Ohyamashimo Formation

Ohyamashimo Formation
Stratigraphic range: Albian ~112.1–106.4 Ma PreꞒ Ꞓ O S D C P T J K Pg N
Type	Geological formation
Unit of	Sasayama Group
Underlies	Sawada Formation
Overlies	Basement (Unconformity)
Thickness	1300 metres
Lithology
Primary	Conglomerate, Sandstone, Mudstone
Other	Tuff
Location
Coordinates	35.1°N 135.1°E / 35.1; 135.1
Approximate paleocoordinates	47.9°N 132.8°E / 47.9; 132.8
Region	Hyogo prefecture
Country	Japan
Type section
Named for	Ohyamashimo, Tamba-Sasayama
Named by	Hayashi et al., 2017[1]
Ohyamashimo Formation (Japan)

The Ohyamashimo Formation(大山下層) is an Early Cretaceous (Albian) geologic formation in Japan. It has been dated to the early-mid Albian, between 112.1 ± 0.4 Ma and 106.4 ± 0.4 Ma. Dinosaur remains have been discovered from this formation, including the sauropod Tambatitanis, the theropod Hypnovenator and the ceratopsian Sasayamagnomus. Other representative fossils from the formation include the monstersaurian lizard Morohasaurus and eutherian mammal Sasayamamylos. The depositional environment represents a fluvial system with a subhumid to semi-arid climate.

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

Ceratopsians

Ceratopsians of the Ohyamashimo Formation
Genus	Species	Location	Stratigraphic position	Material	Notes	Images
Sasayamagnomus[2]	S. saegusai			Skull elements and partial postcranial remains.	A neoceratopcian; closely related to Aquilops americanus.

Ornithopods

Ornithopods of the Ohyamashimo Formation
Genus	Species	Location	Stratigraphic position	Material	Notes	Images
Iguanodontia Indet.	Indeterminate			Dental remains.

Sauropods

Sauropods of the Ohyamashimo Formation
Genus	Species	Location	Stratigraphic position	Material	Notes	Images
Tambatitanis[3]	T. amicitiae			A single partial skeleton.	A titanosauriform sauropod

Theropods

Theropods of the Ohyamashimo Formation
Genus	Species	Location	Stratigraphic position	Material	Notes	Images
Dromaeosauridae Indet.	Indeterminate			Dental remains.
Hypnovenator[4]	H. matsubaraetoheorum			A single partial skeleton.	A troodontid theropod; closely related to Gobivenator mongoliensis.
Therizinosauria Indet.	Indeterminate			Dental remains.
Tyrannosauroidea Indet.	Indeterminate			Dental remains.

Thyreophorans

Thyreophorans of the Ohyamashimo Formation
Genus	Species	Location	Stratigraphic position	Material	Notes	Images
Ankylosauria Indet.	Indeterminate			Dental remains.

Squamates

Squamtes of the Ohyamashimo Formation
Genus	Species	Location	Stratigraphic position	Material	Notes	Images
Morohasaurus[5]	M. kamitakiensis			A nearly complete left dentary.	A monstersaurian lizard
Pachygenys[6]	P. adachii			A single, partially broken right mandible	A scincomorphan lizard

Amphibians

Amphibian of the Ohyamashimo Formation[7]
Genus	Species	Location	Stratigraphic position	Material	Notes	Images
Hyogobatrachus	H. wadai			A nearly complete skeleton.	A frog
Tambabatrachus	T. kawazu			An articulated skeleton lacking the anterior part of the skull, most of the pectoral girdle, manus, and feet.	A frog

Mammals

Mammals of the Ohyamashimo Formation
Genus	Species	Location	Stratigraphic position	Material	Notes	Images
Sasayamamylos[8]	S. kawaii			Three dentaries.	A asioryctitherian eutherian

Crustaceans

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

Molluscs

Bivalves

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

Gastropods

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

Oofossils

Eggs of the Ohyamasimo Formation[9][10]
Genus	Species	Location	Stratigraphic position	Material	Notes	Images
Elongatoolithus	E. sp.
Himeoolithus	H. murakamii
Nipponoolithus	N. ramosus
Prismatoolithus	P. sp.
Subtiliolithus	S. hyogoensis

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. 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.
3. 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.
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.