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.Weishampel, David B; et al. (2004). "Dinosaur distribution (Late Cretaceous, Asia)." In: Weishampel, David B.; Dodson, Peter; and Osmólska, Halszka (eds.): The Dinosauria, 2nd, Berkeley: University of California Press. Pp. 593-600. Kamuysaurus, which is known from the early Maastrichtian Hakobuchi Formation of southern Hokkaido, was also discovered in layers of this group.
Fossil content Reptiles
Dinosaurs Dinosaurs reported from the Yezo Group Genus Species Presence Material Notes ImagesChupkaornisC. keraorumKashima Formation (Coniacian to Santonian)Cervical vertebrae, torso vertebrae, femur, and fibulaA hesperornithiformKamuysaurusK. japonicusHakobuchi FormationA nearly complete skeleton with the skull and mandible (HMG-1219)A hadrosauridNipponopeltaN. yezoensisHikagenosawa Formation. (Cenomanian) A partial skull, teeth, osteoderms, and a cervical vertebraA nodosaurid ankylosaurNipponosaurusN. sachalinensisSakhalinA hadrosaurid ParalitherizinosaurusP. japonicus Osoushinai FormationPartial vertebra and partial right hand A therizinosaur Squamates Turtles reported from the Yezo Group Genus Species Presence Material Notes ImagesAnomalochelysA. angulataHobetsu-cho, Hokkaido (Cenomanian)A nanhsiungchelyidMesodermochelysM. undulatusHakobuchi Formation and upper part of the Osoushinai Formation -NMV-3, a proximal half of right humerus (Osoushinai Formation)A dermochelyid turtleM. sp.Osoushinai Formation.A carapace with boreholes likely made by bivalves, and associated with many mollusc remains.A dermochelyid turtle
Lepidosaurs Lepidosaurs reported from the Yezo Group Genus Species Presence Material Notes ImagesMosasaurusM. hobetsuensisHakobuchi FormationA mosasaurine mosasaurPhosphorosaurusP. ponpetelegansHakobuchi FormationA well-preserved skull with some associated vertebraeA halisaurine mosasaur with proportionately large eyes and binocular visionTaniwhasaurusT. mikasaensisKashima FormationA fragmentary skull, not diagnostic to the species levelA tylosaurine mosasaur also known from New Zealand, Antarctica, and South Africa; originally informally named 'Yezosaurus mikasaensis'.
Plesiosaurs
Plesiosaurs reported from the Yezo Group Genus Species Presence Material Notes ImagesPliosauridae indet.IndeterminateCenomanian to Turonian localities in HokkaidoFour indeterminate remainsPliosaur remains, some with skull sizes comparable to Megacephalosaurus
Pterosaurs Pterosaurs reported from the Yezo Group Genus Species Presence Material Notes ImagesPterodactyloidea indet.IndeterminateFragmentary femur, phalanx, and cervical vertebraA large form with an estimated wingspan of
Fish Fish reported from the Yezo Group Genus Species Presence Material Notes Images ApsopelixA. miyazakii Nakagawa Town and Mikasa area, 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-CampanianOne 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 pluricolumnalA crinoidIsocrinusI. sp.Haborogawa Formation (upper Turonian-Coniacian)One pluricolumnalA crinoidIsselicrinusI. sp.Campanian upper Yezo GroupOne pluricolumnal and five very poorly preserved columnals
Molluscs Molluscs reported from the Yezo Group Genus Species Presence Material Notes Images?Abyssochrysoidea indet.IndeterminateOsoushinai FormationA single specimen found associated with the remains of a Mesodermochelys sp.A gastropodBivalvia indet.UnidentifiedOsoushinai FormationA single specimen found associated with the remains of a Mesodermochelys sp.?Cephalaspidea indet.IndeterminateOsoushinai FormationA single specimen found associated with the remains of a Mesodermochelys sp.A gastropodChelonicerasC. sp.Tsukenai Formation (Aptian)An ammonite ColombicerasC. spathi Tsukenai Formation (Aptian) An ammoniteGaudrycerasG. denseplicatumSatonosawa Creek, Hokkaido (middle Turonian)An ammonoid?Gyrodes?G. sp.Osoushinai FormationA single specimen found associated with the remains of a Mesodermochelys sp.HaboroteuthisH. poseidonUpper Haborogawa Formation (Upper Santonian)KMNH IvP 902,002, a lower jawA teuthidHikideaH. cf. yasukawensisOsoushinai FormationA single specimen found associated with the remains of a Mesodermochelys sp.InoceramusI. (Sphenoceramus) orientalisUppermost Haborogawa Formation (middle-upper part of the lower Campanian)A bivalveI. (Sphenoceramus) schmidtiUppermost Haborogawa Formation (middle-upper part of the lower Campanian)A bivalveI. teshioensisGakkonosawa Creek, Hokkaido (late Turonian)Disarticulated valvesA bivalveNanaimoteuthisN. haggartiUppermost Haborogawa Formation (lower Campanian)KMNH IvP 902,001, a lower jaw, the only specimen availableA cirratan octopusN. jeletzkyiHokkaido (Santonian to lower Campanian)A cirratan octopusN. yokotaiSatonosawa Creek and Gakkonosawa Creek, Hokkaido (middle and late Turonian)Jaw fossilsA cirratan octopusPaleocirroteuthisP. sp.Hokkaido (Santonian to lower Campanian)A cirroctopodidProvannaP. cf. nakagawensisOsoushinai Formation3 specimens found associated with the remains of a Mesodermochelys sp.An abyssochrysoid gastropodRhytidoplitesR. adkinsiTsukenai Formation (Aptian)An ammoniteScalaritesS. mihoensisGakkonosawa Creek, Hokkaido (late Turonian)An ammonoidS. scalarisSatonosawa Creek, Hokkaido (middle Turonian)An ammonoidTetragonitesT. glabrusSatonosawa Creek, Hokkaido (middle Turonian)An ammonoidThyasiraT. tanabeiOsoushinai Formation2 articulated specimens found associated with the remains of a Mesodermochelys sp.A bivalveYezoteuthisY. giganteusOsoushinai FormationJaw fossils A very large oegopsid
Paleoflora
Plants reported from the Yezo Group Genus Species Presence Material Notes ImagesEydeiaE. hokkaidoensisHaborogawa Formation (Coniacian-Santonian)Five perimineralized fruitsA cornale similar to the extant Davidia involucrata Kamikistrobus K. primulus Gakkonosawa Creek, Hokkaido (late Turonian) A bald cypress MikasapterisM. rothwellii Mikasa LocalityA permineralized fertile pinnule A probable stem polypodioid leptosporangiate fern MukawastrobusM. satoiHakobuchi FormationA single cylindrical seed cone 2 cm long, 1.1 cm wideA cypressNilssoniaN. yezoensisHakobuchi FormationA cycadObirafructusO. kokubuniiHaborogawa Formation (Coniacian to Santonian)A permineralized reproductive axis bearing at least 42 spirally arranged folliclesA saxifragaleStockeystrobusS. interdigitataHoborogawa Formation (Coniacian to Santonian)A seed coneA sequoioidZamitesZ. bayeriKashima Formation (Coniacian)A bennettitale
See also List of dinosaur-bearing rock formations
References