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 locality of the Tambatitanis, currently covered by concrete for protection.
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°06′N 135°06′E / 35.1°N 135.1°E / 35.1; 135.1
Approximate paleocoordinates	47°54′N 132°48′E / 47.9°N 132.8°E / 47.9; 132.8
Region	Hyōgo 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, 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.
Sasayamagnomus[3]	S. saegusai	Miyada microvertebrate site, Tamba-Sasayama	Albian	Skull elements and partial postcranial remains.	A neoceratopsian; closely related to Aquilops americanus.

Saurischians

Sauropods

Sauropods of the Ohyamashimo Formation
Genus	Species	Location	Stratigraphic position	Material	Notes	Images
Tambatitanis[2]	T. amicitiae	Kamitaki Bonebed Quarry, Tamba	Albian	A single partial skeleton.	A titanosauriform sauropod

Theropods

Indeterminate theropod dental remains are known from the Kamitaki Bonebed Quarry, Tamba.^[2]

Theropods of the Ohyamashimo Formation
Genus	Species	Location	Stratigraphic position	Material	Notes	Images
Hypnovenator[4]	H. matsubaraetoheorum	Nishikosa, Tamba-Sasayama	Albian	A single partial skeleton.	A troodontid theropod; closely related to Gobivenator mongoliensis.
Therizinosauria 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
Morohasaurus[5]	M. kamitakiensis	Kamitaki Egg Quarry, Tamba	Albian	A nearly complete left dentary.	A monstersaurian lizard
Pachygenys[6]	P. adachii	Miyada microvertebrate site, Tamba-Sasayama	Albian	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	Kamitaki Bonebed Quarry, Tamba	Albian	A nearly complete skeleton.	A frog
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

Mammals

Mammals of the Ohyamashimo Formation
Genus	Species	Location	Stratigraphic position	Material	Notes	Images
Sasayamamylos[8]	S. kawaii	Miyada microvertebrate site, Tamba-Sasayama	Albian	Three dentaries.	A asioryctitherian eutherian

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

Eggs of the Ohyamasimo Formation[9][10]
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

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.