- Tomioka, Gunma, Japan
Yuji TAKAKUWA
Gunma Museum of Natural History, Geology, Faculty Member
- Geology, Systematics (Taxonomy), Mammalogy, Sharks, Fossil sharks, Fossil Deer, and 27 moreDeers, Palaeontology, Mammalian Paleontology, Pleistocene Vertebrate, Cervidae, Vertebrate Paleontology, Taphonomy, Vertebrate Evolution, Morphological evolution, Fossil Elasmobranchii, Fossil Chondrichthyes, Palaeozoic chondrichthyans and other fishes, Chondrichthyes, Vertebrate Palaeontology, Paleontology, Palaeoecology, Paleoecology, Vertebrate taphonomy, Fossil Fish, Miocene, Dinosaurs, Dinosaur Paleontology, Cretaceous, Paleobiology, Jurassic, Triassic, and Sauropodsedit
Research Interests: Geology and Cretaceous
Research Interests: Evolutionary Biology, Geology, Paleontology, Biology, Ecology, and 3 moreAnatomy, Gigantism, and Neogene
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An isolated theropod tooth from the Early Cretaceous Sebayashi Formation is described. It is almost identical to those of the possible spinosauridtheropod Siamosaurus from the Early Cretaceous Sao Khua Formation of Thailand and it is... more
An isolated theropod tooth from the Early Cretaceous Sebayashi Formation is described. It is almost identical to those of the possible spinosauridtheropod Siamosaurus from the Early Cretaceous Sao Khua Formation of Thailand and it is tentatively referred to the Spinosauridae. The tooth represents the second dinosaur species to be identified from the Sebayashi Formation and the second report of the family Spinosauridae in Asia.
Research Interests:
Accumulating fossil records of deep-sea sharks are important for reconstruction of their paleoecology and evolution, because these fossils are generally rare everywhere in the world. This paper reports the newly discovered and diverse... more
Accumulating fossil records of deep-sea sharks are important for reconstruction of their paleoecology and evolution, because these fossils are generally rare everywhere in the world. This paper reports the newly discovered and diverse fossil assemblage of deep-sea sharks from the Miocene deposits in the southwestern part of Gunma Prefecture, central Japan. The specimens are isolated teeth found from seven localities of three Middle Miocene formations. These fossils are identified into twelve species of eleven genera belonging to eight families within three orders: eight species from the Obata Formation (earliest Middle Miocene) of the Tomioka Group, six species from the Haratajino Formation (early Middle Miocene) of the Tomioka Group and four species from the Niwaya Formation (middle Middle Miocene) of the Annaka Group. Four genera, Centrophorus, Deania, Squaliolus and Mitsukurina represent the first fossil record in the Northwest Pacific. Somniosus and Centroscymnus mark the second...
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A new chondrichthyan fossil tooth was found from a float of the black shale of the Lower Jurassic Iwamuro Formation, Numata City (former Shirasawa Village), Gunma Prefecture, central Japan. The specimen (GMNH-PV-2406) is composed of a... more
A new chondrichthyan fossil tooth was found from a float of the black shale of the Lower Jurassic Iwamuro Formation, Numata City (former Shirasawa Village), Gunma Prefecture, central Japan. The specimen (GMNH-PV-2406) is composed of a platform-like flattened crown with rhomboidal outline. The crown has a complex ornamentation pattern. The ornamentation on the occlusal surface is finely reticulate with no occlusal crest. On the lateral surface, numerous striations are on the radical margin of each lateral side. These characteristics show that GMNH-PV-2406 is identified as a lateral tooth of Asteracanthus sp. This specimen marks the third record of the genus from Japan, and the first vertebrate fossil from the Iwamuro Formation and the Jurassic deposits of Gunma Prefecture respectively. The Asian fossil records of the genus Asteracanthus, including GMNH-PV-2406, contribute to the understanding of distributional expansion timing, and the transition of Mesozoic shark fauna in northwestern Panthalassa.
Research Interests:
A new record of fossil chondrichthyan tooth was found from a float of the Early Permian limestone block of the Omama complex of the Ashio tectonic belt, Kiryu City, Gunma Prefecture, central Japan. The specimen (GMNH-PV-2405) is composed... more
A new record of fossil chondrichthyan tooth was found from a float of the Early Permian limestone block of the Omama complex of the Ashio tectonic belt, Kiryu City, Gunma Prefecture, central Japan. The specimen (GMNH-PV-2405) is composed of a flattened root with a median cusp. Though the lingual side of the root (base) is broken, the basolabial shelf is recognized in a labial rim of its radical side. The basolabial shelf is weakly disrupted. The median cusp inclines distally. It has a tall triangular shape with fine striations in the radical half of the lingual surface. The shape of the cusp cross section is convex on its lingual surface. The presence of lateral cusps and intermediate cusplets is uncertain. Through the combination of some characteristics, GMNH-PV-2405 is identified as Ctenacanthidae gen. et sp. indet. GMNH-PV-2405 marks the second record of Paleozoic vertebrate fossils from Gunma Prefecture, and the fifth vertebrate remains from the Ashio tectonic belt. The occurrence of GMNH-PV-2405 provides important paleontological information regarding the paleogeographical setting of the Ashio tectonic belt in the late Paleozoic, and also the radiation in the oceanic environment of the ctenacanthiform shark.
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Diverse vertebrate fossils of more than 160 fragmental bones, teeth and footprints, including cyprinid fishes, reptiles (turtles and crocodiles), and mammals (a rodent, a carnivora and artiodactyls), were collected from the Lower Miocene... more
Diverse vertebrate fossils of more than 160 fragmental bones, teeth and footprints, including cyprinid fishes, reptiles (turtles and crocodiles), and mammals (a rodent, a carnivora and artiodactyls), were collected from the Lower Miocene Koura Formation at Sai and Karubi, Mihonoseki, Matsue, Shimane Prefecture, Southwest Japan. The mode of occurrence and preservation of these fossils imply that sedimentary environment is the lake bottom near to the river mouth were woods and larger vertebrate remains are transported with occasional water currents. Sedimentary facies and sediment compositions of Koura Formation indicate the possibility that the bone beds of the formation were formed with great influence of volcanic activities.
An isolated theropod tooth from the Early Cretaceous Sebayashi Formation is described. It is almost identical to those of the possible spinosauridtheropod Siamosaurus from the Early Cretaceous Sao Khua Formation of Thailand and it is... more
An isolated theropod tooth from the Early Cretaceous Sebayashi Formation is described. It is almost identical to those of the possible spinosauridtheropod Siamosaurus from the Early Cretaceous Sao Khua Formation of Thailand and it is tentatively referred to the Spinosauridae. The tooth represents the second dinosaur species to be identified from the Sebayashi Formation and the second report of the family Spinosauridae in Asia.
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A single vertebra (GMNH-PV-028), classifiable as ornithomimid, was collected from a brackish shale of the Sebayashi Formation (Late Barremian to Aptian), Sanchu Terrane, in Gunma Prefecture, Japan. There are several important... more
A single vertebra (GMNH-PV-028), classifiable as ornithomimid, was collected from a brackish shale of the Sebayashi Formation (Late Barremian to Aptian), Sanchu Terrane, in Gunma Prefecture, Japan. There are several important similarities to the Late Cretaceous Mongolian ornithomimid Gallimimus bullatus. It is only the second representative of the group in Early Cretaceous of Asia.
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The oldest fossil of Echinorhinus in the northwestern Pacific area is described in this paper. The fossil, Echinorhinus sp. （GMNH-PV-3227） , was found in a cobble-size rock block which is estimated that a part of a calcareous nodule of... more
The oldest fossil of Echinorhinus in the northwestern Pacific area is described in this paper. The fossil, Echinorhinus sp. （GMNH-PV-3227） , was found in a cobble-size rock block which is estimated that a part of a calcareous nodule of the Turonian Saku Formation, Yezo Group. The age of the hitherto known oldest fossil Echinorhinus in the northwestern Pacific area was recorded from the Santonian of Japan. However, the GMNH-PV-3227 specimen was accompanied by the specimen of ammonite, Romaniceras (Romaniceras) sp. The range of the subgenus of this ammonite is limited to the middle to upper Turonian. Thus, GMNH-PV-3227 marks the oldest record of this genus in the northwestern Pacific area.
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Eighteen elasmobranch remains from a conglomerate bed of the Coniacian Ashizawa Formation （Obisagawa Member）are described. These specimens consists of five genera of two orders; Hexanchiformes （Hexanchus [Hexanchus sp.]） , Lamniformes... more
Eighteen elasmobranch remains from a conglomerate bed of the Coniacian Ashizawa Formation （Obisagawa Member）are described. These specimens consists of five genera of two orders; Hexanchiformes （Hexanchus [Hexanchus sp.]） , Lamniformes （Cretalamna [C. appendiculata, and C. sp.], Cretoxyrhina [C. mantelli], Scapanorhynchus [S. sp.], and Cretodus [C. crassidens]）. The specimen of Cretodus crassidens marks the first record not only from the Futaba Group, but also the northwestern Pacific region. In addition, this specimen suggests that the genus was distributed widely throughout the northwestern Pacific region at the Late Cretaceous.
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Elasmobranch specimens from the construction site of Mizunami-Kita Junior High School in Mizunami City are described. These specimens consists of five genera; four sharks of Nebrius, Carcharhinus, Galeocerdo, and Megasqualus, and one ray... more
Elasmobranch specimens from the construction site of Mizunami-Kita Junior High School in Mizunami City are described. These specimens consists of five genera; four sharks of Nebrius, Carcharhinus, Galeocerdo, and Megasqualus, and one ray of Rhinoptera. Megasqualus occidentalis (Agassiz, 1856), a new combination is proposed here.
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A new occurrence of fossil brown alga from the Orida tuffaceous sandstone and mudstone Member of the Sawatari Formation assumed to be middle Middle Miocene in Nakanojo Town, Gunma Prefecture, Japan, is reported. The specimen GMNH-PB-2632... more
A new occurrence of fossil brown alga from the Orida tuffaceous sandstone and mudstone Member of the Sawatari Formation assumed to be middle Middle Miocene
in Nakanojo Town, Gunma Prefecture, Japan, is reported. The specimen GMNH-PB-2632 and its counterpart NHFM-KHDS-0002 has round-shaped single pneumacyst in some bases of branches and twigs or short rod-like leaves, and is identified as Cystoseira sp. This alga is also the second recorded fossil of the genus from Japan.
Three living species of Cystoseira are distributed in coastal areas of Japan and are dominantly between cool-temperate and frigid regions of the Northern hemisphere. They prefer cool-temperate marine conditions. Through the basis of the distribution mentioned above, it is suggested that some cold currents also influenced surface water of northeast Japan, including Gunma, in the middle to late Middle Miocene, not only warm currents. The mixing of the biota that preferred cool- and warm-temperate marine conditions in the Sawatari Formation might be the result of climate oscillation
between warming and cooling, after the Middle Miocene Climatic Optimum. And on the basis of the occurrence of Cystoseira and some fish fossils, it is also suggested that there were rocky shores, reefs or some hard substrates near the sedimentary environment of the Sawatari Formation.
in Nakanojo Town, Gunma Prefecture, Japan, is reported. The specimen GMNH-PB-2632 and its counterpart NHFM-KHDS-0002 has round-shaped single pneumacyst in some bases of branches and twigs or short rod-like leaves, and is identified as Cystoseira sp. This alga is also the second recorded fossil of the genus from Japan.
Three living species of Cystoseira are distributed in coastal areas of Japan and are dominantly between cool-temperate and frigid regions of the Northern hemisphere. They prefer cool-temperate marine conditions. Through the basis of the distribution mentioned above, it is suggested that some cold currents also influenced surface water of northeast Japan, including Gunma, in the middle to late Middle Miocene, not only warm currents. The mixing of the biota that preferred cool- and warm-temperate marine conditions in the Sawatari Formation might be the result of climate oscillation
between warming and cooling, after the Middle Miocene Climatic Optimum. And on the basis of the occurrence of Cystoseira and some fish fossils, it is also suggested that there were rocky shores, reefs or some hard substrates near the sedimentary environment of the Sawatari Formation.
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We describe fossil barnacles from the Miocene of Gunma Prefecture, central Japan. The fossils were classified into three species a scalpellid from the Niwaya Formation, an archaeobalanid Striatobalanus sp. from the Haraichi Formation... more
We describe fossil barnacles from the Miocene of Gunma Prefecture, central Japan.
The fossils were classified into three species a scalpellid from the Niwaya Formation,
an archaeobalanid Striatobalanus sp. from the Haraichi Formation and a balanid from the Chayagamatsu Formation and the Mamioka Tuff. The scalpellid, the fifth Japanese record of this family, occurred from muddy deposits, as with other Japanese fossils. Striatobalanus sp. is the first Miocene record of warm-water archaeobalanids in Japan. The species probably distributed in Japan with warm surface currents. Preservation modes of the fossil barnacles imply the further occurrence of additional specimens which will progress their taxonomy and paleobiogeography.
The fossils were classified into three species a scalpellid from the Niwaya Formation,
an archaeobalanid Striatobalanus sp. from the Haraichi Formation and a balanid from the Chayagamatsu Formation and the Mamioka Tuff. The scalpellid, the fifth Japanese record of this family, occurred from muddy deposits, as with other Japanese fossils. Striatobalanus sp. is the first Miocene record of warm-water archaeobalanids in Japan. The species probably distributed in Japan with warm surface currents. Preservation modes of the fossil barnacles imply the further occurrence of additional specimens which will progress their taxonomy and paleobiogeography.
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A new fossil record of shark tooth in Gunma Prefecture was found from an outcrop in the Gunma Safari Park, Tomioka City, Gunma Prefecture. The specimen (GSP-00001) is derived from the late Early Miocene Obata Formation, Tomioka Group. The... more
A new fossil record of shark tooth in Gunma Prefecture was found from an outcrop in the Gunma Safari Park, Tomioka City, Gunma Prefecture. The specimen (GSP-00001) is derived from the late Early Miocene Obata Formation, Tomioka Group. The specimen is only the coronal part of the crown, though the apex is lost. The crown is large and robust. Its shape is almost triangular. The preserved cutting edge is all serrated. The mesial and distal edge show slightly expansion to outside in lingual (or labial) view. And the labial surface of the crown is slightly curved opposite to lingual side. Distal profile slightly shows sigmoid curve in labial view. These characteristics of the specimen and comparison with the natural tooth set of C. megalodon from Saitama (Kawamoto specimen) suggest that the specimen found at the Gunma Safari Park is a lower third or fourth tooth of fossil megatooth shark, Carcharodon megalodon. This specimen is recognized as the first specimen of the species from the Oba...
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A Triceratops specimen, GMNH-PV 124, excavated from the Hell Creek Formation, South Dakota, is composed of nearly complete skull elements, axial skeleton, and partial limb elements. Co-ossification of the skull elements and the... more
A Triceratops specimen, GMNH-PV 124, excavated from the Hell Creek Formation, South
Dakota, is composed of nearly complete skull elements, axial skeleton, and partial limb elements. Co-ossification of the skull elements and the scapula-coracoid contact are not finished and hence the specimen is sub-adult. However, co-ossification has been started or finished among the occipitals, supraorbital horn cores, and parietal, between the rostrum and premaxillae, between the epinasal and nasals contacts, the first to third cervical vertebrae, and sacral vertebrae.
Dakota, is composed of nearly complete skull elements, axial skeleton, and partial limb elements. Co-ossification of the skull elements and the scapula-coracoid contact are not finished and hence the specimen is sub-adult. However, co-ossification has been started or finished among the occipitals, supraorbital horn cores, and parietal, between the rostrum and premaxillae, between the epinasal and nasals contacts, the first to third cervical vertebrae, and sacral vertebrae.
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A new record of fossil shark tooth in Gunma Prefecture was found from the collection of the Iwajuku Museum, Midori City, Gunma Prefecture. The specimen (Iwajuku Museum 2005-A326) was collected by natural historian, Shosaku IWASAWA, from... more
A new record of fossil shark tooth in Gunma Prefecture was found from the collection of the Iwajuku Museum, Midori City, Gunma Prefecture. The specimen (Iwajuku Museum 2005-A326) was collected by natural historian, Shosaku IWASAWA, from the Late Miocene Mamioka Tuff, Tenjinyama, Midori City (former Kasakake Village), Gunma Prefecture, central Japan. The specimen has a relatively large and high triangular shaped crown. The cutting edge is smooth and almost straight in distal profile. Mesial profile slightly shows sigmoid curve in labial view. The labial face of the crown is almost flat, though the apex pointed to labial direction. The root is bulky and stout. These characteristics indicate that the tooth is identified to upper right intermediate tooth of fossil 'mako' shark (" Isurus " hastalis). This specimen is recognized as the first vertebrate fossil of the geological unit. Futhermore, another fossil specimen of shark tooth that Shosaku IWASAWA collected is desc...
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Some dozen well preserved specimens of an extinct fossil sea turtle, Syllomus aegyptiacus, had been unearthed from the Middle Miocene Haratajino Formation and its equivalents of the Tomioka Group from the riverside bed of the Usuigawa... more
Some dozen well preserved specimens of an extinct fossil sea turtle, Syllomus aegyptiacus, had been unearthed from the Middle Miocene Haratajino Formation and its equivalents of the Tomioka Group from the riverside bed of the Usuigawa River in Annaka City and Takadagawa River in Myogi Town, Gunma Prefecture, Central Japan. Almost the entire skeleton of this aberrant sea turtle is first restored based on these newly collectd materials. This species has been reported from the Neogene deposits in North America and Egypt as well as Japan. The fossil record indicates that Syllomus had the midest cosmopolitan distribution of living sea-turtle species. Palatal morphology is described based on a skull collected from the Middle Miocene Nagura Formation of Saitama Prefecture, Central Japan. Its triturating surface suggests that sea grasses might have been a main food of Syllomus.
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This paper reports the newly collected fossil of the brown bear (Ursus arctos) found from the Fuji-do cave, Ueno Village, Tano County, Gunma Prefecture. The cave is known as one of the largest caves for tourism in the Kanto region. The... more
This paper reports the newly collected fossil of the brown bear (Ursus arctos) found from the Fuji-do cave, Ueno Village, Tano County, Gunma Prefecture. The cave is known as one of the largest caves for tourism in the Kanto region. The fossil was collected during the earth removal work of a newly found pathway of the southeastern part of the cave in 1992. The specimen (Fuji-do cave specimen) is a portion of left scapula buried in the cave deposit. The deposit is composed of limestone rubbles that are cemented by calcareous sinter (travertine) with porous structure. The age of the specimen is presumably Late Pleistocene. Based on some morphological characteristics (spine of scapula with linear outline, acromion with proximal expansion, arrangement between lateral margin of glenoid cavity and the spine, and the size), the specimen is identified into Ursus arctos. The living brown bear do not naturally inhabit the Japanese archipelago except on the Island of Hokkaido. Brown bear fossil...
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Sixteen fossil shark teeth were found in close contact with a balaenopterid-whale skeleton of the Late Miocene Pisco Formation in Peru, South America. This whale skeleton (GMNH-PV 1599 was excavated in Aguada de Lomas of western Arequipa... more
Sixteen fossil shark teeth were found in close contact with a balaenopterid-whale skeleton of the Late Miocene Pisco Formation in Peru, South America. This whale skeleton (GMNH-PV 1599 was excavated in Aguada de Lomas of western Arequipa in 1990, and exhibited in Gunma Museum of Natural History, Japan, after 1996. All teeth have relatively large and triangular shaped crowns. The cutting edge is smooth and almost straight in profile. Their roots are bulky and stout with long or expanded lobes. These characteristics indicate that all teeth are identified to fossil 'mako' shark ("Isurus”
hastalis). And the variety of the shape in each tooth implies difference of tooth position in its jaw. The duplication of the teeth at two positions (anterior tooth and intermediate tooth) and the difference of the crown size of anterior teeth suggest that the minimum number of shark individuals which had preyed on the whale is two. And one of them might be smaller than the other one. In addition, it is recognized that one of the sharks sank its upper lateral tooth into the whale skull bone.
hastalis). And the variety of the shape in each tooth implies difference of tooth position in its jaw. The duplication of the teeth at two positions (anterior tooth and intermediate tooth) and the difference of the crown size of anterior teeth suggest that the minimum number of shark individuals which had preyed on the whale is two. And one of them might be smaller than the other one. In addition, it is recognized that one of the sharks sank its upper lateral tooth into the whale skull bone.
Research Interests:
A new record of fossil chondrichthyan tooth was found from a float of the Early Permian limestone block of the Omama complex of the Ashio tectonic belt, Kiryu City, Gunma Prefecture, central Japan. The specimen (GMNH-PV-2405) is composed... more
A new record of fossil chondrichthyan tooth was found from a float of the Early Permian limestone block of the Omama complex of the Ashio tectonic belt, Kiryu City, Gunma Prefecture, central Japan. The specimen (GMNH-PV-2405) is composed of a flattened root with a median cusp. Though the lingual side of the root (base) is broken, the basolabial shelf is recognized in a labial rim of its radical side. The basolabial shelf is weakly disrupted. The median cusp inclines distally. It has a tall triangular shape with fine striations in the radical half of the lingual surface. The shape of the cusp cross section is convex on its lingual surface. The presence of lateral cusps and intermediate cusplets is uncertain. Through the combination of some characteristics, GMNH-PV-2405 is identified as Ctenacanthidae gen. et sp. indet. GMNH-PV-2405 marks the second record of Paleozoic vertebrate fossils from Gunma Prefecture, and the fifth vertebrate remains from the Ashio tectonic belt. The occurrence of GMNH-PV-2405 provides important paleontological information regarding the paleogeographical setting of the Ashio tectonic belt in the late Paleozoic, and also the radiation in the oceanic environment of the ctenacanthiform shark.
Research Interests:
A new chondrichthyan fossil tooth was found from a float of the black shale of the Lower Jurassic Iwamuro Formation, Numata City (former Shirasawa Village), Gunma Prefecture, central Japan. The specimen (GMNH-PV-2406) is composed of a... more
A new chondrichthyan fossil tooth was found from a float of the black shale of the Lower Jurassic Iwamuro Formation, Numata City (former Shirasawa Village), Gunma Prefecture, central Japan. The specimen (GMNH-PV-2406) is composed of a platform-like flattened crown with rhomboidal outline. The crown has a complex ornamentation pattern. The ornamentation on the occlusal surface is finely reticulate with no occlusal crest. On the lateral surface, numerous striations are on the radical margin of each lateral side. These characteristics show that GMNH-PV-2406 is identified as a lateral tooth of Asteracanthus sp. This specimen marks the third record of the genus from Japan, and the first vertebrate fossil from the Iwamuro Formation and the Jurassic deposits of Gunma Prefecture respectively. The Asian fossil records of the genus
Asteracanthus, including GMNH-PV-2406, contribute to the understanding of distributional expansion timing, and the transition of Mesozoic shark fauna in
northwestern Panthalassa.
Asteracanthus, including GMNH-PV-2406, contribute to the understanding of distributional expansion timing, and the transition of Mesozoic shark fauna in
northwestern Panthalassa.
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Accumulating fossil records of deep-sea sharks are important for reconstruction of their paleoecology and evolution, because these fossils are generally rare everywhere in the world. This paper reports the newly discovered and diverse... more
Accumulating fossil records of deep-sea sharks are important for reconstruction of their paleoecology and evolution, because these fossils are generally rare everywhere in the world. This paper reports the newly discovered and diverse fossil assemblage of deep-sea sharks from the Miocene deposits in the southwestern part of Gunma Prefecture, central Japan. The specimens are isolated teeth found from seven localities of three Middle Miocene formations. These fossils are identified into twelve
species of eleven genera belonging to eight families within three orders: eight species from the Obata Formation (earliest Middle Miocene) of the Tomioka Group, six species from the Haratajino Formation (early Middle Miocene) of the Tomioka Group and four species from the Niwaya Formation (middle Middle Miocene) of the Annaka Group. Four genera,
,
,
and
represent the first fossil record in the Northwest Pacific.
and
mark the second
record from the Miocene in the world, and
and
indicate the second in the
circum-Pacific. On the basis of lithofacies, benthic foraminifers and other megafossils, three formations
are thought to have been deposited under outer sublittoral to middle bathyal environments. Since
these environments accord with the Recent species habitats of the eleven shark genera, this fossil deep-
sea shark fauna similar to the recent one in species/generic composition had been habited in bathyal
environment. The presence of this Miocene fauna suggests that the outline of the recent deep-sea
shark fauna in the Northwest Pacific region would have been already established the Miocene. The
establishment of the fauna might have been resulted from the vicariance and the isolation that caused
by the closure of Indonesian seaway and its associated expansion of shallow sea in the Oligo-Miocene
time.
species of eleven genera belonging to eight families within three orders: eight species from the Obata Formation (earliest Middle Miocene) of the Tomioka Group, six species from the Haratajino Formation (early Middle Miocene) of the Tomioka Group and four species from the Niwaya Formation (middle Middle Miocene) of the Annaka Group. Four genera,
,
,
and
represent the first fossil record in the Northwest Pacific.
and
mark the second
record from the Miocene in the world, and
and
indicate the second in the
circum-Pacific. On the basis of lithofacies, benthic foraminifers and other megafossils, three formations
are thought to have been deposited under outer sublittoral to middle bathyal environments. Since
these environments accord with the Recent species habitats of the eleven shark genera, this fossil deep-
sea shark fauna similar to the recent one in species/generic composition had been habited in bathyal
environment. The presence of this Miocene fauna suggests that the outline of the recent deep-sea
shark fauna in the Northwest Pacific region would have been already established the Miocene. The
establishment of the fauna might have been resulted from the vicariance and the isolation that caused
by the closure of Indonesian seaway and its associated expansion of shallow sea in the Oligo-Miocene
time.