Yoichi Azuma & Yukimitsu Tomida. Encyclopedia of Dinosaurs. Editor: Philip J Currie & Kevin Padian. Amsterdam: Academic Press, 1997.
Research History
A hadrosaur found in southern Sakhalin in 1935 and described as Nipponosaurus sachalinensis (Nagao, 1936) was the first dinosaur ever found in Japan. However, Sakhalin became a part of the USSR after World War II, and no further dinosaur fossils were found in the current Japanese territory for more than 30 years. In 1978, a fragment of a sauropod humerus was found in the Upper Cretaceous of Iwate Prefecture; this became the first dinosaur fossil found within the current Japanese territory (Hasegawa et al., 1991). Since then, a number of dinosaur body and footprint fossils have been found in the Cretaceous at various localities in Japan. These discoveries were made in part by children and amateur collectors, but in recent years prefectural museums, universities, and local education boards began to organize expeditions, and dinosaur fossil material has been accumulated with accelerated speed.
The Tetori Group, which ranges from the Late Jurassic to the Early Cretaceous of central Japan, has produced most of the dinosaur fossils found in Japan to date. For about the past 10 years the Fukui Prefectural Museum, Gifu Prefectural Museum, and the Shiramine-mura Education Board have been organizing expeditions to the Cretaceous part of the Tetori Group and have collected many dinosaur specimens. A small expedition to the Cretaceous Mifune Group, distributed in west-central Kyushu, produced some theropods and ornithischians and further finds of dinosaur fossils seem promising.
Dinosaur Fossils and Their Horizons
Dinosaur fossils are from 14 different localities or areas in Japan. They are explained briefly from north to south.
Iwate Pref., Iwaizumi-cho; Miyako Group (Late Aptian to Early Albian)
A fragmentary humerus of a sauropod was found. Its Japanese name is “Moshiryu.” It was once questionably identified as Mamenchisaurus sp. (Diplodocidae) (Hasegawa et al., 1991). However, the recent discovery of M. sinocanadorumfrom Xinjiang, China, revealed that Mamenchisaurus is not a member of Diplodocidae (Russell and Zheng, 1993). We believe that the fragmentary nature of the specimen does not hold any characters to identify it at a generic level or probably at the family level either. It is probably best identified as Sauropoda indet.
Fukushima Pref., Iwaki City and Hirono-cho; Futaba Group (Coniacian to Santonian)
A hadrosaur tooth, an ornithischian cervical vertebra, a theropod tibia (Hasegawa et al., 1987), and a sauropod tooth have been found.
Gumma Pref., Nakazato-mura; Sebayashi Fm. (Aptian)
A theropod cervical vertebra (Hasegawa et al., 1984) and a number of dinosaur footprints (Matsukawa and Obata, 1985) have been reported. The “footprints” are about 50 imprints on a bedding plane with ripple marks. It is said that they include four different kinds of dinosaurs. However, the preservation is so poor and the outline of each imprint is so unclear that the possibility remains that they may not be dinosaur footprints.
Toyama Pref., Oyama-cho; Tetori Group, Itoshiro Subgroup (Valanginian)
A theropod and a sauropod footprint have been reported (Goto, 1992).
Ishikawa Pref., Shiramine-mura; Tetori Group, Itoshiro Subgroup (Valanginian) and Akaiwa Subgroup (Barremian to Aptian)
Two theropod teeth and several ornithischian teeth have been reported (Azuma, 1991). Two footprints each of an iguanodont and a theropod have also been found (Azuma and Takeyama, 1991; Azuma, 1993). These co-occur with a number of ganoid scales and turtle shell fragments.
Fukui Pref., Katsuyama City; Tetori Group, Akaiwa Subgroup (Barremian to Aptian)
A dromaeosaurid (claws, several phalanges, humerous, ulna, femur, tibia, metatarsus, astragalus, maxilla, dentary, and isolated teeth) and an iguanodontid (premaxilla, maxilla, dentaries, angular, quadrate, parietal, several isolated teeth, cervical and dorsal vertebrae, synsacrum, caudal vertebrae, phalanges, etc.) are known. The dromaeosaurid is an extremely large species of the family and is unusual among Early Cretaceous coelurosaurs. The iguanodontid may be reconstructed to be about 5 m in length, and some of its skull characters are considered in between Iguanodon and Probactrosaurus. More than 300 bones and teeth from taxa other than the previously mentioned two species have been excavated, including teeth and vertebrae of theropods, spoon-shaped teeth of a sauropod, and teeth and bones of a few ornithischian taxa. In addition to the dinosaurs, numerous ganoid scales and turtle shell fragments and a nearly complete skeleton of a crocodile have been found. All these fossils were collected from a sandstone bed 1.5–2 m thick.
Several horizons about 1.5 m above the dinosaur-bearing sandstone contain a number of footprints of dinosaurs and other vertebrates, including birds (Azuma, 1993). Dinosaur footprints include theropods, sauropods, and ornithopods, and some of them are continuous trackways. Ornithopod footprints are mainly iguanodontids, and they range from 18 to 70 cm in length, probably representing different growth stages. Furthermore, these iguanodont footprints include both two-legged and four-legged gaits. The largest theropod footprint is 68.5 cm in length, indicating the presence of a large carnosaur.
Fukui Pref., Izumi-mura; Tetori Group, Itoshiro Subgroup (Valanginian) and Akaiwa Subgroup (Barremian to Aptian)
Several footprints of theropods and ornithopods, as well as a number of bird footprints, have been found (Azuma et al., 1992; Azuma and Takeyama, 1991; Tomida and Azuma, 1992).
Gifu Pref., Shirakawa and Shokawa-mura; Tetori Group, Itoshiro Subgroup (Valanginian)
Iguanodontid footprints preserved on a ripple-marked sandstone bed (Kunimitsu et al., 1990), and teeth of theropods, a hypsilophodontid, and another ornithopod have been found (Gifu Pref. Dinosaur Research Group, 1992a,b; Hasegawa et al., 1990).
Yamaguchi Pref., Shimonoseki City; Toyonishi Group, Kiyosu-e Fm. (Latest Jurassic or Earliest Cretaceous)
Footprints of an iguanodontid and a theropod were recently reported to occur on a sandstone bed (Okazaki, 1994), but we believe that both sets of footprints are iguanodontids. The geologic age of the fossil-bearing bed is uncertain; most likely it is earliest Cretaceous.
Fukuoka Pref., Miyata-cho; Kammon Group, Sengoku Fm. (Neocomian)
An incomplete tooth of a large theropod has been found. A new genus and species of Megalosauridae, Wakinosaurus satoi, was proposed based on this poorly preserved specimen (Okazaki, 1992a). However, because no diagnostic characters taxonomically distinguish the specimen from other theropods at the genus and family levels, this proposed name should be abandoned. It is best identified as Theropoda indet.
Fukuoka Pref., Kitakyushu City; Kammon Group. Wakino Subgroup (Neocomian)
Several teeth of a small theropod, a sauropod tooth, and two ceratopsian-like teeth have been reported (Okazaki, 1992b), co-occurring with fragmentary bones of turtles and crocodiles and ganoid scales.
Kumamoto Pref., Mifune-cho; Mifune Group (Cenomanian)
Discoveries of a tooth (Hasegawa et al., 1992) and a metatarsal, tibia, and phalanx of possibly the same individual of an allosaurid-like theropod (Tamura et al., 1991) have been reported, but both identifications are questionable. Turtles, crocodiles, a pterosaur bone, and a jaw of an eutherian mammal also co-occurred.
Other Localities
Discoveries of a hadrosaur femur and ischium from the Ezo Group (Coniacian) of Obira-cho, northwestern Hokkaido and an iguanodontid tooth fragment from the Tazukawa Formation of Monobe Group (late Hauterivian to Barremian) of Katsu-ura-cho Tokushima Pref. have been announced in the newspapers.
Summary
Japanese dinosaur fossils, except in a very few cases, are mostly isolated teeth and bones. Therefore, taxonomic studies are very diffcult, and current identifications in the literature include a number of doubts and questions. The dinosaur localities other than the Tetori Group, Kammon Group, and Mifune Group are located within the Outer Belt of the Japanese tectonic divisions. The localities in the Outer Belt are of shallow marine sediments, and the dinosaur fossils are allochthonous. Therefore, there is not much hope for more dinosaur fossils from those localities. On the other hand, the Tetori, Kammon, and Mifune groups are terrestrial sediments, and dinosaur fossils are more or less autochthonous. Thus, more dinosaur material can be expected from further excavations. Also, dinosaur fossils from those groups are all from the Early Cretaceous, and it is expected that in the near future they will provide information on the dinosaurs and paleoenvironments of the Early Cretaceous, which is sometimes called the “twilight zone” of dinosaur fossils.
The Tetori Group is subdivided into three subgroups (Kuzuryu, Itoshiro, and Akaiwa in ascending order) and shows a characteristic transition of sedimentary environments from marine to freshwater through brackish water environments (Maeda, 1961). The Kuzuryu Subgroup has yielded only a skeleton of a land lizard, Tedorosaurus asuwaensis (Shikama, 1969). Dinosaur fossils have been found from the upper part of the Itoshiro Subgroup and the Akaiwa Subgroup. They include theropods, sauropods, and ornithopods, and important members include a dromaeosaurid and an iguanodontid. Co-occurring taxa include crocodiles, turtles, ganoid fishes, and birds. The dinosaur fauna of the Tetori Group is Early Cretaceous in age, and it represents a fauna that lived in a wet lowland area in a warm, humid climate located on the Asian continental edge. It also represents the dinosaur fauna of the Far East, and its significance should be emphasized.