# Yezo 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. 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.
Numerous fossils are known from the unit, mostly ammonites and bivalves, but also marine vertebrates such as mosasaurs, plesiosaurs 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. Nipponosaurus is known from an unnamed unit of the group from Southern Sakhalin, probably late Santonian or early Campanian in age. Kamuysaurus, which is known from the early Maastrichtian Hakobuchi Formation of southern Hokkaido, was also discovered in layers of this group.

## Fossil content
### Reptiles

Reptiles reported from the Yezo Group :
| Genus | Species | Presence | Material | Notes |
| Anomalochelys | A. angulata | Hobetsu-cho, Hokkaido. (Cenomanian) | | A nanhsiungchelyid. |
| Chupkaornis | C. keraorum | Kashima Formation (Coniacian to Santonian). | | A hesperornithiform. |
| Kamuysaurus | K. japonicus | Hakobuchi Formation. | "A nearly complete skeleton with the skull and mandible (HMG-1219)". | A hadrosaur. |
| Mesodermochelys | M. undulatus | Hakobuchi Formation and upper part of the Osoushinai Formation. | -NMV-3, a proximal half of right humerus (Osoushinai Formation). | A dermochelyid turtle. |
| Mesodermochelys | M. sp. | Osoushinai Formation. | A carapace with boreholes likely made by bivalves, and associated with many mollusc remains. | A dermochelyid turtle. |
| Mosasaurus | M. hobetsuensis | Hakobuchi Formation. | | A mosasaurine mosasaur. |
| Nipponosaurus | N. sachalinensis | Sakhalin. | | A hadrosaur. |
| Nodosauridae | Indeterminate | Hikagenosawa Formation. (Cenomanian) | Left rear half of the skull (MCM A522) | |
| Paralitherizinosaurus | P. japonicus | Osoushinai Formation. | Partial vertebra and partial right hand. | A therizinosaur. |
| Phosphorosaurus | P. ponpetelegans | Hakobuchi Formation. | 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. | 4 indeterminate remains. | Pliosaur remains, some with skull sizes comparable to Megacephalosaurus. |
| Taniwhasaurus | T. sp. | Kashima Formation. | 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). |

### Fish

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

### Echinoderms

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

### Molluscs

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

### Paleoflora

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