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Cretaceous Research 37 (2012) 319e340

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Cretaceous Research

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Review paper A review of the Upper Cretaceous marine reptiles from Japan

T. Sato a,*, T. Konishi b, R. Hirayama c, M.W. Caldwell d,e a Department of Astronomy and Earth Sciences, Tokyo Gakugei University, Nukui-Kita-Machi 4-1-1, Koganei City, Tokyo 184-8501, Japan b Royal Tyrrell Museum of Palaeontology, PO Box 7500, Drumheller, Alberta T0J 0Y0, Canada c School of International Liberal Studies, Waseda University, Nishiwaseda 1-6-1, Shinjuku-ku, Tokyo 169-8050, Japan d Department of Biological Sciences, University of Alberta, Edmonton, Alberta T6G 2E9, Canada e Department of Earth and Atmospheric Sciences, University of Alberta, Edmonton, Alberta T6G 2E9, Canada article info abstract

Article history: Taxonomy and stratigraphic distribution of the Upper Cretaceous marine reptiles from Japan are Received 10 March 2011 reviewed. Remains of the Chelonioidea (sea turtles), Mosasauridae, and Plesiosauria are known in various Accepted in revised form 9 March 2012 parts of Japan, including the holotypes of the dermochelyid Mesodermochelys undulatus, mosasaurine Available online 19 April 2012 Mosasaurus hobetsuensis and M. prismaticus, tylosaurine Taniwhasaurus mikasaensis, and elasmosaurid Futabasaurus suzukii. Less diagnostic materials of other groups such as protostegiids, plioplatecarpines, Keywords: polycotylids, pliosauroids, were also collected. Mesodermochelys dominates the chelonioid fauna, and in Chelonioidea comparison with European and North American faunas, suggests a rather restricted geographical Japan Mosasauridae distribution of chelonioid species during the Late Cretaceous. The mosasaurid records support the world- Plesiosauria wide trend of increasingly mosasaurine-dominated post-Santonian assemblages, and demonstrate Reptilia suprageneric-level compositional changes in the northwestern Pacific through time. Elasmosaurid fossils are known from all stages of the Upper Cretaceous in Japan and indicate their continuous presence in the northwestern Pacific. Polycotylid remains are fewer in number and limited to the lower Upper Cretaceous. Pliosauroid specimens are even rarer but raise the upper limit of the stratigraphic range of the group in Northern Pacific to the Turonian. Ó 2012 Elsevier Ltd. All rights reserved.

1. Introduction 1988; Oji, 1985; Chinzei, 1986; Maeda, 1991; Tanabe et al., 2003), have been conducted on the marine invertebrates from the Creta- Palaeontological study of Cretaceous marine sediments in Japan ceous of Japan. began in the 19th century with one of the earliest publications being In contrast, the vertebrate fossils from the same strata seem to a descriptive treatise on fossil molluscs (Yokoyama, 1890). A large have attracted much less attention, at least when the number of number of later studies, in particular those by Matsumoto and his academic publications and researchers is compared to those students, established the Japanese Cretaceous as a standard section focused on fossil invertebrates. Early publications date back at least in the circum-Paciﬁc region (e.g., Matsumoto, 1942; Kimura et al., to the 1920s (e.g., Tokunaga and Shimizu, 1926), but scholarly 1993a and references therein). The early emphasis on molluscan articles on Cretaceous marine reptiles have remained few in fossils (e.g., ammonites, bivalves), and later on microfossils number. Those that were published were often provisional studies (e.g., foraminifers, radiolarians) have continued to play major roles in (e.g., Shikama, 1963; Obata et al., 1970, 1972), most likely owing to biostratigraphic zonation, sometimes in combination with magne- the limited availability of specimens and resources (facilities for tostratigraphy for correlation with the global geological timescale preparation, references, comparative material) necessary for the (Toshimitsu et al., 1995). Chemostratigraphy (e.g., Hasegawa, 1997; proper study of Mesozoic reptiles. Uramoto et al., 2007) is a relatively new but promising tool for The situation began to change in the 1980s when descriptive correlation and environmental interpretation. Research in various papers started appearing in professional journals (e.g., Suzuki, other disciplines, such as theoretical and functional morphology, 1985a; Nakaya, 1989a, b), with the number of publications palaeoecology, and taphonomy (e.g., Tanabe, 1979; Okamoto, 1984, increasing dramatically in the 1990s (e.g., Hirayama and Chitoku, 1994, 1996). This trend continued into the ﬁrst decade of the 21st century (Obata et al., 2007), with descriptions of famous plesiosaur

* Corresponding author. Tel.: þ81 42 329 7537; fax: þ81 42 329 7504. and mosasaur specimens, respectively known to the Japanese E-mail address: [email protected] (T. Sato). public as “Futaba Suzuki Ryu” and “Ezo Mikasa Ryu”, ﬁnally being

320 T. Sato et al. / Cretaceous Research 37 (2012) 319e340 published more than 30 years after their initial discoveries (Sato a large number of previously proposed lithostratigraphic nomen- et al., 2006a; Caldwell et al., 2008). clatures (e.g., Kimura et al., 1993a). Thus, after nearly six decades of steady effort by a small but In the classic scheme, the Yezo Supergroup is divided into Lower committed group of researchers, the temporal and spatial biodi- Yezo, Middle Yezo, Upper Yezo, and Hakobuchi Groups, in versity of the Cretaceous marine reptiles of Japan is beginning to ascending order (e.g., Kimura et al., 1993a); this system is widely reveal itself. In this contribution, we review the state of under- used for the current collection records of many fossil specimens. standing of the taxonomy and stratigraphic distribution of the Recently, Takashima et al. (2004) integrated lithostratigraphic and aquatically adapted animals, and attempt to analyze their biogeo- biostratigraphic data and proposed a new scheme in which the graphical significance. entire sequence, now referred to as the Yezo Group, is divided into several formations (the names of certain formations differ in 2. Geological background different areas because of facies changes). The unit boundaries in the two nomenclatures do not necessarily match, and it could lead Japanese Upper Cretaceous marine reptile fossils occur in five to unintentional errors if information contained in one nomencla- major stratigraphical units: the Yezo, Kuji, Futaba, Izumi, and ture was simply converted into another. The stratigraphic infor- fl Goshoura groups (Figs. 1and 2; Table 1). Many of these specimens mation of each specimen provided in this paper re ects the original were excavated by private collectors and donated to local assignments as given in collection data or in publication which museums; therefore, their locality data is based solely on museum often employs the old nomenclature. A generalized, schematic records and/or published accounts. Many specimens from Hok- presentation of the stratigraphic nomenclatures by Kimura et al. fi kaido were collected as float along rivers and creeks, and their (1993a, table 2.13) and Takashima et al. (2004, g. 8) in Fig. 2 fi original horizons cannot be precisely verified; however, the creeks gives rough but suf cient information for an overview of the in the fossil producing areas are generally restricted, and we stratigraphic distribution of the specimens from Hokkaido. consider it is fairly safe to assume that the original horizon is not The Kuji Group in Iwate Prefecture is known for commercial very far from the point of collection. quantities of amber. The group is geologically divided into the The Yezo Group (or Supergroup) exposed in central Hokkaido Tamagawa (non-marine), Kunitan (marine), and Sawayama (non- has so far yielded the largest number of specimens of marine marine) formations (e.g., Kimura et al., 1993a). A few specimens of reptiles from Japan (for reviews of the stratigraphy, see Takashima marine reptiles are known from the Kunitan Formation, a unit that et al., 2004 and Kimura et al., 1993a). The group represents consists largely of sandstones and interpreted as having been AptianeMaastrichtian sediments deposited in a forearc basin, and deposited in nearshore to inner shelf environments; the formation has been subject to intensive biostratigraphic studies because of is dated as Santonian based on molluscan fossils (Terui and the abundant micro- and macrofossils found there since the early Nagahama, 1995). part of the 20th century, and especially after the 1940s. The One of the oldest published accounts on Japanese fossil marine lithology of these units is rather monotonous (primarily consisting reptiles came from the Upper Cretaceous in Fukushima Prefecture of sand- and mudstone), and there are considerable regional (Tokunaga and Shimizu, 1926). The Futaba Group is divided into differences in the lateral extent and thickness of the units owing to three formations, i.e., Ashizawa and Kasamatsu (both Coniacian), lateral facies changes and tectonic deformation (e.g., faults and and Tamayama (Coniacian to Santonian), and these represent fl folds). The thickness of recognized lithological units varies laterally, uvial to shallow marine environments (Kimura et al., 1993a; Ando and there is no guarantee that lithological boundaries are time et al., 1995; Kubo et al., 2002). Various ammonites and bivalves, synchronous. These problems have challenged stratigraphers who pollen, shark teeth, and reptiles including dinosaurs and marine attempted to establish a stratigraphic framework, and there are reptiles, have been reported from the Ashizawa and Tamayama formations (Ando et al., 1995; Kubo et al., 2002; Manabe et al., 2003). All of the marine reptiles from Fukushima with precise stratigraphic information came from these two formations. Various marine invertebrates, mainly molluscs such as ammonites and inoceramids, have been reported from the CampanianeMaastrichtian Izumi Group. Outcrops in Osaka, Wakayama, Hyogo (including Awaji Island), and Kagawa prefectures have yielded marine reptiles; there are a number of publication on these reptilian fossils, but only a limited number of specimens are in public repositories. The Izumi Group consists of an extremely thick marine sequence; the strata form a plunging syncline, and the base of the group is time-transgressive towards the east (Morozumi, 1985; Kodama, 1990; Kimura et al., 1993a). There are regional differences in stratigraphic nomenclature, and lithological boundaries of certain fossil-yielding units are considerably diachronous owing to lateral facies change. We follow Nakazawa et al. (1987) for the nomenclature of rock units in the Osaka-Wakayama area; see Itihara et al. (1986) for a review of palaeontological studies in this area. On Awaji Island of Hyogo Prefecture, we follow Morozumi (1985) who identified five ammonite zones of the Izumi Group in the southern part of the island and correlated them with the uppermost portion of the Yezo Group. Yamasaki (1986) divided the Izumi Group in Kagawa Prefecture into six formations, but his system has not been widely used; instead, collection records of the reptilian fossils usually Fig. 1. Localities of Cretaceous marine reptiles from Japan. mention the stage or biozone to indicate the fossil-yielding Author's personal copy

T. Sato et al. / Cretaceous Research 37 (2012) 319e340 321

Fig. 2. Geological units yielding Cretaceous marine reptiles in Japan. Stratigraphy compiled from Kimura et al. (1993a) and Takashima et al. (2004).

Table 1 Stratigraphic distribution of the marine reptiles in the Japanese Cretaceous. Note that the abbreviation(s) in parentheses indicate the stage(s) of the fossil-yielding horizon and not the unit as a whole. Taxa without a stage designation are those for which the stage of the fossil-yielding stratum was not speciﬁed in the original reference.

Area Geological unit Chelonioidea Mosasauridae Plesiosauria Hokkaido Hakobuchi Gp Mesodermochelys undulatus (Cam, Maa) Mosasaurus hobetsuensis (Maa) Elasmosauridae indet. (Maa) Chelonioidea indet. Mosasaurus prismaticus (Cam or Maa) Mosasaurinae indet. (Cam, Maa) Mosasauridae indet. (Cam or Maa)

Upper Yezo Gp M. sp. cf. M. undulatus (San, Cam) Taniwhasaurus mikasensis (San or Cam) Elasmosauridae indet. (incl. Kashima and (Con, San, Cam) Haborogawa Fms) Protostegidae indet. (San) Tylosaurus? sp. (Cam) Elasmosauridae? (Con, San) Protostegidae indet. (San?) Mosasaurinae indet. (Cam) Polycotylidae (Tur, Con, or San) Chelonioidea indet. (Tur?, Con, San?) Plioplatecarpinae indet. (Con or San) Russellosaurina indet. (Con or San) Mosasauridae indet (San)

Middle Yezo Gp Desmatochelys sp. cf. D. lowi (Tur) Elasmosauridae indet. (Cen) (incl. Mikasa Fm) Chelonioidea indet. (Tur, Con) Polycotylidae indet. (Cen, Tur?) Pliosauroidea (Cen, Tur)

Iwate Kunitan Fm Russellosaurina indet. (San) Plesiosauria indet. (San) Mosasauridae indet. (San)

Fukushima Tamayama Fm Chelonioidea indet. (San) Mosasauridae (San) Futabasaurus suzukii (San) Elasmosauridae indet. (San) Polycotylidae? (San)

Ashizawa Fm Plioplatecarpinae indet. (Con) Elasmosaridae indet. (Con) Polycotylidae? (Con)

Osaka-Wakayama Kokawa Fm Mosasaurinae indet. (Maa)

Shintachi Fm Mosasaurinae indet. (Maa)

Mutsuo Fm Mosasaurus sp. (Maa) Mosasaurinae indet. (Maa) Russellosaurina? (Maa) Mosasauridae indet. (Maa)

Awaji Is Shimonada Fm Mosasaurinae indet. (Maa) Elasmosauridae? (Maa)

Kita-ama Fm Mesodermochelys undulatus (Maa) Mosasauridae indet. (Maa)

Seidan Fm Russellosaurina? (Cam)

Kagawa Izumi Gp Mesodermochelys undulatus (Cam) Mosasaurinae indet. (Cam) Elasmosauridae? (Cam) M. sp. cf. M. undulatus (Cam) Polycotylidae? (Cam)

Kagoshima Hegushi Fm Elasmosauridae indet. (Cen) Author's personal copy

322 T. Sato et al. / Cretaceous Research 37 (2012) 319e340 horizons. Therefore, we did not designate the fossil-yielding Dermochelydae Gray, 1825 formation for the material from Kagawa in Table 1, although the Mesodermochelys Hirayama and Chitoku, 1996 formation name is mentioned in the systematic palaeontology section below if provided in original references. Mesodermochelys undulatus Hirayama and Chitoku, 1996 The AlbianeCenomanian Goshoura (or “Goshonoura” in old Fig. 3AeC references) Group in Kyushu consists of clastic sediments in non-marine to shallow marine environments, yielding marine and Holotype. HMG 5, supraoccipital, vertebrae, limb elements, cara- non-marine fossils (Komatsu and Maeda, 2005,andreferences pace and plastron. therein). The only published record of a marine reptile is the plesiosaurian specimen from the upper portion of the Hegushi Type locality and horizon. Shirafunenosawa Creek, Hobetsu district Formation, Shishi Island, Kagoshima Prefecture, dated as of Mukawa Town, Hokkaido; Maastrichtian, Hakobuchi Group. Cenomanian based on co-occurring ammonites (Nakaya et al., 2006). Referred material. GSJ uncatalogued specimen (coracoids, plastron): 3. Material and method Sumoto, Awaji Island, Hyogo; Maastrichtian, Kita-ama Formation, Izumi Group. Hirayama (1993), Hirayama and Chitoku (1994). A large number of fossils of marine reptiles have been discovered HMG 6 (partial carapace and limbs): Ohmagarinosawa Creek, from the Japanese Cretaceous, but readers should note that many Hobetsu district of Mukawa Town, Hokkaido; Hakobuchi Group. specimens are privately owned and often remain unknown to science. Hirayama and Chitoku (1996). In this review, we focus on the specimens that (1) we have examined HMG 8, 9, 363, 365, 1053, 1129, 1220 (partial skeletons and personally, (2) are in public repositories, and/or (3) have published shells): Pankerusanosawa Creek, Hobetsu district of Mukawa Town, records including conference abstracts if in private collection. There- Hokkaido; Hakobuchi Group. Suzuki (1984), Hirayama and Suzuki fore, the specimens in private collections mentioned in this contribu- (1985), Chitoku (1987, 1990), Hirayama and Chitoku (1996), tion are those with sufficient published information for review. Sakurai (2005), personal observation (RH). In the following systematic palaeontology section, locality or HMG 368, 369 (partial shells and limb elements): a branch of stratigraphic information is simplified for some specimens, because Pankerusanosawa Creek, Hobetsu district of Mukawa Town, Hok- the limited numbers of very specific data do not necessarily kaido; Hakobuchi Group. Chitoku (1990), Hirayama and Chitoku improve the discussion of this review; their details are available in (1996), Sakurai (2005). the original publication or on file at the repository. Recent munic- HMG 7, 342, 362, 1054 (partial skeletons and shells): Hobetsu ipal mergers have affected many of the locality names at the city River, Hobetsu district of Mukawa Town, Hokkaido; Hakobuchi level and below. The municipality names in this review reflect the Group. Hirayama and Suzuki (1985), Chitoku (1987, 1990), current state of their organization. Hirayama and Chitoku (1996), Sakurai (2005). HMG 1062, 1063, 1064 (mandible, vertebrae, limb and shell Institutional abbreviations. ECN, Educational Congress of Numata, elements): Sososhisawa Creek, Hobetsu district of Mukawa Town, Numata Town, Hokkaido Prefecture; GPM, Gifu Prefectural Hokkaido; Hakobuchi Group. Sakurai (2005). Museum, Seki City, Gifu Prefecture; GSJ, Geological Survey of Japan, HMG 1230 (partial carapace): Shirafunenosawa Creek, Hobetsu Tsukuba City, Ibaraki Prefecture; HMH, Historical Museum of district of Mukawa Town, Hokkaido: Hakobuchi Group. Sakurai Hokkaido, Sapporo, Hokkaido Prefecture; HMG, Hobetsu Museum (2005), personal observation (RH). (geology collection), Mukawa City, Hokkaido Prefecture; HMC, HMG 1078 (shell fragments): Hobetsu River, Hobetsu district of Hidaka Mountain Center, Hidaka Town, Hokkaido Prefecture; IMCF, Mukawa Town, Hokkaido. Hakobuchi Group? Sakurai (2005), Iwaki Museum of Coal and Fossils, Iwaki City, Fukushima Prefec- personal observation (RH). ture; IPMM, Iwate Prefectural Museum, Morioka City, Iwate HMG 1195 (carapace, shell and girdle fragments): a branch of Prefecture; KCU, Kochi University, Kochi City, Kochi Prefecture; Makkashimappu River, Hobetsu district of Mukawa Town, Hokkaido; KSNHM, Natural History Museum, Kishiwada City, Osaka Prefec- Hakobuchi Group. Sakurai (2005), personal observation (RH). ture; KGU, Kagoshima University, Kagoshima City, Kagoshima HMG 1225, 1454, 1474, 1476, 1526, 1527, 1531, 1533, 1534, 1535, Prefecture; MCM, Mikasa City Museum, Mikasa City, Hokkaido 1536, 1537 (partial skeletons and shells): Hobetsu district of Town, Prefecture; NFL, Numata Fossils Laboratory, Numata Town, Hokkaido; Maastrichtian, Hakobuchi Formation. Personal obser- Hokkaido Prefecture; NM, Nakagawa Museum, Nakagawa City, vation (RH). Hokkaido Prefecture; NSM, National Museum of Nature and Science HPMHN uncatalogued specimens (coracoids, pubis, shell frag- (formerly National Science Museum), Tokyo; OM, Obira Museum, ments). Sumoto, Awaji Island, Hyogo; Maastrichtian, Kita-ama Obira Town, Hokkaido Prefecture; OMNH, Osaka Museum of Formation, Izumi Group. Hirayama (1993), Hirayama and Chitoku Natural History, Osaka City, Osaka Prefecture; TUTg and THUg, (1994). Teikyo Heisei University (formerly Teikyo University of Technology) NSM PV15022 (left quadrate, opisthotic, and columella auris): (geology), Tokyo; SMAC, Sapporo Museum Activity Center, Sapporo Panketosanosawa Creek, Hobetsu district of Mukawa Town, City, Hokkaido Prefecture; TUM, Tohoku University Museum, Hokkaido; Maastrichtian, Hakobuchi Group. Obata et al. (1972). Sendai City, Miyagi Prefecture; UHR, Hokkaido University Museum, TKPM GFV126 (partial skeleton): Koutou River in Shionoe, Sapporo City, Hokkaido Prefecture; UMUT, University Museum of Takamatsu City, Kagawa; Campanian, Zone B of basal University of Tokyo, Tokyo. conglomerate-sand bed (Bando and Hashimoto, 1984), Izumi Group. Hirayama et al. (2006). 4. Systematic palaeontology Private collections in Hirayama (1993) (various postcranial fragments): Sumoto, Awaji Island, Hyogo; Maastrichtian, Kita-ama 4.1. Chelonioidea Formation, Izumi Group. Hirayama (1993), Hirayama and Chitoku (1994). Testudines Linnaeus, 1758 Cryptodira Gray, 1825 Remarks. See Hirayama and Chitoku (1996) for the full description Chelonioidea Aggasiz, 1857 of this species, and a series of studies by Hirayama (e.g., 1997, Author's personal copy

T. Sato et al. / Cretaceous Research 37 (2012) 319e340 323

2006a) and his co-authors for the biogeographical significance of Turonian, Saku Formation, Middle Yezo Group. Hirayama (1992a, the findings. M. undulatus is an aberrant chelonioid, especially 1995, 1997a, b), Hirayama and Chitoku (1994). characterized by thick peripheral plates with undulating medial borders, although its limb morphology shows dermochelyid Remarks. The family-level affinity of Desmatochelys has been affinity. M. undulatus is the most common marine turtle from the debated (e.g., cheloniid in Carroll, 1988; desmatochelyid in Japanese Cretaceous, especially the Maastrichtian of Hakobuchi and Nicholls, 1992), but we follow the cladistic analysis of Hirayama Izumi Groups, and is represented by a large number of individuals (1992a, b, 1998a, b) in assigning this genus to the Protostegidae. of various sizes. The largest shell is estimated to be up to 2 m long, Its protostegid synapomorphies include the medial meeting of possibly comparable with the well-known Archelon ischyros palatines, the reduced vomer, extensive connection of pterygoid to Wieland, 1896 from North America. quadrate, the C-shaped pterygoid, the biconvex cervical vertebra, and a radius with outward curvature. THU g262 is an articulated Mesodermochelys sp. cf. M. undulatus skeleton, including skull, hyoids, complete cervical series, and Figs. 3D, 4A, B partial shell. The specimen is identified as this genus based on the fi Referred specimens. HMH 151807 (partial skull and a peripheral): uniquely reduced ossi cation of the shell. New osteological Kotanbetsu River, Tomamae Town, Hokkaido; Santonian, Upper information, such as the cervical vertebrae and entoplastron, is Yezo Group. Hirayama (2007), Hirayama et al. (2007). included in the reconstructions of D. lowi by Hirayama and HMG 366, 367 (shell fragments): Kotanbetsu River, Tomamae Chitoku (1994) and Hirayama (1997a, b). Desmatochelys was only Town, Hokkaido; Santonian, Upper Yezo Group? Chitoku (1990), known from the Cenomanian and Turonian of western North Sakurai (2005). America (Hirayama 1997a, b), and the discovery of the Japanese NMV 3 (proximal portion of right humerus): Rubeshobezawa of specimen widened the geographic distribution of the genus Abeshinai River, Nakagawa Town, Hokkaido; Campanian, Osoush- considerably. inai Formation, Upper Yezo Group. Hirayama and Hikida (1998). Protostegidae gen. et sp. indet. NMV 50, 53 (partial carapace and femur): Nio River, Nakagawa Fig. 4C, D Town, Hokkaido; Campanian, Osoushinai Formation, Upper Yezo Group. Personal observation (RH). Referred material. NSM PV20024 (partial shell, hind limb and NMV 51 (neurals): Wakkawenbetsu River, Nakagawa Town, girdle): Horotatezawa of Kotabetsu River, Tomamae Town, Hok- Hokkaido; Santonian, Upper Yezo Group. Personal observation (RH). kaido; Santonian, Upper Yezo Group. Hirayama (1995). NSM PV15017 (partial shell and skeleton): Akanosawa of Obir- THU g2161 (cervical, rib, radius): Kaminosawa of Kotanbetsu ashibe River, Obira Town, Hokkaido; Santonian, Upper Yezo Group. River, Tomamae Town, Hokkaido; Santonian, Upper Yezo Group. Obata et al. (1972), Hirayama (1994). Hirayama (1995). THU g2162 (partial carapace): Horodatezawa of Kotanbetsu THU g2163 (partial skull): Nubinaizawa of Kenomai River, Mon- River, Tomamae Town, Hokkaido; Santonian, Upper Yezo Group. betsu area of Hidaka Town, Hokkaido; Santonian? Hirayama (1995). Hirayama (1994). HMG 1055 (costals, centrum, neural arch): Hanbanosawa of MCM A133 (carapace): Haboro River, Haboro Town, Hokaido; Ikushunbetsu River, Mikasa City, Hokkaido; Santonian, Upper Yezo Santonian?, Upper Yezo Group. Personal observation (RH). Group. Hirayama and Chitoku (1994), Sakurai (2005). MCM A570 (partial carapace, vertebrae): Kaminosawa of Kotanbetsu River, Tomamae Town, Hokkaido; Santonian, Upper Remarks. Hirayama (1995) suggested these fragmentary speci- Yezo Group. Personal observation (RH). mens from the Santonian of Hokkaido Prefecture would represent Private collection in Tanimoto et al. (2006) (humerus): Shionoe, rather small protostegids; their estimated shell lengths are less Takamatsu City, Kagawa; Campanian, Izumi Group. Tanimoto et al. than 60 cm. They share protostegid synapomorphies such as the (2006), personal observation (RH). keeled, thickened neurals and the extensive contact between pterygoid and quadrate. This material should represent an Remarks. Mesodermochelys undulatus and/or its close relatives are unknown taxon of this family, because no protostegid species also known from the Santonian and Campanian sediments of with identical characteristics has ever been reported. Its unique Hokkaido and Kagawa prefectures, although they are generally features include rather extensive costals and a large central collected as rather fragmentary specimens. They are similar to fontanel of the plastron. Maastrichtian taxa, whereas minor differences (e.g., plastron and pelvic girdle) suggest at least one Santonian form would represent Chelonioidea gen. et sp. indet. a different species. The HMH specimen represents the first reasonably preserved skull referred to this genus, though this is not Referred material. HMG 361 (shell fragment): Pankerusanosawa associated with good postcranial material and the taxonomic Creek, Hobetsu district of Mukawa Town, Hokkaido; Hakobuchi reference remains tentative; in fact, a preliminary analysis indicates Group. Chitoku (1987, 1990), Sakurai (2005). similarities between this specimen and the Protostegidae, rather HMG 364 (shell fragments): Yubari City, Hokkaido; horizon than with the Dermochelyidae (Hirayama, 2007). The total length unknown. Hirayama and Chitoku (1994), Sakurai (2005). of the humerus reported in Tanimoto et al. (2006) is about 50 cm, HMG 1072 (egg): Hobetsu River, Hobetsu district of Mukawa and the shell length is estimated to be about 2 m. Town, Hokkaido; Turonian, Middle Yezo Group. Sakurai (2005). HMG 1128 (vertebra, shell fragments): Takikawanosawa Creek, Protostegidae Cope, 1872 Hobetsu district of Mukawa Town, Hokkaido; Upper Yezo Group. Desmatochelys Williston, 1894 Sakurai (2005), personal observation (RH). Desmatochelys lowi Williston, 1894 HMG 1529 (partial shell), Tosanosawa of Hobest River, Hobest district of Mukawa town, Hokkaido; Upper Yezo Group ? Personal Desmatochelys cf. D. lowi observation (RH). HMG 1530 (partial shell), Shisanushubetsuzawa, of Hobest Referred material. THU g262 (skull, cervicals, forelimb and girdle, River, Hobest district of Mukawa town, Hokkaido; Upper Yezo partial shell): Pankemoyuuparo River, Yubari City, Hokkaido; Group ? Personal observation (RH). Author's personal copy

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Fig. 3. Representative specimens of Cretaceous marine turtles from Japan, Part I. AeC, Mesodermochelys undulatus Hirayama and Chitoku from the Hakobuchi Formation (Early Maastrichtian) of Hobetsu, Mukawa-cho, Hokkaido. A, a mounted skeleton in dorsal view, displayed in the Hobetsu Museum, largely based on HMG 5 (holotype) and 342 (modiﬁed from Hirayama, 2006a). B, HMG-369, plastron in ventral view. C, HMG 1062, dentary in dorsal view. D, NMV 3, Mesodermochelys cf. undulatus Hirayama and Chitoku from the Osoushinai Formation (Early Campanian) of Yasukawa, Nakagawa-cho, Hokkaido, proximal portion of right humerus in ventral view (after Hirayama and Hikida, 1998).

IMCF uncatalogued specimen (shell fragments): Yachi, Iwaki SMAC 2572 (cervicals, pectoral girdle, shell fragments): Haboro City, Fukushima; Santonian, “Upper Formation” (¼ Tamayama River, Haboro Town, Hokaido; TuronianeSantonian, Upper Yezo Formation), Futaba Group. Hirayama and Chitoku (1994). Group. Tahara et al. (in press). MCM uncatalogued specimen (partial skeleton): Kamiki- TUT g87 (thoracic centrum): Upper Yezo Group. Hirayama and nenbetsu River, Obira town, Hokkaido; Coniacian, Upper Yezo Chitoku (1994). Group. Personal observation (RH). NFL 035 (shell fragments): Koshin Second Dam of Ponponita- Remarks. These are very fragmentary or unprepared remains and nishibetsu River, Numata Town, Hokkaido; Turonian? Middle Yezo cannot be identiﬁed at a lower taxonomic level. Turtle egg shells Group. Hirayama and Chitoku (1994), personal observation (RH). have been often collected from the TuronianeSantonian marine NSM PV15018 (egg): Kechikauen-obirashibe River, Obira sediments of Hokkaido Prefecture (Fukuda and Obata, 1991;RH Town, Hokkaido; Coniacian, Upper Yezo Group. Fukuda and Obata pers. obs.). These eggs may belong to extinct protostegids, because (1991). living chelonioids do not have a rigid calcareous egg shell. Author's personal copy

Fig. 4. Representative specimens of Cretaceous marine turtles from Japan, Part II. A, HNH-151807, Mesodermochelys sp. from the Upper Yezo Group (Santonian) of Tomamae-cho, Hokkaido, skull in dorsal view. B, NSM PV15017, Mesodermochelys sp. from the Upper Yezo Group (Santonian) of Obira-cho, Hokkaido, partial carapace in dorsal view. C, D, NSM PV20024, Protostegidae gen. et sp. indet. from the Upper Yezo Group (Santonian) of Tomamae-cho, Hokkaido. C, partial carapace in dorsal view. D, right hypoplastron and xiphiplastron, ventral view. E, Desmatochelys cf. lowii Williston, THUg 262 (skull, hyoids, cervicals, pectoral girdle, and partial carapace and plastron in ventral view) from the Saku Formation (Middle Turonian) of the Nakanosawa of Pankemoyuuparo River, Yuubari, Hokkaido. Author's personal copy

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4.2. Mosasauridae vertebrae, 21 with a rib), 22, 23 (two dorsal vertebrae) (all considered part of single individual): Sobura area, Osaka; Maas- Squamata Oppel, 1811 trichtian, Azenotani mudstone bed of Mutsuo Formation, Izumi Mosasauridae Gervais, 1852 Group. Tanimoto et al. (1994a, b, 2004a), Tanimoto (2005). Mosasaurinae Gervais, 1852 Private collection in Tanimoto and Yokoi (2005) (left dentary): Mosasaurus Conybeare, 1822 locality and horizon as above. Tanimoto and Yokoi (2005), Tanimoto (2005). Mosasaurus hobetsuensis Suzuki, 1985a Remarks. Many of the specimens (KSNHM-F6-10e23) are described Holotype. HMG 12, single marginal tooth, dorsal vertebrae, ribs, and figured in Tanimoto et al. (1994a). They were all found within pectoral girdle elements (right scapula, right coracoid, clavicle?, a small area and at the same horizon, and were regarded as interclavicle), articulated right forelimb, isolated left manual belonging to a single individual (see Tanimoto et al., 2004a, fig. 1). A elements. Relatively small individual with estimated body length of lack of overlapping elements and their matching sizes further 5.5 m (Suzuki, 1985a). justify this assumption. The left dentary described by Tanimoto and Yokoi (2005) could be part of the former specimens as well Type locality and horizon. Pankerusanosawa Creek in the Hobetsu (Tanimoto, 2005; Tanimoto and Yokoi, 2005). The generic assign- district of Mukawa Town, Hokkaido; Maastrichtian, Hakobuchi ment of all of these elements to Mosasaurus is supported by a clear Group. presence of heterodonty, various marginal teeth exhibiting a vari- able but consistent posterior curvature, and the braincase showing Remarks. To date, the holotype is the only known specimen refer- mosasaurine characteristics (e.g., the basisphenoid possessing able to this species. The original diagnosis by Suzuki (1985a) a narrow basipterygoid process, with anterolaterally facing artic- includes characteristics of the marginal tooth and the front limb. ular surface; Bell, 1997, p. 312). Although only the first two char- As Suzuki (1985a) pointed out however, some conspicuous features acters apply to the dentary specimen in private hands, it is here of the propodial and epipodial elements of the front limb are var- assigned to Mosasaurus sp. given the probability of it pertaining to iably lacking in other congeners, including the pectoral crest the same individual as the other specimens. continuous with the glenoid condyle surface, the poorly developed Mosasaurinae gen. et sp. indet. postglenoid process, the well-developed anterodistal flange of the Figs. 5C, D, 6A radius, the round antebrachial foramen, and the posteriorly protruding olecranon process. All of these characters are instead Referred material.HMG10(caudalvertebrae,limbelements;Fig. 5D): shared with Clidastes (Russell, 1967) and, where present, with Shirofunenosawa Creek, Hobetsu district of Mukawa Town, Hokkaido; Prognathodon overtoni (Konishi et al., 2011). In addition, the Maastrichtian, Hakobuchi Formation, Yezo Group. Sakurai (2005, smooth-enameled, laterally highly compressed, and nearly straight 2008). tooth crown shares similarities with the teeth of Liodon mosa- HMG 1077 (skull fragments, pterygoid teeth, left ulnare; sauroides. The current generic assignment to this mosasaurine Fig. 6A): Shirakabanosawa Creek, Hobetsu district of Mukawa taxon should be considered tenuous. Town, Hokkaido; CampanianeMaastrichtian, Hakobuchi Group, Yezo Group. Sakurai and Ono (2004) , Sakurai (2005, 2008). Mosasaurus prismaticus Sakurai et al., 1999 NFL 33 (large skull fragment including upper and lower teeth in Fig. 5A, B occlusion: Fig. 5C): Shisennosawa River, Numata Town, Hokkaido; Campanian, Upper Yezo Group. Kimura et al. (1993b). Holotype. HMG 1065, single marginal tooth, six braincase elements KSNHM-F6-2 (associated premaxilla, maxillae, and dentaries all articulated except the parietal, epipterygoid, and supratemporal with teeth): Kanewari, Sanuki City, Kagawa; Campanian, Hiketa (Fig. 5A, B) Formation, Izumi Group. Tanimoto and Kanazawa (2001), Tanimoto et al. (2004b), Tanimoto (2005), Konishi et al. (2010). Type locality and horizon. Hobetsu River in the Hobetsu district of KSNHM-F6-3 (fragment of large tooth bearing element): Mukawa Town, Hokkaido; CampanianeMaastrichtian, Hakobuchi Showaike, Sennan City, Osaka; Maastrichtian, Mutsuo Formation, Group. Izumi Group. Konishi et al. (in press). OMNH MV-0066 (marginal tooth): Hashiramoto City, Remarks. The holotype is the only known specimen referred to this Wakayama; Maastrichtian, Kokawa Formation, Izumi Group. species. In the original work by Sakurai et al. (1999), the generic Kishimoto (1991), Tanimoto (2005), Ohara (2004). identification was based on the combination of the braincase and Private collection in Tanimoto and Kikyo (2001) (section of 11 marginal tooth characters, whereas the specific identification was articulated terminal caudal vertebrae): Nandan, Minami-Awaji City, solely based on the characteristics of the single marginal tooth. Awaji Island, Hyogo; Maastrichtian, Shimonada Formation, Izumi Although the parietal characters listed by Sakurai et al. (1999) and Group. a slender, well-faceted tooth crown support assignment of HMG Private collection in Tanimoto et al. (2005) (single intermediate 1065 to Mosasaurus, most mosasaurines including Mosasaurus also caudal vertebra): Hakotsukuri, Hannan City, Osaka; Maastrichtian, exhibit heterodonty, and thus single tooth morphology is of little Shintachi Formation, Izumi Group. Tanimoto (1993), Tanimoto et al. value in the alpha-level taxonomy of mosasaurines. As Sakurai et al. (1995, 2005). (1999) demonstrated, based on the available information, there is no exact morphological match among known mosasaur teeth with Remarks. Each haemal arch-spine complex is fused to the respective the tooth of M. prismaticus; at the same time, at least a set of teeth is vertebral body ventrally in HMG-10. The well-ossified left ulnare required from this taxon to augment its specific distinction. (identified as a right ulnare by Sakurai, 2008, but it is morpholog- Mosasaurus sp. ically nearly identical to the left ulnare of the Mosasaurus hobetsuensis holotype; Fig. 6A) of HMG 1077 assigns it to a mosasaurine Referred material. KSNHM-F6-10e15 (six teeth), 16 (left splenial), unequivocally. We agree with Sakurai (2008) that this specimen 17 (right articular), 18 (basisphenoid), 19e21 (three cervical could be referred to Mosasaurus hobetsuensis, but since the generic Author's personal copy

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Fig. 5. Representative mosasaurid specimens from Japan, Part I: material assignable to Mosasaurinae. A, B, HMG 1065, the holotype of Mosasaurus prismaticus Sakurai, Chitoku, and Shibuya, 1999 from Campanian or Maastrichtian Hakobuchi Group/Formation in Hokkaido. A, braincase in right lateral view. B, parietal in dorsal view. C, NFL 33, Mosasaurinae gen. et sp. indet. from the Campanian Upper Yezo Group in Hokkaido, occluded left maxilla and dentary in lateral view. D, HMG-10, Mosasaurinae gen. et sp. indet. from the Maas- trichtian Hakobuchi Formation in Hokkaido, anterior intermediate caudal vertebrae (note haemal arch-spine complex fused to centrum). E, F, HMG 1077, Mosasaurinae gen. et sp. indet. from the CampanianeMaastrichtian Hakobuchi Group/Formation in Hokkaido. E, ulnare. F, pterygoid teeth. identity of this taxon is uncertain, it is here only referred to the lacking obvious faceting or striation characteristic of halisaurine subfamily. Kimura et al. (1993b) assigned NFL 33 to Mosasauridae and russellosaurine dentition (e.g., Caldwell et al., 2008, ﬁg. 5; gen. et sp. indet. Here, we reassign it to a mosasaurine based on the Konishi and Caldwell, 2011; Fig. 5C). OMNH MV-0066 constitutes marginal teeth exhibiting a consistently smooth enamel surface, a single marginal tooth that is laterally highly compressed and Author's personal copy

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Fig. 6. Representative mosasaurid specimens from Japan, Part II: material assignable to Russellosaurina (AeE), and Mosasauridae (F). A, MCM.M0009, holotype of Taniwhasaurus mikasaensis Caldwell, Konishi, Obata, and Muramoto, 2008 from the Santonian/Campanian boundary interval, Kashima Formation in Hokkaido, skull in left lateral view. B, HMG 11, Plioplatecarpinae gen. et sp. indet. from the Coniacian or Santonian Upper Yezo Group, Hokkaido, snout portion in dorsal view. C, HMG 371, Tylosaurus? sp. from the Campanian Upper Yezo Group in Hokkaido, right jugal, partial right mandible, and marginal tooth in lateral view. D, E, IPMM 30881, Russellosaurina gen. et sp. indet. from the Santonian Kunitan Formation in Iwate, marginal tooth in D, medial and E, lateral views. F, KSNHM-F6-480, Mosasauridae gen. et sp. indet. from the Maastrichtian Mutsuo Formation in Osaka, posterior dorsal vertebra in anterior view. Scale bar shared between D and E. Author's personal copy

T. Sato et al. / Cretaceous Research 37 (2012) 319e340 329 posteriorly curved. It is considered mosasaurine due not only to the Referred material. HMG 371 (fragmentary skull elements including highly laterally compressed nature, but also to the smooth crown lower jaw, vertebra, phalanx; Fig. 5E): Sanushupe River, Hobetsu surface on both sides (the medial side shows broad, weak faceting district of Mukawa Town; Campanian, Upper Yezo Group. Chitoku but is otherwise smooth) (Kishimoto, 1991, fig. 12). KSNHM-F6-2 (1987, 1990, 1994), Kito and Chitoku (1991), Sakurai (2005, 2008). was originally reported as Kourisodon sp., but Konishi et al. (2010) tentatively re-assigned this specimen to Mosasaurus prismaticus Remarks. Based largely on the marginal dentition and phalangeal based on the laterally compressed and narrowly faceted tooth morphology, HMG 371 was previously referred to as Tylosaurus sp. crowns that characterize all the preserved, in situ marginal teeth of (Chitoku, 1990, 1994; Kito and Chitoku, 1991). Based on other the specimen. KSNHM-F6-3 constitutes a fragment of a deep anatomical features of the specimen such as a jugal and an angular, mosasaur jaw with four alveoli. Based mainly on the straight crown the Tylosaurus affinity was questioned by Caldwell and Konishi with its surface exhibiting anastomosing texture, Konishi et al. (in (2007). However, a concentration of distinct striations is present press) assigned it to a mosasaurine, and suggested that it prob- near the base of the medial crown surface in HMG 371, which is ably represented a Prognathodon-like form. Two private specimens a clear russellosaurine feature (e.g., Polcyn and Bell, 2005; Konishi listed above both belong to the subfamily as they each exhibit and Caldwell, 2007). Among russellosaurines, the marginal denti- haemal arch(es) fused to the centrum/centra. The section of the 11 tion of plioplatecarpines does not conform to that of HMG 371 articulated vertebrae represents the down-turned portion of the mainly because of its stout morphology and lack of a tight poster- caudal skeleton (Tanimoto, 2005, fig. 3.4; Lindgren et al., 2007). The omedial crown curvature. Among tylosaurines, the marginal tooth single intermediate caudal vertebra is one of the four vertebrae of HMG 371 lacks a coarse lateral fluting (grooving) structure unlike originally reported in Tanimoto et al. (1995) as Mosasauridae gen. et Taniwhasaurus spp. (e.g., Caldwell et al., 2005; Martin and sp. indet. (Obata et al., 2007; Tanimoto, pers. comm., 2012). While Fernández, 2007; Caldwell et al., 2008). In this light, Chitoku’s the foregoing character unequivocally assigns this vertebra to (1994) generic assignment of Tylosaurus to this specimen appears Mosasaurinae, identification of the remaining three elements reasonable, but we think that both non-dental characters and more below the family level has yet to be confirmed pending further comparative material from and beyond Japan need be examined preparation (Tanimoto pers. comm., 2012). more rigorously. Taking a cautious taxonomic approach, we tentatively refer HMG 371 to Tylosaurus sp. Russellosaurina Polcyn and Bell, 2005 Tylosaurinae Williston, 1897 Plioplatecarpinae Dollo, 1884 Taniwhasaurus Hector, 1874 Plioplatecarpinae gen. et sp. indet. Taniwhasaurus mikasaensis Caldwell et al., 2008 Fig. 6C Fig. 6B Referred material. HMG 11 [skull elements (anterior upper jaw and right angular), cervical vertebrae including atlas; Fig. 6C]: Hobetsu Holotype. MCM.M0009, partial, articulated skull and lower jaws River, Hobetsu district of Mukawa Town, Hokkaido; Coniacian or (Fig. 6B). Santonian, Upper Yezo Group. Suzuki (1985b), Chitoku (1987, 1990, 1994), Sakurai (2005, 2008). Type locality and horizon. “Kikumen” (sic) Creek near Mikasa City, IMCF uncatalogued specimen (tooth): Tsurubo area of Iwaki Hokkaido; Kashima Formation, Yezo Group; Santonian-Campanian City, Fukushima; Coniacian, Ashizawa Formation of Futaba Group. boundary interval, Upper Cretaceous (Caldwell et al., 2008). A. Sato in Iwaki City Board of Education and Kairyu-no-Sato Fossil Excavation Research Group (1991, 1992). Referred material. MCM.A600 (skull elements including partial jugal, postorbitofrontal, and quadrate): Haboro area, Hokkaido; Remarks. HMG 11 was referred to as Platecarpus sp. in Sakurai upper Santonianelower Campanian, Haborogawa Formation, Yezo (2005),butSakurai (2008) re-assigned the specimen to Pliopla- Group. Caldwell et al. (2008). tecarpinae? gen. et sp. indet., based on personal communications MCM.A1008 and M10 (vertebrae): Ikushunbetsu River, Mikasa with one of us (MWC) in April, 2006. The marginal tooth crown of City, Hokkaido; upper Santonianelower Campanian, Kashima this specimen is distinctly curved posteriorly from the mid- Formation, Yezo Group. Caldwell et al. (2008). height, bearing strong facets laterally and weak facets with fine striations medially, a set of characters that diagnoses plioplate- Remarks. The holotype specimen was nicknamed as “Ezo Mikasa carpines (e.g., Konishi and Caldwell, 2011). Combined with the Ryu” shortly after its discovery. All specimens of Taniwhasaurus dorsoventrally compressed articulation surface of the cervical mikasaensis known so far were collected from the Kashima and vertebra (Suzuki, 1985b, pl. 3), the plioplatecarpine affinity of Haborogawa formations of the Yezo Group. See Caldwell et al. HMG 11 is well corroborated. No other preserved elements (2008) for more details on the locality and anatomy of the speci- e mens listed above, and the diagnosis and comparison with two including the premaxilla maxillae complex of the specimen fi other species of Taniwhasaurus, i.e., T. oweni from New Zealand and con dently assign it to a known plioplatecarpine genus or diag- T. antarcticus from Antarctica. Because the genus Taniwhasaurus nose it as a new member of the subfamily; hence, we refer the was only known from middle Campanianelower Maastrichtian specimen to an indeterminate plioplatecarpine. The IMCF specimen mentioned by A. Sato is a pterygoid tooth with two trans- strata of the Southern Hemisphere prior to the discovery of the Japanese species, it was hypothesized to be endemic to the austral versely oriented carinae, characteristic of the plioplatecarpines (Konishi and Caldwell, 2011, p. 770). Weddellian Province (Martin and Fernández, 2007; Martin et al., 2007). Recognition of T. mikasaensis refuted this hypothesis, as Russellosaurina gen. et sp. indet. well as extended the range of the genus to cover the entire Cam- Fig. 6D, E panian (Caldwell et al., 2008). Referred material. NSM-PV15003 (caudal vertebrae): Abare Creek, Tylosaurus? sp. Hobetsu district of Mukawa Town, Hokkaido; Coniacian or Santo- Fig. 5E nian, Kashima Formation? Upper Yezo Group. Author's personal copy

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IPMM 30881 (marginal tooth; Fig. 6D, E): Motokizawa, Kuji City, IPMM 40041 (tooth): Maita Coast, Noda Village, Iwate; Santo- Iwate; Santonian, Kunitan Formation, Kuji Group. Kawakami et al. nian, Kunitan Formation, Kuji Group. Kawakami et al. (1985). (1985). NSM-PV22125 (vertebra): Irumazawa, Iwaki City, Fukushima; lower Santonian, Tamayama Formation, Futaba Group. Kawakami Remarks. The first-known Japanese mosasaur specimen NSM- et al. (1985). PV15003 consists of four caudal vertebrae including haemal KSNHM-F6-5 (marginal tooth base attached to tooth bearing arches, and their taxonomic referral bases on the presence of element): Sobura, Kaizuka City, Osaka; Maastrichtian, Azenotani haemapophyses, to which haemal arches articulate (Caldwell and mudstone bed of Mutsuo Formation, Izumi Group. Konishi, 2007). IPMM 30881 represents a moderately large tooth KSNHM-F6-6 (left angular, cervical rib, and vertebral centrum): crown (>3 cm high) that exhibits distal tapering, lateral fluting, and locality and horizon same as KSNHM-F6-5. medial striations (Fig. 6D, E). These conditions are consistent with KSNHM-F6-7 (rib fragment): locality and horizon same as russellosaurine dentition (e.g., Polcyn and Bell, 2005). KSNHM-F6-5. KSNHM-F6-8 (rib fragment): locality and horizon same as Russellosaurina? KSNHM-F6-5. KSNHM-F6-9 (rib fragment): locality and horizon same as Referred material. OMNH MV-0076 (pre-caudal vertebra): Nagata, KSNHM-F6-5. Minami-Awaji City, Awaji Island, Hyogo; late Campanian, Seidan KSNHM-F6-480 (posterior dorsal vertebra; Fig. 6F): Sobura, Formation, Izumi Group. Tanimoto and Miyamoto (1996), Tanimoto Kaizuka City, Osaka; Maastrichtian, Azenotani mudstone bed of (2005, 2007). Mutsuo Formation, Izumi Group. Takata (2004). Private collection in Tanimoto and Date (2000) (tooth): Sobura, KCU uncatalogued specimen (tooth): Mitsukawa Quarry in Kaizuka City, Osaka; Maastrichtian, Mutsuo Formation, Izumi Sumoto City, Awaji Island, Hyogo; Maastrichtian, “Formation D” Group. (¼ Kita-ama Formation), Izumi Group. Iwaki and Maeda (1989). Private collection in Tanimoto et al. (1998) (teeth): Sobura, Remarks. OMNH MV-0076 exhibits an elongate centrum and an Kaizuka City, Osaka; Mutsuo Formation, Izumi Group. Tanimoto anteroposteriorly narrow, posteriorly inclined neural spine. While et al. (1998), Tanimoto (2005). it was regarded as a caudal vertebra previously (Tanimoto and Private collection in Tanimoto and Ueno (2000) (cervical Miyamoto, 1996; Tanimoto, 2007), the elongate centrum, which vertebra): Sobura, Kaizuka City, Osaka; Maastrichtian, Azenotani is twice as long as high, the dorsoventrally deep transverse mudstone bed of Mutsuo Formation, Izumi Group. Tanimoto and processes, and their position high in the centrum, argue more Ueno (2000), Tanimoto (2005). favourably for the interpretation that this vertebra belonged to Private collection in Tanimoto and Nunoya (2000, 2003) (single a pre-caudal region. Among mosasaurs, plioplatecarpine Selma- marginal tooth): Showaike, Sennan City, Osaka; Maastrichtian, saurus spp. exhibit elongate cervical vertebrae with a slender Mutsuo Formation, Izumi Group. Tanimoto (2005). neural spine that slants strongly posteriorly, although their known Private collection in Tanimoto et al. (2001) (external mold of range is Santonianelower Campanian (Wright and Shannon, 1988; marginal tooth): Takinoike, Izumisano City, Osaka; Maastrichtian, Kiernan, 2002; Polcyn and Everhart, 2008). The referral of the Mutsuo Formation, Izumi Group. Tanimoto (2005). tooth from Kaizuka City, Osaka to this parafamily is at best Private collection in Tanimoto and Kishimoto (2002) (verte- tentative. Lacking the root, the tooth preserves most of its crown brae): near Orodani, Sumoto City, Awaji Island, Hyogo; Maas- and is approximately 30 mm in height. The crown is narrowly trichtian, Kita-ama Formation, Izumi group. faceted on both lateral and medial sides, which are divided by carinae that are positioned anteroposteriorly, features that occur Remarks.FirstreportedasMosasaurus(?) sp. by Kawakami et al. (1985), in plioplatecarpines. At the same time, the crown is less curved NSM-PV22125 was collected before 1964 (I. Obata, pers. comm., 2011), and does not taper distally as readily as in IPMM 30881, a russel- which makes it the first mosasaur fossil discovered in Japan losaurine. The latter features are characteristic of mosasaurine (cf. Caldwell and Konishi, 2007). The specimen consists of a large (ca. mosasaurs; hence, our referral of this tooth to Russellosaurina is 10 cm long), elongate procoelous centrum lacking various processes equivocal. post-mortem, and is thus only assignable to family level. KSNHM-F6- 5e9 were collected, along with the left dentary in Tanimoto and Yokoi Mosasauridae gen. et sp. indet. (2005) (see above), by the same collector in the same vicinity. Given Fig. 6F the consistent size range and no overlapping of these elements, these Referred material. HMG 1075 (caudal vertebrae): Sounushibetsu five specimens and the left dentary bone in Tanimoto and Yokoi River, Hobetsu district of Mukawa Town, Hokkaido; (2005) may pertain to the same individual of a mosasaur. If this is ConiacianeMaastrichtian, Hakobuchi or Upper Yezo Group. Sakurai the case, the five KSNHM specimens can be identified as Mosasaurus and Ono (2004), Sakurai (2005, 2008). sp., though this only remains possibility. The KCU specimen was HMG 1076 (caudal vertebrae): Hobetsu River, Hobetsu district of reported as a marine reptile tooth, but the well-developed carinae Mukawa Town, Hokkaido; CampanianeMaastrichtian, Hakobuchi indicate a mosasaurid affinity. Likewise, various other specimens in Group. Sakurai and Ono (2004), Sakurai (2005, 2008). the list can be only identifiable down to family level. The single HMG 370 (cervical vertebra): Chibeshinaisawa Creek in Mon- marginal tooth reported by Tanimoto and Nunoya (2000) was first betsu district of Hidaka Town, Hokkaido; Santonian, Upper Yezo identified to Mosasaurus sp., but Tanimoto and Nunoya (2003) Group. Chitoku (1987, 1990), Sakurai (2005, 2008). referred to it as cf. Platecarpus sp. The Maastrichtian age of the spec- HMC uncatalogued specimen (“Hidaka specimen #1” in Sakurai imen precludes it from being Platecarpus sp., however (Konishi and and Ono, 2004) (posterior skull elements, axis): Hidaka Town, Caldwell, 2011). While Tanimoto (2005) re-assigned it to Mosasaurus Hokkaido; Yezo Group? Sakurai and Ono (2004). sp., the tooth exhibits distinct striae that also characterize russello- HMC uncatalogued specimen (“Hidaka specimen #2” in Sakurai saurinedentition.Hence,weonlyassignittothefamilylevelhere, and Ono, 2004) (three dorsal vertebrae and rib fragment): Hidaka pending a future discovery of more complete specimens with the Town, Hokkaido; Yezo Group? Sakurai and Ono (2004). tooth morphology. Author's personal copy

Fig. 7. Representative specimens of Cretaceous plesiosaurs from Japan. AeD, Elasmosauridae. EeG,Polycotylidae.H,Pliosauroidea. A, skull of the holotype of the Futabasaurus suzukii,NSM PV15025, from the Santonian Tamayama Formation in Fukushima. B, anterior view and C, right lateral view of an indeterminate elasmosaurid cervical vertebra, IMCF temporary number 2482, from the Coniacian Ashizawa Formation in Fukushima. D, ventral view of the right coracoid of an indeterminate elasmosaurid, IMNHP 400.012, Tamayama FormationinFukushima.E,anterior view and F, ventral view of the anterior portion of the interclavicles of an indeterminate polycotylid, HMG 1079, CenomanianeTuronian Mikasa Formation, Hokkaido. G, possible polycotylid tooth, IMCF temporary number 2477, from the Coniacian Ashizawa Formation in Fukushima. H, indeterminate pliosauroid tooth, NSM 15006, from the Cenomanian Mikasa Formation in Hokkaido. Author's personal copy

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4.3. Plesiosauria Hokkaido; Maastrichtian, Hakobuchi Formation, Yezo Group. Ogawa and Nakaya (1998). Sauropterygia Owen, 1840 NSMPV 15004 (tooth): Urakawa area, Hokkaido; Campanian, Plesiosauria de Blainville, 1835 Upper Yezo Group. Obata et al. (1972). Elasmosauridae Cope, 1869 OM 1: (partial skeleton): Obirashibe River, Obira Town, Hok- Futabasaurus Sato et al., 2006a kaido; Santonian, Upper Yezo Group. Nakatani (2009), Nakatani and Nakaya (2008). Futabasaurus suzukii Sato et al., 2006a Specimen in Tokunaga and Shimizu (1926) (vertebra, lost during Fig. 7A WWII): Shinyashiki, Iwaki City, Fukushima. Santonian, Tamayama Formation? Futaba Group. Tokunaga and Shimizu (1926). Holotype. NSM PV15025, partial skeleton lacking posterior portion Private collection in Matsumoto et al. (1982) currently deposited of the skull, scapulae and coracoids, and most of the tail (Fig. 7A). at GPF (vertebrae, ribs, limb bones): Obirashibe (“Obirashibetsu” in Matsumoto et al., 1982) River, Obira Town, Hokkaido; Lower Santo- Type locality and horizon. Irumazawa, Iwaki City, Fukushima; nian, Upper Yezo Group. “Obira specimen no.1” in Kanie et al. (1998). Santonian, Tamayama Formation, Futaba Group. Private collection mentioned in Obata et al. (1989) (partial skeleton): Chenaibo, Wakkanai City, Hokkaido; Cenomanian, Referred material. None. Middle Yezo Group. Obata et al. (1989).

Remarks. The holotype specimen was discovered in 1968 (Obata Remarks. HMG 1, also known as “Hobetsu Araki Ryu” or “Hoppy”,is et al., 1970) and has been known to the Japanese public as the first fully described plesiosaurian skeleton from Japan (Nakaya, “Futaba Suzuki Ryu”. Sato et al. (2006a) described this specimen 1989a, b). The specimen includes a considerable number of and established a new genus and species. Despite the large number elements, but the lack of the skull and damage to girdle and limb of plesiosaurian specimens known from Japan, the holotype of bones do not allow identification at lower taxonomic levels. HMG F. suzukii is so far the only specimen that can be clearly identified to 4 includes a proximal half (femur, epipodials, tarsals) of a partially the species level, and is also the oldest elasmosaurid taxon known articulated hind limb. from the northern Pacific. A preliminary phylogenetic analysis (Sato The indeterminate elasmosaurid specimens from the Futaba et al., 2006b) recovered F. suzukii as a relatively basal member of the Group listed here are mentioned in government reports by the Elasmosauridae, but the support was low and the branching Iwaki City Board of Education. The IMNHP specimens were pattern was sensitive to the selection of OTUs. Over 80 shark teeth collected during a geological survey of the area near the locality of were associated with the holotype skeleton, including the broken the Futabasaurus holotype, at approximately the same horizon. The teeth embedded in the bones, demonstrating scavenging/predation IMCF specimen is an isolated cervical that came from a different by several individuals of the shark Cretalamna appendiculata locality and a lower horizon than the IMNHP specimens. (Shimada et al., 2010). The uncatalogued KGU specimen includes the mandible and anterior cervicals, and is currently under preparation (Nakaya et al., Elasmosauridae indet. 2006). This is the first Cretaceous marine reptile to be reported Fig. 7BeD from Kyushu, and its early Cenomanian age (based on the co-occurring ammonite Graysonites) makes it one of the oldest Referred material. HMG 1 (partial postcranial skeleton): a tributary occurrences of elasmosaurids in Japan. of Sanushube River, Hobetsu district of Mukawa Town, Hokkaido; Mochizuki et al. (2006) distinguished two elasmosaurid indi- Campanian, Upper Yezo Group. Nakaya and Hobetsu Plesiosaurian viduals in the 320 bones of NMV 1 based on the size and overlap of Fossil Excavation Research Group (1981), Nakaya (1989a, b). some of them. Ogawa and Nakaya (1998) described NMV 2 which HMG 4 (ribs, gastroliths, limb): Azumi, Hobetsu district of consists of the occipital condyle, a dentary, teeth, vertebrae, ribs, Mukawa Town, Hokkaido; Upper Yezo Group. Sakurai (2005). partial pectoral girdle elements, limb elements, and gastralia. IMCF temporary number 2482 (vertebra; Fig. 7B, C): Tsurubo, Much of the non-vertebral bones of this specimen were found Iwaki City, Fukushima; Coniacian, Ashizawa Formation, Futaba disarticulated. Based on the morphology of the vertebrae, scapula, Group. A. Sato in Iwaki City Board of Education and Kairyu-no-Sato and humerus, Ogawa and Nakaya (1998) considered that it is Research Group (1992). similar to Morenosaurus stocki Welles, 1943 among known IMNHP 400.002, 003, 004 (vertebrae): Irumazawa, Iwaki City, elasmosaurids. Hikida (1998) compared the sphericity of the Fukushima; Santonian, Tamayama Formation, Futaba Group. Suzuki gastroliths associated with the two NMV specimens and sug- and Hasegawa in Iwaki City Board of Education (1988). gested that those with NMV 1 came from nearshore environ- IMNHP 400.008, 012 (coracoids; Fig. 7D): Irumazawa, Iwaki City, ments, whereas those with NMV 2 originated in river (estuarine) Fukushima; Santonian, Tamayama Formation, Futaba Group. Suzuki environments. and Hasegawa in Iwaki City Board of Education (1988). NSM PV15004 is an isolated tooth, and Obata et al. (1972), IMNHP 400.011 (femur): Irumazawa, Iwaki City, Fukushima; referring it to Elasmosaurus (?) sp., noted its morphological simi- Santonian, Tamayama Formation, Futaba Group. Suzuki and Hase- larity to the teeth of Futabasaurus. gawa in Iwaki City Board of Education (1988). OM 1 is composed of fragmentary skull elements including KGU uncatalogued specimen in Nakaya et al. (2006) (mandible a partial braincase, as well as a considerable portion of the post- and cervical vertebrae): Shishi Island, Nagashima Town, Kagosh- cranial skeleton, that is currently under study by D. Nakatani at ima; Cenomanian, Hegushi Formation, Goshoura Group. Nakaya Kagoshima University. Nakatani (2009) and Nakatani and Nakaya et al. (2006). (2009) carried out a preliminary cladistic analysis including OM NMV 1 (two individuals): Hidarinosawa River, Nakagawa Town, 1 in which they found it to be nested within the Elasmosauridae. Hokkaido; Coniacian, Upper Yezo Group. Mochizuki et al. (2006). Obata et al. (1989) and Obata (1993) reported on the occurrence NMV 2 (fragmentary skull elements and partial postcranial of the partial postcranial skeleton of a juvenile elasmosaur from the skeleton): Suminosawa of Abeshinai River, Nakagawa Town, Cenomanian, along with a detailed biostratigraphic study of the Author's personal copy

T. Sato et al. / Cretaceous Research 37 (2012) 319e340 333 fossil-yielding section. The specimen represents a mostly articu- Formation, Yezo Group. Echizenya and Minoura (2005), Sakurai lated torso, and includes girdle and limb bones. (2005). The cervical centrum reported in Tokunaga and Shimizu (1926) SMAC 2872 (ilium and limb fragments): Kamikinenbetsu River, as Plesiosaurus sp. was unfortunately lost during the WWII, but its Obira Town, Hokkaido; TuronianeSantonian, Saku or Haborogawa peculiar morphology with a slight ventral notch and proportion Formation, Upper Yezo Group. Tahara et al. (in press). suggest that it belongs to a juvenile elasmosaurid. UMUT MV 19965 (partial postcranial skeleton): Obirashibe The private specimens referred to in Matsumoto et al. (1982) River, Obira Town, Hokkaido; Cenomanian, Middle Yezo Group. Ta. and Kanie et al. (1998, listed in “Elasmosauridae?” below) include Sato (1995), Sato and Storrs (2000). cephalopod jaw apparatuses interpreted as stomach contents. Some of the apparatuses were separated from the elasmosaurid Remarks. HMG 357 is fragmentary but includes a characteristic bones and registered at the Yokosuka City Museum, Kanagawa clavicular arch of the Polycotylidae (Sato and Nishimura, 2012), (including the holotype YCM-GP 693; see Kanie, 1998, for a list of whereas HMG 1067 includes a considerable number of partly paratype and referred specimens), and Kanie (1998) established articulated bones and is currently under study by one of us (TS). a new genus and species of the vampyromorph coleoid Pro- HMG 1079 consists of fragmentary remains of a juvenile individual, vampyroteuthis giganteus. and was described in detail by Echizenya and Minoura (2005) who identified the specimen as an indeterminate polycotylid based on Elasmosauridae? the vertebral proportion and morphology of the interclavicle. SMAC 2872 was collected as a float; the exact horizon from Referred material. HMG 352 (gastralia): Sanushube River, Hobetsu which it was derived cannot be determined. Our identification is district of Mukawa Town, Hokkaido; Upper Yezo Group. Sakurai tentative and primarily based on the morphology of the damaged (2005). ilium and tibia (or radius), which is comparable to the UMUT pol- HMG 354 (fragmentary postcranial skeleton): Hobetsu River, ycotylid (Tahara et al., in press). Hobetsu district of Mukawa Town, Hokkaido; Upper Yezo Group. UMUT MV 19965 is the first polycotylid to be described from East Sakurai (2005). Asia (Sato and Storrs, 2000); note that this specimen was originally HMG 1194 (girdle bone?): ?Hobetsu district of Mukawa Town, reported as a pliosauroid sensu Brown (1981) (Ta. Sato, 1995). Hokkaido; Yezo Group. Sakurai (2005). Sato and Storrs (2000) noted potentially new characters of this HPMHN D1-5386 (isolated tooth): Nandan, Minami-Awaji City, specimen but put off establishing a new taxon because the lack of so Awaji Island, Hyogo; Maastrichtian, Shimonada Formation, Izumi many skeletal parts causes difficulties in effectively diagnosing it. Group. Ts. Sato (1995). The ammonite jaw apparatuses found in this specimen presented the UHR 33109 (vertebrae, ribs, ilium): Shirochiunezawa Creek, first direct evidence of the diet of the polycotylids (Sato and Tanabe, Haboro, Hokkaido; Coniacian, Lower Haborogawa Formation 1998). A phylogenetic analysis by Albright et al. (2007) found the (Okamoto et al., 2003), Upper Yezo Group. Kaim et al. (2008), UMUT specimen in a sister-taxon relationship with Palmulasaurus, Echizenya (2010). but it was involved in a major polytomy near the base of the Poly- Private collection (“Obira specimen no. 2”)inKanie et al. (1998) cotylidae in the revised analysis of O’Keefe (2008). currently deposited at GPM (vertebrae, ribs, girdle bone): Shimo- kinenbetsu River, Obira Town, Hokkaido; Lower Santonian, Upper Polycotylidae? Yezo Group? Fig. 7G Private collection in Tanimoto and Kanazawa (2001) (tooth): Kanewari, Sanuki City, Kagawa; Campanian, Hiketa Formation? Referred material. HMG 355 (fragmentary postcranial): Nagawa, Izumi Group. Tanimoto and Kanazawa (2001). Hobetsu district of Mukawa Town, Hokkaido; Middle Yezo Group. Private collection in Tanimoto and Ogawa (2002) (humerus): Sakurai (2005). Nandan, Minami-Awaji City, Awaji Island, Hyogo; Maastrichtian, IMCF temporary number 2871 (vertebra): Irumazawa River, Shimonada Formation, Izumi Group. Iwaki City, Fukushima; Santonian, Tamayama Formation, Futaba Group. Personal observation (TS). Remarks. All specimens are too fragmentary for confident identifi- IMCF temporary number 2477 (tooth: Fig. 7G): Tsurubo, Iwaki cation. The HPMNH specimens and two private collections from the City, Fukushima; Coniacian, Ashizawa Formation, Futaba Group. Izumi Group were reported as plesiosauroids; they may represent Iwaki City Board of Education and Kairyu-no-Sato Fossil Excavation elasmosaurids but are too fragmentary to exclude other possibili- Research Group (1990). ties. Echizenya (2010) suggested an elasmosaurid affinity for the KGU uncatalogued specimen (femur): Shirotori, Higashi-kagawa UHR specimen based on the morphology of the ilium. City, Kagawa; Campanian, Izumi Group. Nakatani and Nakaya (2010).

Polycotylidae Cope, 1868 Remarks. There is a peculiar perforation in the coracoid(?) of the HMG specimen, which is seen among certain polycotylids such as Polycotylidae indet. Dolichorhynchops, but the fragmentary nature of the specimen Fig. 7E, F creates uncertainty. The overall morphology of the IMCF vertebra and the slenderness and striation pattern of the IMCF tooth may Referred material. HMG 357 (vertebrae and fragmentary girdle suggest a polycotylid afﬁnity for these specimens. The identiﬁca- bones): Tappu, Obira Town, Hokkaido; Middle Yezo Group? Sakurai tion of the KGU specimen by Nakatani and Nakaya (2010) is based (2005), Sato and Nishimura (2012). on the slenderness of the femur, which is characteristic of short- HMG 1067 (teeth, partial postcranial skeleton): Nagawa, necked plesiosaurs, and they assumed that the Polycotylidae is Hobetsu district of Mukawa Town, Hokkaido; Upper Yezo Group? the only short-necked plesiosaur to survive into Campanian. Sakurai (2005). HMG 1079 (vertebrae, interclavicles; Fig. 7E, F): Ikushunbetsu Pliosauroidea indet. River, Mikasa City, Hokkaido; CenomanianeTuronian, Mikasa Fig. 7H Author's personal copy

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Referred material. UHR 33108 (partial postcranial skeleton): Shir- Shikama (1963) described UHR 32184, consisting of four partial ochiunezawa Creek, Haboro Town, Hokkaido; Turonian, Shirochi centra, as the holotype of an ichthyosaur “Miopterygius (?) yezoensis Formation (Okamoto et al., 2003), Middle Yezo Group. Kaim et al., sp. nov.”. The centra bear a pair of foramina subcentralia charac- (2008), Echizenya (2011). teristic of the plesiosaurs. NSM PV15006 (tooth; Fig. 7H): Katsurazawa, Mikasa City, Hok- Terui and Nagahama (1995: 239) mentioned the occurrence of kaido; Cenomanian, Mikasa Formation, Middle Yezo Group. Obata plesiosaurian fossil(s) from a lower portion of the Kunitan Forma- et al. (1972). tion, but no further information (e.g., number of specimen(s), repository) was provided. The report is the only published account Remarks. Use of the term “Pliosauroidea” requires additional on the plesiosaurs from this formation. explanation to clarify the taxonomic meaning. Our reference of Japanese specimens to the Pliosauroidea is exclusively based on 5. Discussion their similarity to the taxon specified herein and consistently referred to as a member of the Pliosauroidea in the studies So far, all confirmed cases of Cretaceous marine reptiles from mentioned below. Traditional classifications included the poly- Japan are from Cenomanian and younger strata. Azuma (1995) cotylid plesiosaurs within the Pliosauroidea, but recent phyloge- reported on a few possible plesiosaurian teeth from the Lower netic studies have given conflicting results: Druckenmiller and Cretaceous Kitadani Formation in Fukui Prefecture, but a close Russell (2008) supported the monophyly of short-necked plesio- examination suggests that they are more likely to be crocodilian saurs (i.e., traditional members of the Pliosauroidea), whereas the teeth (Kobayashi, pers. comm., 2011). There are a few other analyses of O’Keefe (2001, 2004) and Ketchum and Benson (2010) occurrences of possible plesiosaurian teeth from (possible) Lower recognized their polyphyly, i.e., the Polycotylidae among long- Cretaceous deposits (e.g., Tanimoto and Kaede, 1998; Toyama necked forms (Plesiosauroidea) and the Pliosauroidea consisting Prefecture Board of Education, 2003; TUM 38469), but unfortu- of the rest of the short-necked forms. nately they are not diagnostic enough to confirm their plesio- Obata et al. (1972) noted the similar striation patterns in the saurian affinity and/or the stratigraphic level from which they come teeth of Polyptychodon and NSM PV15006, and identified the remains undetermined. latter as Polyptychodon sp. cf. P. interruptus Owen 1841. Echizenya Taxonomic diversity of the marine reptiles is highest in the (2011) regarded the UHR specimen as possibly referable to Poly- Cretaceous sediments on Hokkaido. In general, the taxonomic ptychodon interruptus based on the pattern of tooth striation and content of marine reptilian fauna in a geological unit elsewhere in Kaim et al. (2008) examined the chemosynthetic gastropods Japan can be explained as a subset of those in coeval strata in associated with this specimen. The taxonomic status of the genus Hokkaido (Table 1). The difference in diversity is, however, prob- Polyptychodon, however, remains unclear. Welles (1962) treated ably not “real”; it is more likely to reflect the large number of all species of this genus, including P. interruptus,as“nomina specimens that have been accumulated during a long history of dubia”, but soon established a new species of this genus, P. hud- intensive collecting and the continuous curatorial work of local soni Welles and Slaughter, 1963. A few later studies (Storrs, 1981; museums. We are aware of the presence of material that is yet to be Carpenter, 1996) treated the genus and/or P. hudsoni as a valid studied and not mentioned in this contribution, both from Hok- pliosauroid taxon, but Welles’ (1962) claim was never seriously kaido and elsewhere. Further study of these specimens and future refuted and Polyptychodon was never included in phylogenetic discoveries will certainly contribute to a better understanding of studies. In addition to the uncertainty regarding the taxonomic Japanese Cretaceous marine vertebrates. For now, summarized status of Polyptychodon, little is known about the variation below are the characteristics of each of the three groups reviewed in tooth morphology among pliosauroid taxa. It is, therefore, here, based on the currently available information. reasonable to regard these specimens as indeterminate pliosauroids. 5.1. Chelonioidea

Plesiosauria indet. The oldest confirmed occurrences of Cretaceous chelonioids from Japan are the protostegid Desmatochelys and an indeterminate Referred material. There are numerous indeterminate plesiosaurian chelonioid from the Turonian. The two are late occurrences when fossils (e.g., Obata et al., 1972, 2007; Sakurai, 2005), and it is not compared with the fossil records of other protostegids or chelo- practical to list them all. The specimens noted below are of nioids as a whole, which appeared and diversified during the late historical or geographical significance. Early Cretaceous (Hirayama, 1997a, b, 1998a, b; Kear and Lee, 2006; NFL 34 (tooth, vertebrae, ischium): Shisennosawa River, Numata Tong et al., 2006). Records of marine macrofossils are generally Town, Hokkaido; Campanian, Upper Yezo Group. Kimura et al. poor in Japanese Lower Cretaceous sediments, and we consider the (1993b). paucity of adequate sediments to be partly responsible for the UHR 32184 (vertebrae): Yubari area, Hokkaido; horizon absence of earlier chelonioids, although the lack of Cenomanian unknown. material is puzzling when compared with the Cenomanian records Specimen(s?) mentioned in Terui and Nagahama (1995): locality of plesiosaurian fossils. Chelonioids are known from all stages of unknown; Kunitan Formation, Kuji Group. the rest of the period, and remains of the dermochelyid Meso- dermochelys dominate the chelonioid fauna of the latest Remarks. Kimura et al. (1993b) identified the NFL specimen as Cretaceous. a plesiosauroid based on the tooth characteristics and relative Many living taxa of sea turtles are cosmopolitan in their distri- length of the cervical centra, but available information is not bution (Caretta, Chelonia, Eretmochelys, Lepidochelys and Der- sufficient for identification at a lower taxonomic level. In addition, mochelys; Zangerl et al., 1988; Iverson, 1992; Hirayama, 1997a, b). the taxonomic content of the Plesiosauroidea has changed This is also the case in Tertiary chelonioids such as Syllomus considerably in recent phylogenetic studies (e.g., O’Keefe, 2001; (MioceneePliocene, Cheloniidae) and Psephophorus (Eocenee Druckenmiller and Russell, 2008; Ketchum and Benson, 2010). For Pliocene, Dermochelyidae). During the Cretaceous, however, the these reasons, we classify this specimen as an indeterminate provincialism among the Late Cretaceous chelonioids was plesiosaur. pronounced (Hirayama, 2006b). Cheloniids dominate the fossil Author's personal copy

T. Sato et al. / Cretaceous Research 37 (2012) 319e340 335 records of marine turtles in North America and Europe (e.g., Parham is largely congruent with what has been recognized globally during and Stidham, 1999; Mulder, 2003; Lehman and Tomlinson, 2004). In the same time period with respect to mosasaur diversity. Japan, however, the primitive dermochelyid Mesodermochelys is the In the Western Interior of North America and circum- most common fossil sea turtle in the SantonianeMaastrichtian Mediterranean regions (Europe and North Africa), various russel- (Hirayama and Chitoku, 1996; Hirayama 1997a, b; Hirayama and losaurine genera are known from the ConiacianeSantonian (e.g., Hikida, 1998), but very few protostegids coexisted, and no Creta- Ectenosaurus, Plesioplatecarpus, Platecarpus, Selmasaurus, Tylosau- ceous cheloniid material has been reported. Taxonomic components rus, Hainosaurus), while only one mosasaurine genus (Clidastes)is of Maastrichtian chelonioid faunas differ considerably even between known from this period (e.g., Lingham-Soliar, 1992; Everhart, 2001; the eastern coast of North America (represented by the cheloniids Polcyn and Everhart, 2008; Konishi and Caldwell, 2011; but see Osteopygis and Peritresius and the protostegid Atlantochelys)and Bullard and Caldwell, 2010). Throughout the Campanian however, Western Europe (represented by cheloniids Allopleuron and Glypto- mosasaurine mosasaurs began diversifying at the generic level chelone)asnotedbyHirayama (1997a, b). while Clidastes persisted into the late Campanian (Diedrich and Mulder, 2004). On the other hand, generic-level russellosaurine diversity began to decline in the middle Campanian and remained 5.2. Mosasauridae low thereafter (e.g., Kiernan, 2002). In the Maastrichtian, the mosasaurine genera that had evolved during the Campanian Thus far, nearly 40 mosasaur specimens from Japan have been persisted and further speciated (i.e., Globidens, Prognathodon, reported in the literature. Most come from the Yezo Group Mosasaurus) and new genera arose (e.g., Carinodens), whereas outcropping in Hokkaido and the Izumi Group in western Japan. Plioplatecarpus and Hainosaurus constituted the sole russellosaur- Tanimoto (2005) noted that all the known mosasaur specimens ine genera (e.g., Russell, 1967; Welles and Gregg, 1971; Lingham- from the upper CampanianeMaastrichtian Izumi Group repre- Soliar, 1992, 1999; Christiansen and Bonde, 2002; Dortangs et al., sented mosasaurine mosasaurs or were Mosasauridae gen. et sp. 2002; Bardet et al., 2005; Schulp et al., 2008). In the Maas- indet. Our taxonomic review of the mosasaur specimens from Japan trichtian, mosasaurine mosasaurs also appear to have expanded supports Tanimoto’s (2005) conclusion, but our survey additionally their geographic distribution; both Mosasaurus and Prognathodon demonstrates that all the mosasaurine mosasaurs from Japan are are known from New Zealand [Welles and Gregg, 1971; see restricted to the (late) Campanian and Maastrichtian. Conversely, Caldwell et al., 2005 for synonymization of Tylosaurus haumuriensis temporal occurrences of russellosaurine mosasaurs are limited to (Hector, 1874) with Taniwhasaurus oweni Hector, 1874], and less older Coniacian and Santonian rocks except for HMG 371 (Tylo- unequivocally, Mosasaurus from Japan (e.g., Suzuki, 1985a, b and saurus? sp.) and the specimens of recently described Taniwhasaurus herein). mikasaensis, although the youngest possible age for T. mikasaensis is In summary, our comprehensive review of mosasaur taxa from earliest Campanian and the formations that yielded these speci- Japan provides: (1) further support for the world-wide trend of an mens are predominantly ConiacianeSantonian in age (Takashima increasingly mosasaurine-dominated post-Santonian mosasaur et al., 2004; Caldwell et al., 2008). Fig. 8 shows these observa- fauna; and (2) the ﬁrst report of supergeneric-level compositional tions, and demonstrates that this pattern of temporal distribution changes through time in mosasaurs that inhabited the north- in mosasaurs across the last four stages of the Late Cretaceous Japan western Paciﬁc.

Fig. 8. Temporal distribution of mosasaur specimens from Japan across the last four stages in the Late Cretaceous (ConiacianeMaastrichtian). A, Mosasaurinae. B, Russellosaurina. Total sample size ¼ 20. No taxonomically equivocal specimens are included in this analysis. Of the 20 specimens, four (20%) are in private collections. Where the estimated age of a given mosasaur specimen ranges across two adjacent ages, the older of the two was selected. The majority of the russellosaurine specimens were derived from Con- iacianeSantonian strata (as for Taniwhasaurus mikasaensis, see main text), whereas all of the mosasaurine specimens are from CampanianeMaastrichtian strata. See main text for discussion of the patterns. Author's personal copy

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5.3. Plesiosauria pliosauroids in the northern Pacific from the previous record of Albian (Haggart et al., 2003) to Turonian, and it is concordant with The Plesiosauria have the widest stratigraphic distribution of the latest record of the pliosauroid genus (Brachauchenius from the the three reviewed groups, ranging from the Cenomanian to USA; Ketchum and Benson, 2010 and references therein). Note that Maastrichtian, and material referred to the Elasmosauridae, Pol- the short-necked plesiosaurs are known from the ycotylidae, and Pliosauroidea is documented. No specimens CampanianeMaastrichtian of New Zealand (Wiffen and Moisley, referable to the aberrant plesiosaurian taxa from southern high 1986) as well, but they were identified according to a traditional latitudes, such as Aristonectes and Kaiwhekea, whose relationship classification (e.g., Brown, 1981) in which polycotylids are not with other plesiosaurs are debated (e.g., Chatterjee and Small, distinguished from the Pliosauroidea as in more recent studies. In 1989; Gasparini et al., 2003; O’Keefe and Street, 2009; Ketchum addition to the clarification of the Pliosauroidea as a taxon, re- and Benson, 2010), have been reported from the Japanese Creta- evaluation of the taxonomic status of Polyptychodon and the ceous. Japanese plesiosaurian fossils are usually too poorly “short-necked” plesiosaurian fossils from New Zealand is necessary preserved for identification at lower taxonomic levels, but their to determine the significance of the Japanese pliosauroids. stratigraphic occurrences are well-documented and often dated to a stage. Early Cretaceous plesiosaurs are known from the Amer- 6. Conclusions icas and Australia (e.g., Welles, 1962; Kear, 2003; Druckenmiller and Russell, 2008, 2009) and the lack of their definite record in The taxonomy and stratigraphic distribution of nearly all spec- Japan is rather conspicuous, but the paucity of adequate sedi- imens of Cretaceous marine reptiles with published records are ments may be the main reason for this. The presence of possible reviewed. Chelonioid, mosasaurid, and plesiosaurian fossils are Early Cretaceous material mentioned above calls for further known from the Yezo Group (Supergroup) in Hokkaido, Kuji Group search. in Iwate, Futaba Group in Fukushima, Izumi Group in Osaka, Material referred to the Elasmosauridae are known from all Wakayama, Hyogo, and Kagawa, and/or Goshoura Group in stages and most geological units reviewed in this study, suggesting Kagoshima, ranging from the Cenomanian to the Maastrichtian. their relative abundance throughout the Late Cretaceous in the Probable owing to the large number of specimens that have been northwestern Pacific. Elasmosaurid fossils are known from the recovered, the taxonomic diversity of marine reptiles is highest in Cretaceous in various parts of the world, and their presence in the Hokkaido; those in other units are basically subsets of those of Japanese Cretaceous is not surprising. A closer look at the strati- equivalent strata in Hokkaido. There are occurrences of possible graphic range of known elasmosaurid faunas on a regional scale, plesiosaurian teeth in the Lower Cretaceous, but they are too however, reveals the importance of the Japanese record as a proof fragmentary for confident identification and/or the stratigraphic of their continuous presence in the northwestern Pacific information pertaining to them is inadequate. throughout the Late Cretaceous. Published accounts on elasmo- The dermochelyid Mesodermochelys undulatus and protostegid saurid fossils from other areas in the northern Pacific are limited in Desmatochelys are known, along with less diagnostic chelonioid the number of specimens recovered and their stratigraphic distri- material. Mesodermochelys undulatus dominates the Japanese che- bution (Maastrichtian of California in Welles, 1943; Santonian of lonioid fauna in the uppermost Cretaceous, whereas remains of British Columbia in Nicholls and Meckert, 2002). The Japanese Cretaceous dermochelyids are very rare in the rest of the world. Futabasaurus suzukii is the oldest elasmosaurid material identifi- Comparison between the taxonomic composition of the chelonioid able at species level in the northern Pacific(Sato et al., 2006a). faunas in Japan, Europe, and North America suggest a restricted Cretaceous elasmosaurids have been reported from Australia but geographical distribution of chelonioid species during the Late their stratigraphic range is limited to the middle Cretaceous Cretaceous. (AptianeTuronian; Kear, 2003, 2006), whereas remains from New Most mosasaurid specimens come from the Yezo Group in Zealand, Chile, and Antarctic are of the Santonian or later age Hokkaido and the Izumi Group in western Japan. Our review (Chatterjee and Small, 1989; Hiller et al., 2005; Otero et al., 2009). supports Tanimoto’s (2005) earlier notion of the dominance of There are many fewer specimens of polycotylids than elasmo- mosasaurine materials in the collection from the upper saurids in Japan, and their confirmed stratigraphic occurrences are CampanianeMaastrichtian, whereas russellosaurine mosasaurs are limited to the lower part of the Upper Cretaceous (Cenomanian and limited to older strata (Coniacianeearly Campanian). Similar Turonian). This also contrasts with the Western Interior Seaway of patterns are known in the Western Interior of North America and North America where the stratigraphic distribution of the Poly- the circum-Mediterranean regions. Therefore, the Japanese mosa- cotylidae ranges from the Albian to the CampanianeMaastrichtian saurid records support the world-wide trend of an increasingly (Sato, 2005; Druckenmiller and Russell, 2009), and polycotylid mosasaurine-dominated post-Santonian fauna, at the same time remains outnumber those of the elasmosaurids in certain local revealing suprageneric-level compositional changes through time plesiosaurian faunas (Nicholls and Russell, 1990). Nevertheless, it is in the northwestern Pacific for the first time. obviously too early to conclude that their absence from or paucity The plesiosaurian record ranges throughout the Upper Creta- in younger strata in Japan, because polycotylid material (including ceous. Remains of elasmosaurids are the commonest and known possible remains) are known from the Coniacian, Santonian, and from all stages and almost all geological units reviewed in this Campanian stages. The relative abundance of the polycotylids has study. Although most of them are not diagnostic at lower taxo- been used to characterize regional plesiosaurian faunas in North nomic levels, collectively they demonstrate a continuous presence America (e.g., Nicholls and Russell, 1990; Nicholls and Meckert, of the Elasmosauridae in the northwestern Pacific during the Late 2002), but few other regions in the world have comparable data. Cretaceous. In contrast, known occurrences of polycotylids are Further research on Japanese polycotylids will provide new limited both in number and stratigraphic distribution biogeographical perspectives on the evolution of the Polycotylidae. (CenomanianeTuronian for those with confirmed stratigraphy). Pliosauroid remains from the Japanese Cretaceous are rare and Polycotylids are more common and have a longer range in the very fragmentary, and their identification as Pleiosauroidea is Western Interior Seaway, although the small sample size of Japa- typological owing to the continuing lack of consensus regarding the nese remains and occurrences of possibly younger specimens taxonomic content of the Pliosauroidea as a group. Nevertheless, caution against a firm conclusion. The pliosauroid specimens are Japanese specimens widen upwards the stratigraphic range of the even fewer, but the Turonian specimen raises the upper limit of the Author's personal copy

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