Article · Wikipedia archive · Last revised Jul 7, 2026

Ohyamashimo Formation

The Ohyamashimo Formation(大山下層) is an Early Cretaceous (Albian) geologic formation, distributed in Tamba and Tamba-Sasayama, Hyogo Prefecture, Japan. It has been dated to the early-mid Albian, between 112.1 ± 0.4 Ma and 106.4 ± 0.4 Ma. The depositional environment represents a fluvial system with a subhumid to semi-arid climate. The Ohyamashimo Formation was called the 'Lower Formation' of the Sasayama Group before it was named in 2017.

Last revised
Jul 7, 2026
Read time
≈ 4 min
Length
971 w
Citations
17
Source
Wikipedia ↗
Ohyamashimo Formation
Stratigraphic range: Albian
~
Kamitaki Bonebed Quarry, currently covered by concrete for protection.
TypeGeological formation
Unit ofSasayama Group
UnderliesSawada Formation
OverliesBasement (Unconformity)
Thickness1300 metres
Lithology
PrimaryConglomerate, Sandstone, Mudstone
OtherTuff
Location
Coordinates35°06′N 135°06′E / 35.1°N 135.1°E / 35.1; 135.1
Approximate paleocoordinates47°54′N 132°48′E / 47.9°N 132.8°E / 47.9; 132.8
RegionHyōgo Prefecture
CountryJapan
Type section
Named forOhyamashimo, Tamba-Sasayama
Named byHayashi et al., 20171
Ohyamashimo Formation (Japan)

The Ohyamashimo Formation(大山下層) is an Early Cretaceous (Albian) geologic formation, distributed in Tamba and Tamba-Sasayama, Hyogo Prefecture, Japan. It has been dated to the early-mid Albian, between 112.1 ± 0.4 Ma and 106.4 ± 0.4 Ma. The depositional environment represents a fluvial system with a subhumid to semi-arid climate. The Ohyamashimo Formation was called the 'Lower Formation' of the Sasayama Group before it was named in 2017.1

Fossil content

Color key
Taxon Reclassified taxon Taxon falsely reported as present Dubious taxon or junior synonym Ichnotaxon Ootaxon Morphotaxon
Notes
Uncertain or tentative taxa are in small text; crossed out taxa are discredited.

Dinosaurs

Ornithischians

Ornithischians of the Ohyamashimo Formation
Genus Species Location Stratigraphic position Material Notes Images
Ankylosauria Indet.2 Indeterminate Kamitaki Bonebed Quarry, Tamba Albian Dental remains.
Iguanodontia Indet.2 Indeterminate Kamitaki Bonebed Quarry, Tamba Albian Dental remains.
Sasayamagnomus3 S. saegusai Miyada microvertebrate site, Tamba-Sasayama Albian Skull elements and partial postcranial remains. A neoceratopsian; closely related to Aquilops americanus.
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Saurischians

Sauropods
Sauropods of the Ohyamashimo Formation
Genus Species Location Stratigraphic position Material Notes Images
Tambatitanis2 T. amicitiae Kamitaki Bonebed Quarry, Tamba Albian A single partial skeleton. A titanosauriform sauropod
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Theropods
Theropods of the Ohyamashimo Formation
Genus Species Location Stratigraphic position Material Notes Images
Dromaeosauridae Indet.2 Indeterminate Kamitaki Bonebed Quarry, Tamba Albian Dental remains.
Hypnovenator4 H. matsubaraetoheorum Nishikosa, Tamba-Sasayama Albian A single partial skeleton. A troodontid theropod; closely related to Gobivenator mongoliensis.
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Therizinosauria Indet.2 Indeterminate Kamitaki Bonebed Quarry, Tamba Albian Dental remains.
Theropoda Indet.2 Indeterminate Kamitaki Bonebed Quarry, Tamba Albian Dental remains.
Tyrannosauroidea Indet.2 Indeterminate Kamitaki Bonebed Quarry, Tamba Albian Dental remains.

Squamates

Squamtes of the Ohyamashimo Formation
Genus Species Location Stratigraphic position Material Notes Images
Morohasaurus5 M. kamitakiensis Kamitaki Egg Quarry, Tamba Albian A nearly complete left dentary. A monstersaurian lizard
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Pachygenys6 P. adachii Miyada microvertebrate site, Tamba-Sasayama Albian A single, partially broken right mandible A scincomorphan lizard
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Amphibians

Amphibian of the Ohyamashimo Formation7
Genus Species Location Stratigraphic position Material Notes Images
Hyogobatrachus H. wadai Kamitaki Bonebed Quarry, Tamba Albian A nearly complete skeleton. A frog
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Tambabatrachus T. kawazu Kamitaki Bonebed Quarry, Tamba Albian An articulated skeleton lacking the anterior part of the skull, most of the pectoral girdle, manus, and feet. A frog
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Mammals

Mammals of the Ohyamashimo Formation
Genus Species Location Stratigraphic position Material Notes Images
Sasayamamylos8 S. kawaii Miyada microvertebrate site, Tamba-Sasayama Albian Three dentaries. A asioryctitherian eutherian
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Crustaceans

Crustaceans of the Ohyamashimo Formation
Genus Species Location Stratigraphic position Material Notes Images
Mongolocypris M. sp. Albian A ostracod
Conchostraca Indet. Indeterminate Albian A clam shrimp

Molluscs

Bivalves

Bivalves of the Ohyamashimo Formation
Genus Species Location Stratigraphic position Material Notes Images
Sphaerium S. coreanicum Albian A sphaeriid bivalve

Gastropods

Gastropods of the Ohyamashimo Formation
Genus Species Location Stratigraphic position Material Notes Images
Viviparus V. cf. keishoensis Albian A viviparid gastropod

Oofossils

Oofossils of the Ohyamasimo Formation910
Genus Species Location Stratigraphic position Material Notes Images
Elongatoolithus E. sp. Kamitaki Bonebed Quarry, Tamba Likely laid by oviraptorosaur.
Himeoolithus H. murakamii Kamitaki Egg Quarry, Tamba Likely laid by a small theropod.
Nipponoolithus N. ramosus Kamitaki Bonebed Quarry and Kamitaki Egg Quarry, Tamba Likely laid by non-avian maniraptoran theropod.
Prismatoolithus P. sp. Kamitaki Bonebed Quarry and Kamitaki Egg Quarry, Tamba Likely laid by either troodontids or birds.
Subtiliolithus S. hyogoensis Kamitaki Bonebed Quarry and Kamitaki Egg Quarry, Tamba Likely laid by a small theropod.
Spheroolithus S. sp. Kamitaki Bonebed Quarry, Tamba Likely laid by hadrosaurs or other ornithopods.
References

References

  1. Hayashi, Keiichi; Fujita, Saki; Koarai, Kazuto; Matsukawa, Masaki (2017-09-15). "Stratigraphy and paleoenvironment of the Cretaceous Sasayama Group in the Sasayama area, Hyogo Prefecture, western Japan". The Journal of the Geological Society of Japan. 123 (9): 747–764. doi:10.5575/geosoc.2017.0016. ISSN 0016-7630.
  2. Saegusa, Haruo; Ikeda, Tadahiro (12 August 2014). "A new titanosauriform sauropod (Dinosauria: Saurischia) from the Lower Cretaceous of Hyogo, Japan". Zootaxa. 3848 (1). doi:10.11646/zootaxa.3848.1.1.
  3. Tanaka, Tomonori; Chiba, Kentaro; Ikeda, Tadahiro; Ryan, Michael J. (September 2024). "A new neoceratopsian (Ornithischia, Ceratopsia) from the Lower Cretaceous Ohyamashimo Formation (Albian), southwestern Japan". Papers in Palaeontology. 10 (5). doi:10.1002/spp2.1587.
  4. Kubota, Katsuhiro; Kobayashi, Yoshitsugu; Ikeda, Tadahiro (25 July 2024). "Early Cretaceous troodontine troodontid (Dinosauria: Theropoda) from the Ohyamashimo Formation of Japan reveals the early evolution of Troodontinae". Scientific Reports. 14 (1). doi:10.1038/s41598-024-66815-2. PMC 11272788.
  5. Ikeda, Tadahiro; Ota, Hidetoshi; Tanaka, Tomonori; Ikuno, Kenji; Kubota, Katsuhiro; Tanaka, Kohei; Saegusa, Haruo (February 2022). "A fossil Monstersauria (Squamata: Anguimorpha) from the Lower Cretaceous Ohyamashimo Formation of the Sasayama Group in Tamba City, Hyogo Prefecture, Japan". Cretaceous Research. 130 105063. doi:10.1016/j.cretres.2021.105063.
  6. Ikeda, Tadahiro; Ota, Hidetoshi; Saegusa, Haruo (2 January 2015). "A new fossil lizard from the Lower Cretaceous Sasayama Group of Hyogo Prefecture, western Honshu, Japan". Journal of Vertebrate Paleontology. 35 (1) e885032. doi:10.1080/02724634.2014.885032.
  7. Ikeda, Tadahiro; Ota, Hidetoshi; Matsui, Masafumi (June 2016). "New fossil anurans from the Lower Cretaceous Sasayama Group of Hyogo Prefecture, Western Honshu, Japan". Cretaceous Research. 61: 108–123. doi:10.1016/j.cretres.2015.12.024.
  8. Kusuhashi, Nao; Tsutsumi, Yukiyasu; Saegusa, Haruo; Horie, Kenji; Ikeda, Tadahiro; Yokoyama, Kazumi; Shiraishi, Kazuyuki (22 May 2013). "A new Early Cretaceous eutherian mammal from the Sasayama Group, Hyogo, Japan". Proceedings of the Royal Society B: Biological Sciences. 280 (1759) 20130142. doi:10.1098/rspb.2013.0142. PMC 3619506.
  9. Tanaka, Kohei; Zelenitsky, Darla K.; Saegusa, Haruo; Ikeda, Tadahiro; DeBuhr, Christopher L.; Therrien, François (January 2016). "Dinosaur eggshell assemblage from Japan reveals unknown diversity of small theropods". Cretaceous Research. 57: 350–363. doi:10.1016/j.cretres.2015.06.002.
  10. Tanaka, Kohei; Zelenitsky, Darla K.; Therrien, François; Ikeda, Tadahiro; Kubota, Katsuhiro; Saegusa, Haruo; Tanaka, Tomonori; Ikuno, Kenji (October 2020). "Exceptionally small theropod eggs from the Lower Cretaceous Ohyamashimo Formation of Tamba, Hyogo Prefecture, Japan". Cretaceous Research. 114 104519. doi:10.1016/j.cretres.2020.104519.