnal.[21]	A crinoid.
Isselicrinus	I. sp.	Campanian upper Yezo Group.[21]	One pluricolumnal and five very poorly preserved columnals.[21]

Molluscs

Molluscs reported from the Yezo Group
Genus	Species	Presence	Material	Notes	Images
?Abyssochrysoidea		Osoushinai Formation.[12]	A single specimen found associated with the remains of a Mesodermochelys sp.[12]	A gastropod.
Bivalvia	Unidentified.[12]	Osoushinai Formation.[12]	A single specimen found associated with the remains of a Mesodermochelys sp.[12]
?Cephalaspidea		Osoushinai Formation.[12]	A single specimen found associated with the remains of a Mesodermochelys sp.[12]	A gastropod.
Cheloniceras	C. sp.	Tsukenai Formation (Aptian).[22]		An ammonite.
Colombiceras	C. spathi	Tsukenai Formation (Aptian).[22]		An ammonite.
Gaudryceras	G. denseplicatum	Satonosawa Creek, Hokkaido (middle Turonian).[23]		An ammonoid.
?Gyrodes	?G. sp.	Osoushinai Formation.[12]	A single specimen found associated with the remains of a Mesodermochelys sp.[12]
Haboroteuthis	H. poseidon	Upper Haborogawa Formation (Upper Santonian).[24]	"KMNH IvP 902,002, a lower jaw."[24]	A teuthid.
Hikidea	H. cf. yasukawensis	Osoushinai Formation.[12]	A single specimen found associated with the remains of a Mesodermochelys sp.[12]
Inoceramus	I. (Sphenoceramus) orientalis	Uppermost Haborogawa Formation (middle-upper part of the lower Campanian).[25]		A bivalve.
	I. (Sphenoceramus) schmidti	Uppermost Haborogawa Formation (middle-upper part of the lower Campanian).[25]		A bivalve.
	I. teshioensis	Gakkonosawa Creek, Hokkaido (late Turonian).[23]	Disarticulated valves.[23]	A bivalve.
Nanaimoteuthis	N. hikidai	Uppermost Haborogawa Formation (lower Campanian).[24]	"KMNH IvP 902,001, a lower jaw, the only specimen available."[24]	A vampyromorph.
	N. jeletzkyi	Hokkaido (Santonian to lower Campanian.)[26]		A vampyromorph.
	N. yokotai	Satonosawa Creek and Gakkonosawa Creek, Hokkaido (middle and late Turonian).[23]	Jaw fossils.	A vampyromorph.
Paleocirroteuthis	P. sp.	Hokkaido (Santonian to lower Campanian).[26]		A cirroctopodid.
Provanna	P. cf. nakagawensis	Osoushinai Formation.[12]	3 specimens found associated with the remains of a Mesodermochelys sp.[12]	An abyssochrysoid gastropod.
Rhytidoplites	R. adkinsi	Tsukenai Formation (Aptian).[22]		An ammonite.
Scalarites	S. mihoensis	Gakkonosawa Creek, Hokkaido (late Turonian).[23]		An ammonoid.
	S. scalaris	Satonosawa Creek, Hokkaido (middle Turonian).[23]		An ammonoid.
Tetragonites	T. glabrus	Satonosawa Creek, Hokkaido (middle Turonian).[23]		An ammonoid.
Thyasira	T. tanabei	Osoushinai Formation.[12]	2 articulated specimens found associated with the remains of a Mesodermochelys sp.[12]	A bivalve.
Yezoteuthis	Y. giganteus	Osoushinai Formation.[27]	Jaw fossils.	A very large oegopsid.

Paleoflora

Plants reported from the Yezo Group
Genus	Species	Presence	Material	Notes	Images
Eydeia	E. hokkaidoensis	Haborogawa Formation (Coniacian-Santonian).[28]	5 perimineralized fruits.[28]	A cornale similar to the extant Davidia involucrata.
Mikasapteris	M. rothwellii	Mikasa Locality.[29]	A permineralized fertile pinnule.[29]	A probable stem polypodioid leptosporangiate fern.
Mukawastrobus	M. satoi	Hakobuchi Formation.[30]	"A single cylindrical seed cone 2 cm long, 1.1 cm wide."[30]	A cypress.
Nilssonia	N. yezoensis	Hakobuchi Formation.[31]		A cycad.
Obirafructus	O. kokubunii	Haborogawa Formation (Coniacian to Santonian).[32]	"A permineralized reproductive axis bearing at least 42 spirally arranged follicles."[32]	A saxifragale.
Stockeystrobus	S. interdigitata	Hoborogawa Formation (Coniacian to Santonian).[33]	A seed cone.[33]	A sequoioid.
Zamites	Z. bayeri	Kashima Formation (Coniacian).[34]		A bennettitale.

See also

• List of dinosaur-bearing rock formations