The Bayanshiree Formation (also known as Baynshiree/Baynshire, Baynshirenskaya Svita, Baysheen Shireh, or Bayan Shireh) is a

geological formation A geological formation, or simply formation, is a body of rock having a consistent set of physical characteristics (lithology) that distinguishes it from adjacent bodies of rock, and which occupies a particular position in the layers of rock expo ...

Mongolia Mongolia is a landlocked country in East Asia, bordered by Russia to the north and China to the south and southeast. It covers an area of , with a population of 3.5 million, making it the world's List of countries and dependencies by po ...

, that dates to the

Cretaceous The Cretaceous ( ) is a geological period that lasted from about 143.1 to 66 mya (unit), million years ago (Mya). It is the third and final period of the Mesozoic Era (geology), Era, as well as the longest. At around 77.1 million years, it is the ...

period. It was first described and established by Vasiliev et al. 1959.

Description

The Bayanshiree Formation is primarily composed by varicoloured

claystones Mudrocks are a class of fine-grained Sedimentary rock#Clastic sedimentary rocks, siliciclastic sedimentary rocks. The varying types of mudrocks include siltstone, claystone, mudstone and shale. Most of the particles of which the stone is compo ...

and

sandstones Sandstone is a clastic sedimentary rock composed mainly of sand-sized (0.0625 to 2 mm) silicate grains, cemented together by another mineral. Sandstones comprise about 20–25% of all sedimentary rocks. Most sandstone is composed o ...

with calcareous concretions and characterized by grey mudstones and yellowish-brown medium grained sandstones. Up to thick, the most complete sections are found in the eastern

Gobi Desert The Gobi Desert (, , ; ) is a large, cold desert and grassland region in North China and southern Mongolia. It is the sixth-largest desert in the world. The name of the desert comes from the Mongolian word ''gobi'', used to refer to all of th ...

, consisting of fine-grained, often cross-stratified gray sandstone interbedded with claystone and concretionary, intraformational conglomerates with relatively thick units of red to brown

mudstone Mudstone, a type of mudrock, is a fine-grained sedimentary rock whose original constituents were clays or muds. Mudstone is distinguished from ''shale'' by its lack of fissility.Blatt, H., and R.J. Tracy, 1996, ''Petrology.'' New York, New York, ...

in the upper part. The Baynshire and Burkhant localities are mainly composed by mudstone,

siltstone Siltstone, also known as aleurolite, is a clastic sedimentary rock that is composed mostly of silt. It is a form of mudrock with a low clay mineral content, which can be distinguished from shale by its lack of fissility. Although its permeabil ...

, sandstone, and conglomerates, with most of their sedimentation being fluvial. The environments that were present on the Bayanshiree Formation consisted mainly on semi-arid climates with large water bodies, such as rivers or

lakes A lake is often a naturally occurring, relatively large and fixed body of water on or near the Earth's surface. It is localized in a basin or interconnected basins surrounded by dry land. Lakes lie completely on land and are separate from t ...

. Although it is considered to be partially

lacustrine A lake is often a naturally occurring, relatively large and fixed body of water on or near the Earth's surface. It is localized in a basin or interconnected basins surrounded by dry land. Lakes lie completely on land and are separate from t ...

, largescale cross-stratification in many of the sandstone layers at the Baynshire locality seem to indicate that a large

meander A meander is one of a series of regular sinuous curves in the Channel (geography), channel of a river or other watercourse. It is produced as a watercourse erosion, erodes the sediments of an outer, concave bank (cut bank, cut bank or river cl ...

ing

fluvial A river is a natural stream of fresh water that flows on land or inside caves towards another body of water at a lower elevation, such as an ocean, lake, or another river. A river may run dry before reaching the end of its course if it ru ...

system was present. It has been implied that during the late Bayan Shirehnian times, large

rivers A river is a natural stream of fresh water that flows on land or inside caves towards another body of water at a lower elevation, such as an ocean, lake, or another river. A river may run dry before reaching the end of its course if it ru ...

with direct connections to the

sea A sea is a large body of salt water. There are particular seas and the sea. The sea commonly refers to the ocean, the interconnected body of seawaters that spans most of Earth. Particular seas are either marginal seas, second-order section ...

drained a prominent part of the eastern Gobi region.

Stratigraphy

According to Jerzykiewicz and Russell, the Bayanshiree Formation can be divided into 2 informal units: upper and lower beds. Danilov and colleagues have suggested that the lower beds are Cenomanian to early Turonian, and the upper beds are late Turonian to Santonian in age. While the lower beds are composed by extensive conglomerate that indicates the ancient presence of very active rivers, the upper beds are mainly composed by mudstone and claystone that is

interbedded In geology, interbedding occurs when beds (layers of rock) of a particular lithology lie between or alternate with beds of a different lithology. For example, sedimentary rocks may be interbedded if there were sea level variations in their sedimen ...

by sandstone, indicating again, the presence of rivers but also lakes and other

water bodies A body of water or waterbody is any significant accumulation of water on the surface of Earth or another planet. The term most often refers to oceans, seas, and lakes, but it includes smaller pools of water such as ponds, wetlands, or more rare ...

. Based on comparisons with other formations, Jerzykiewicz and Russell suggested that the Bayanshiree paleofauna seems to correspond best with the

Turonian The Turonian is, in the International Commission on Stratigraphy, ICS' geologic timescale, the second age (geology), age in the Late Cretaceous epoch (geology), Epoch, or a stage (stratigraphy), stage in the Upper Cretaceous series (stratigraphy), ...

through early

Campanian The Campanian is the fifth of six ages of the Late Cretaceous epoch on the geologic timescale of the International Commission on Stratigraphy (ICS). In chronostratigraphy, it is the fifth of six stages in the Upper Cretaceous Series. Campa ...

stages of the Late Cretaceous, about 93 to 80

million years ago Million years ago, abbreviated as Mya, Myr (megayear) or Ma (megaannum), is a unit of time equal to (i.e. years), or approximately 31.6 teraseconds. Usage Myr is in common use in fields such as Earth science and cosmology. Myr is also used w ...

. However, examination of the

magnetostratigraphy Magnetostratigraphy is a geophysical correlation technique used to date sedimentary and volcanic sequences. The method works by collecting oriented samples at measured intervals throughout the section. The samples are analyzed to determine their ' ...

of the formation indicates that the entire Bayanshiree lies within the Cretaceous Long Normal, which lasted only until the end of the

Santonian The Santonian is an age in the geologic timescale or a chronostratigraphic stage. It is a subdivision of the Late Cretaceous Epoch or Upper Cretaceous Series. It spans the time between 86.3 ± 0.7 mya ( million years ago) and 83.6 ± 0.7 m ...

stage, giving a possible

Cenomanian The Cenomanian is, in the International Commission on Stratigraphy's (ICS) geological timescale, the oldest or earliest age (geology), age of the Late Cretaceous epoch (geology), Epoch or the lowest stage (stratigraphy), stage of the Upper Cretace ...

through Late Santonian age, between 98 and 83 million years ago. In 2012, Averianov and Sues re-examined many formations from the

and using

biostratigraphic Biostratigraphy is the branch of stratigraphy which focuses on correlating and assigning relative ages of rock strata by using the fossil assemblages contained within them.Hine, Robert. "Biostratigraphy." ''Oxford Reference: Dictionary of Biology ...

occurrences and previous dating, the Bayanshiree Formation was considered to be

in age. The lower beds dating to 98 million and 90 million years ago (early Cenomanian to late Turonian), and the upper beds dating to 90 million and 83 million years ago (late

to late Santonian).

Calcite Calcite is a Carbonate minerals, carbonate mineral and the most stable Polymorphism (materials science), polymorph of calcium carbonate (CaCO3). It is a very common mineral, particularly as a component of limestone. Calcite defines hardness 3 on ...

U–Pb measurements performed by Kurumada and colleagues in 2020 have estimated the age of the Bayanshiree Formation between 95.9 ± 6.0 Ma and 89.6 ± 4.0 Ma.

Correlations

A potential correlation between the

Iren Dabasu Formation The Iren Dabasu Formation (also known as Erlian Formation) is a Late Cretaceous geologic Formation (geology), formation in the Erenhot, Iren Nor region of Inner Mongolia. Dinosaur remains diagnostic to the genus level are among the fossils that h ...

has been long suggested by most authors, mainly based on the highly similar vertebrate assemblages. However, Van Itterbeeck et al. 2005 argued against this correlation concluding that instead, the Iren Dabasu Formation was coeval with the younger

Nemegt Formation The Nemegt Formation (also known as Nemegtskaya Svita) is a geological formation in the Gobi Desert of Mongolia, dating to the Late Cretaceous. The formation consists of river channel sediments and contains fossils of fish, turtles, crocodilians ...

based on the charophyte and ostracode assemblages; therefore, these formations were dated to the Late Campanian-Early Maastrichtian. Averianov and Sues instead proposed a correlation between the Bayanshiree, Iren Dabasu and Bissekty formations. In 2015, Tsuihiji and team found the Iren Dabasu-Nemegt correlation to be inconsistent since the microfossil assemblages used by Van Itterbeeck and colleagues were not restricted to the Maastrichtian period and the similarities between these assemblages were most probably due to a similar deposition and climate settings.

Paleobiota of the Bayanshiree Formation

In terms of

biodiversity Biodiversity is the variability of life, life on Earth. It can be measured on various levels. There is for example genetic variability, species diversity, ecosystem diversity and Phylogenetics, phylogenetic diversity. Diversity is not distribut ...

therizinosaurs Therizinosaurs (; once called segnosaurs) are an extinct group of large herbivorous theropod dinosaurs whose fossils have been mainly discovered from Cretaceous deposits in Asia and North America. Potential fragmentary remains have also been foun ...

and

turtles Turtles are reptiles of the order Testudines, characterized by a special shell developed mainly from their ribs. Modern turtles are divided into two major groups, the Pleurodira (side necked turtles) and Cryptodira (hidden necked turtle ...

were the most abundant vertebrates across the formation, as evidenced on numerous remains. Nevertheless, hadrosauroids were fairly abundant too, particularly at the Baynshire locality with numerous remains unearthed from this area and a new unnamed hadrosauroid. In addition, most specimens of ''

Gobihadros ''Gobihadros'' is a genus of basal hadrosauroid dinosaur that lived in Asia during the Late Cretaceous period in what is now the Bayan Shireh Formation (Cenomanian-Santonian). It contains only the type species ''Gobihadros mongoliensis''. It has ...

'' come from this locality. Also,

niche partitioning In ecology, a niche is the match of a species to a specific environmental condition. Three variants of ecological niche are described by It describes how an organism or population responds to the distribution of resources and competitors (for e ...

has been reported within the therizinosaurids '' Erlikosaurus'' and ''

Segnosaurus ''Segnosaurus'' is a genus of therizinosaurid dinosaur that lived in what is now southeastern Mongolia during the Late Cretaceous, about 102–86 million years ago. Multiple incomplete but well-preserved specimens were discovered in the Go ...

'', and the

ankylosaurs Ankylosauria is a group of herbivorous dinosaurs of the clade Ornithischia. It includes the great majority of dinosaurs with Armour (zoology), armor in the form of bony osteoderms, similar to turtles. Ankylosaurs were bulky quadrupeds, with short ...

Talarurus ''Talarurus'' ( ; meaning "basket tail" or "wicker tail") is a genus of ankylosaurid dinosaur that lived in Asia during the Late Cretaceous period, about 96 million to 89 million years ago. The first remains of ''Talarurus'' were discovered in 1 ...

'' and '' Tsagantegia''.

Mammals A mammal () is a vertebrate animal of the class Mammalia (). Mammals are characterised by the presence of milk-producing mammary glands for feeding their young, a broad neocortex region of the brain, fur or hair, and three middle e ...

however, are extremely uncommon; '' Tsagandelta'' is the only mammal described so far. Besides vertebrate fossils, abundant fossil

fruits In botany, a fruit is the seed-bearing structure in flowering plants (angiosperms) that is formed from the ovary after flowering. Fruits are the means by which angiosperms disseminate their seeds. Edible fruits in particular have long propaga ...

have been collected from the Bor Guvé and Khara Khutul localities and they are especially abundant at Bor Guvé. Although they resemble ''

Abelmoschus esculentus Okra (, ), ''Abelmoschus esculentus'', known in some English-speaking countries as lady's fingers, is a flowering plant in the mallow family native to East Africa. Cultivated in tropical, subtropical, and warm temperate regions around the wo ...

'' their taxonomic position remains unclear and further examination is required. Bayan Shireh Formation Fauna

Crocodylomorphs

Fish

Flora

Mammals

Pterosaurs

Turtles

Dinosaurs

Ornithischians

= Ankylosaurs

= Ceratopsians

= Hadrosaurs

= Pachycephalosaurs

Sauropods

Theropods

= Dromaeosaurids

= Ornithomimosaurs

= Oviraptorosaurs

= Therizinosaurs

= Troodontids

= Tyrannosauroids

= Other Theropods

References

{{Reflist, refs= {{cite book, last=Benton, first=M.J., author-link=Michael J. Benton, chapter=Mongolian Place Names and Stratigraphic Terms, pages=xxii-xxviii, editor-last=Benton, editor-first=M.J., editor2-last=Shishkin, editor2-first=M.A., editor3-last=Unwin, editor3-first=D.M., editor4-last=Kurochkin, editor4-first=E.N., title=The Age of Dinosaurs in Russia and Mongolia, publisher=Cambridge University Press, isbn=0-521-55476-4 Geologic formations of Mongolia Upper Cretaceous Series of Asia Cenomanian Stage Turonian Stage Coniacian Stage Santonian Stage Sandstone formations Shale formations Fluvial deposits Lacustrine deposits Ichnofossiliferous formations Ooliferous formations Fossiliferous stratigraphic units of Asia Paleontology in Mongolia