The Kitadani Formation ( ja, 北谷層 ''Kitadani-sō'') is a unit of Lower Cretaceous sedimentary rock which crops out near the city of Katsuyama in Fukui Prefecture,

Japan Japan ( ja, 日本, or , and formally , ''Nihonkoku'') is an island country in East Asia. It is situated in the northwest Pacific Ocean, and is bordered on the west by the Sea of Japan, while extending from the Sea of Okhotsk in the north ...

, and it is the primary source of Cretaceous-aged non-marine vertebrate fossils in Japan. Dinosaur remains are among the fossils that have been recovered from the formation, but it also preserves a diverse assemblage of plants, invertebrates, and other vertebrates.Weishampel, D.B.; et al. (2004). "Dinosaur distribution (Early Cretaceous, Asia)." In: Weishampel, D.B.; Dodson, P.; and Osmólska, H. (eds.): The Dinosauria, 2nd, Berkeley: University of California Press. Pp. 563-570. . Most, if not all, of the fossil specimens collected from the Kitadani Formation are reposited at the

Fukui Prefectural Dinosaur Museum The , located in Katsuyama, Fukui, Japan, is one of the leading dinosaur museums in Asia that is renowned for its exhibits of fossil specimens of dinosaurs and paleontological research. It is sited in the Nagaoyama Park ( Katsuyama Dinosaur For ...

. The Kitadani Formation is a unit within the

Tetori Group The Tetori Group is a stratigraphic group in Japan, found within several basins in and around Fukui Prefecture. It is Early Cretaceous in age. It primarily consists of freshwater continental deposits, with some beds of volcanic tuffite. It primari ...

, a major sequence of Lower Cretaceous rocks that is distributed across Fukui, Ishikawa, and

Gifu is a city located in the south-central portion of Gifu Prefecture, Japan, and serves as the prefectural capital. The city has played an important role in Japan's history because of its location in the middle of the country. During the Sengoku ...

prefectures of western-central Honshu. The Tetori Group exhibits marked lateral variation, and the Kitadani Formation is only present in Fukui Prefecture. The Kitadani Formation comprises interbedded tuffs, sandstones, and

shale Shale is a fine-grained, clastic sedimentary rock formed from mud that is a mix of flakes of clay minerals (hydrous aluminium phyllosilicates, e.g. kaolin, Al2 Si2 O5( OH)4) and tiny fragments (silt-sized particles) of other minerals, especial ...

s and reaches a maximum thickness of approximately one hundred meters (~328 feet). It conformably overlies the

Akaiwa Formation The Akaiwa Formation ( ja, 赤岩層, translit=Akaiwa-sō) is an Early Cretaceous (Hauterivian-Barremian) geologic formation in central Honshu, Japan.Omichidani Formation. The Kitadani Formation is significant because it is the major source of dinosaur fossils in Japan and because of Japan's unique position along the northeastern margin of Eurasia during the Early Cretaceous.

Geology

The Kitadani Formation is a unit within the

, a Lower Cretaceous sequence of predominantly sedimentary rock which crops out in the

Fukui is a Japanese name meaning "fortunate" or sometimes "one who is from the Fukui prefecture". It may refer to: Places * Fukui Domain, a part of the Japanese han system during the Edo period * Fukui Prefecture, a prefecture of Japan located in ...

, Ishikawa, and

prefectures of west-central Honshu, Japan in the region surrounding Mount Haku. The formations present within the Tetori Group vary laterally, and the Kitadani Formation crops out only in the Kuzuryū River district of Fukui Prefecture. In this region, the sequence comprises, in ascending stratigraphic order: Gomijima Formation, the Kuwajima Formation, the

Akaiwa Formation The Akaiwa Formation ( ja, 赤岩層, translit=Akaiwa-sō) is an Early Cretaceous (Hauterivian-Barremian) geologic formation in central Honshu, Japan. The Kitadani Formation comprises alternating horizons of red-brown tuffs, blackish

s and sandstones, and thin coal beds. The sandstones within the Kitadani Formation are light gray and green and range in clast size from fine to coarse. The type section of the Kitadani Formation occurs along the Nakanomatadani branch of the near the city of Katsuyama, where it is approximately 100 m (~328 feet) in thickness. The Kitadani Formation conformably overlies the Akaiwa Formation and is unconformably overlain by the Omichidani Formation. The Kitadani Formation has had varying nomenclature throughout the history of its study. In the early stratigraphic literature on the Tetori Group, the Kitadani Formation was variably referred to as the "Lower part of the Omichidani Formation", the "Chinaboradani Alternation of Tuff, Shale, and Sandstone", the "Kitadani Alternation of Sandstone, Shale, and Tuff", and simply the "Kitadani Alternation" prior to its designation as a formation.

Age

The Kitadani Formation was biostratigraphically dated to the late Barremian and early

Aptian The Aptian is an age in the geologic timescale or a stage in the stratigraphic column. It is a subdivision of the Early or Lower Cretaceous Epoch or Series and encompasses the time from 121.4 ± 1.0 Ma to 113.0 ± 1.0 Ma (million years ago), a ...

ages of the Early Cretaceous Epoch in 2002 based upon the presence of the freshwater

bivalve Bivalvia (), in previous centuries referred to as the Lamellibranchiata and Pelecypoda, is a class of marine and freshwater molluscs that have laterally compressed bodies enclosed by a shell consisting of two hinged parts. As a group, bival ...

''Nippononaia ryosekiana''. In 2005, part of the Kitadani Formation was biostratigraphically dated to the Barremian Age based upon the occurrence of the charophyte gyrogonite ''Clavator harrisii reyi'' in association with other charophytes. These biostratigraphic age assignments are supported by zircon fission track radioisotopic ages of tuff, which date the Kitadani Formation to 127-115 Ma.

Fossil assemblage

The Kitadani Formation preserves a diverse assemblage of plant fossils; invertebrate fossils; and vertebrate body and trace fossils, including mammals, turtles, neosuchian reptiles, and

dinosaurs Dinosaurs are a diverse group of reptiles of the clade Dinosauria. They first appeared during the Triassic period, between 243 and 233.23 million years ago (mya), although the exact origin and timing of the evolution of dinosaurs is t ...

. Many vertebrate specimens from the Kitadani Formation are incomplete and poorly preserved, so taxonomic diversity is likely higher than it seems.

Plant fossils

The plant fossil assemblage of the Kitadani Formation is characterized by a rarity of ferns and an abundance of cycadales and

conifers Conifers are a group of cone-bearing seed plants, a subset of gymnosperms. Scientifically, they make up the division Pinophyta (), also known as Coniferophyta () or Coniferae. The division contains a single extant class, Pinopsida. All extan ...

represented mostly by cones and shoots. A palynological study in 2013 resulted in the identification of greater than 40 species of spores, pollen grains, and plant fragments from the Kitadani Formation representing gymnosperms, freshwater algae, and epiphyllous fungus; however, no angiosperm pollen was identified. Branches of the conifer ''Brachyphyllum obesum'' have been recovered, which was interpreted to represent the warming and possible drying of the climate toward the upper Tetori Group. This interpretation is supported by the lack of plants from lower in the Tetori Group, such as ginkgos, in the Kitadani Formation.

Invertebrate fossils

The invertebrate fossil assemblage of the Kitadani Formation mostly comprises

freshwater Fresh water or freshwater is any naturally occurring liquid or frozen water containing low concentrations of dissolved salts and other total dissolved solids. Although the term specifically excludes seawater and brackish water, it does include ...

and brackish water

and

gastropod The gastropods (), commonly known as snails and slugs, belong to a large taxonomic class of invertebrates within the phylum Mollusca called Gastropoda (). This class comprises snails and slugs from saltwater, from freshwater, and from land. T ...

mollusks Mollusca is the second-largest phylum of invertebrate animals after the Arthropoda, the members of which are known as molluscs or mollusks (). Around 85,000 extant species of molluscs are recognized. The number of fossil species is esti ...

Vertebrate fossils

Mammalia

At least three mammalian taxa have been recovered from the Kitadani Formation, represented by rare teeth and partial jaws. In 2004, the

spalacotheriid Spalacotheriidae is a family of extinct mammals belonging to the paraphyletic group ' Symmetrodonta'. They lasted from the Early Cretaceous to the Campanian in North America, Europe, Asia and North Africa. Spalacotheriids are characterised by ha ...

symmetrodont Symmetrodonta is a group of Mesozoic mammals and mammal-like synapsids characterized by the triangular aspect of the molars when viewed from above, and the absence of a well-developed talonid. The traditional group of 'symmetrodonts' ranges in a ...

" ''Symmetrolestes parvus'' was reported based upon a fragmentary right mandible with the first incisor and five postcanine teeth preserved. Two non-therian mammal specimens were reported in 2015, including an

eobaatarid Eobaataridae is a family of fossil mammals within the order Multituberculata. Remains are known from the Lower Cretaceous of Europe and Asia. They are among the most derived representatives of the informal suborder "Plagiaulacida", and closely re ...

multituberculate and a

triconodontid Triconodontidae is an extinct family of small, carnivorous mammals belonging to the order Eutriconodonta, endemic to what would become Asia, Europe, North America and probably also Africa and South America during the Jurassic through Cretaceous p ...

eutriconodontan Eutriconodonta is an order of early mammals. Eutriconodonts existed in Asia, Africa, Europe, North and South America during the Jurassic and the Cretaceous periods. The order was named by Kermack ''et al.'' in 1973 as a replacement name for the p ...

. These specimens were noted as possessing non-tribosphenic dentition, and were interpreted being taxonomically distinct to closely related taxa from elsewhere in the Tetori Group, but neither specimen was named. The eobaatarid is represented only by a damaged left

premolar The premolars, also called premolar teeth, or bicuspids, are transitional teeth located between the canine and molar teeth. In humans, there are two premolars per quadrant in the permanent set of teeth, making eight premolars total in the mouth ...

. The triconodontid is represented by a partial right dentary possessing a faint

Meckelian groove The Meckelian groove (or Meckel's groove, Meckelian fossa, or Meckelian foramen, or Meckelian canal) is an opening in the medial (inner) surface of the mandible (lower jaw) which exposes the Meckelian cartilage.Adocus ''Adocus'' is an extinct genus of aquatic turtles belonging to the family Adocidae. ''Adocus'' was once considered to belong to the family Dermatemyidae. Description Species of the genus ''Adocus'' had flattened and smoothly contoured shells w ...

'', ''

Basilemys ''Basilemys'' () is a large, terrestrial trionychoid turtle from the Upper Cretaceous. In Greek, the word "Basil" means royal or kingly and the word "Emys" means turtle. Therefore, ''Basilemys'' means King Turtle. ''Basilemys'' has been found in r ...

'', and Trionychidae. A more recent evaluation of all 700+ turtle specimens from the Kitadani Formation was conducted in 2015. In that study, the authors concluded that the turtle assemblage of the Kitadani Formation consists of four trionychoid taxa and two other eucryptodires. In addition to the genera reported in 2002, ''Perochelys'', ''Gobiapalone'', ''Apalonina'', and an unnamed nanhsiungchelyid were identified.

Eusuchia

A nearly complete skeleton of a

goniopholidid Goniopholididae is an extinct family of moderate-sized semi-aquatic neosuchian crocodyliformes. Their bodyplan and morphology are convergent on living crocodilians. They lived across Laurasia (Asia, Europe and North America) between the Middle Ju ...

eusuchian has been noted from the Kitadani Formation, but this material remains formally unpublished and unnamed. This specimen was discovered in 1982, and it was hypothesized to be closely related to ''

Sunosuchus ''Sunosuchus'' is an extinct genus of goniopholidid mesoeucrocodylian. Fossils are known from China, Kyrgyzstan, and Thailand and are Jurassic in age, although some may be Early Cretaceous. Four species are currently assigned to the genus: the ...

'', ''

Goniopholis ''Goniopholis'' (meaning "angled scale") is an extinct genus of goniopholidid crocodyliform that lived in Europe and Africa during the Late Jurassic and Early Cretaceous. Being semi-aquatic it is very similar to modern crocodiles. It ranged from ...

'', and ''

Eutretauranosuchus ''Eutretauranosuchus'' is an extinct genus of goniopholidid crocodyliform. ''E. delfsi'' is the only known species within the genus. Discovery and history The holotype, ''Eutretauranosuchus delfsi'', was discovered by Edwin Delfs in 1957 amo ...

'' based on a preliminary phylogenetic analysis.

Dinosauria

Dinosaurs are among the most well-known vertebrate taxa from the Kitadani Formation. Taxa from all three major dinosaurian clades —

Theropoda Theropoda (; ), whose members are known as theropods, is a dinosaur clade that is characterized by hollow bones and three toes and claws on each limb. Theropods are generally classed as a group of saurischian dinosaurs. They were ancestrally ca ...

, Sauropodomorpha, and

Ornithischia Ornithischia () is an extinct order of mainly herbivorous dinosaurs characterized by a pelvic structure superficially similar to that of birds. The name ''Ornithischia'', or "bird-hipped", reflects this similarity and is derived from the Greek s ...

— have been recovered. Among theropods, two taxa have been named, with additional indeterminate dromaeosaurid and other theropod material also preserved. '' Fukuiraptor kitadanensis'' was reported in 2000 based upon a partial skull and associated partial postcranium. The phylogenetic relationships of ''Fukuiraptor'' are uncertain. Although confidently assigned to the Megaraptora, various phylogenetic analyses have suggested that megaraptorans may either be

allosauroids Carnosauria is an extinct large group of predatory dinosaurs that lived during the Jurassic and Cretaceous periods. Starting from the 1990s, scientists have discovered some very large carnosaurs in the carcharodontosaurid family, such as '' G ...

tyrannosauroids Tyrannosauroidea (meaning 'tyrant lizard forms') is a superfamily (or clade) of coelurosaurian theropod dinosaurs that includes the family Tyrannosauridae as well as more basal relatives. Tyrannosauroids lived on the Laurasian supercontinent be ...

or non-tyrannosauroid coelurosaurs. ''

Fukuivenator paradoxus ''Fukuivenator'' ("hunter of Fukui Prefecture") is an extinct genus of therizinosaurian theropod dinosaur from the Early Cretaceous of Japan. Discovery and naming The type species ''Fukuivenator paradoxus'' was in 2016 named and descri ...

'' was reported in 2016, and it is interpreted to be an early-diverging maniraptoran. However, its unique combination of ancestral and derived features associated with several different coelurosaurian clades precludes referral to a more exclusive clade than Maniraptora. The

titanosaurian Titanosaurs (or titanosaurians; members of the group Titanosauria) were a diverse group of sauropod dinosaurs, including genera from all seven continents. The titanosaurs were the last surviving group of long-necked sauropods, with taxa still thr ...

sauropod dinosaur ''

Fukuititan nipponensis ''Fukuititan'' (meaning "Fukui giant") is a genus of titanosauriform sauropod dinosaur that lived in the Early Cretaceous (either Barremian or Aptian age) in what is now Japan. It is known from FPDM-V8468, the associated partial skeleton o ...

'' was reported from the Kitadani Formation in 2010 based upon a single partial skeleton including cranial and postcranial material. Other indeterminate sauropod material is also preserved. Ornithischian dinosaurs are represented in the Kitadani Formation by two named taxa and other indeterminate specimens, including an unnamed psittacosaurid ceratopsian. ''

Fukuisaurus tetoriensis ''Fukuisaurus'' (meaning "Fukui lizard") is a genus of herbivorous ornithopod dinosaur that lived during the Early Cretaceous in what is now Japan. The type species is ''F. tetoriensis'', which was named and described in 2003. Discovery and ...

'' was named in 2003 based upon sparse cranial material, but more complete specimens have been recovered in the time since. ''Fukuisaurus'' is interpreted to be a non-

hadrosauroid Hadrosauroidea is a clade or superfamily of ornithischian dinosaurs that includes the "duck-billed" dinosaurs, or hadrosaurids, and all dinosaurs more closely related to them than to ''Iguanodon''. Their remains have been recovered in Asia, Eu ...

hadrosauriform Ankylopollexia is an extinct clade of ornithischian dinosaurs that lived from the Late Jurassic to the Late Cretaceous. It is a derived clade of iguanodontian ornithopods and contains the subgroup Styracosterna. The name stems from the Greek word ...

., Another hadrosauriform ''

Koshisaurus katsuyama ''Koshisaurus'' is a monospecific genus of basal hadrosauroid from the Kitadani Formation in Japan. The discovery of the genus suggests that hadrosauroids had higher diversity along the eastern margin of Asia in the Early Cretaceous. "Koshi" mea ...

'' was reported in 2015 and interpreted to be an early-branching hadrosauroid, not especially closely related to ''Fukuisaurus''. Additionally, non-avialan theropod, avialan theropod, sauropod, ornithischian tracks have been reported from the Kitadani Formation. Dinosaur egg fragments have also been reported from the Kitadani Formation, including '' Plagioolithus fukuensis'', a three-layered eggshell interpreted as a fossil

avian Avian may refer to: *Birds or Aves, winged animals *Avian (given name) (russian: Авиа́н, link=no), a male forename Aviation *Avro Avian, a series of light aircraft made by Avro in the 1920s and 1930s *Avian Limited, a hang glider manufacture ...

egg. If indeed avian, ''Plagioolithus'' would represent the oldest known fossil bird egg.

References

{{Reflist Geologic formations of Japan Lower Cretaceous Series of Asia Aptian Stage Barremian Stage Sandstone formations Shale formations Tuff formations Coal formations Coal in Japan Fluvial deposits Ichnofossiliferous formations Ooliferous formations Geography of Fukui Prefecture