The Bayanshiree Formation (also known as Baynshiree/Baynshire, Baynshirenskaya Svita, Baysheen Shireh, or Bayan Shireh) is a geological formation in Mongolia, that dates to the Cretaceous period. It was first described and established by Vasiliev et al. 1959.
Description The Bayanshiree Formation is primarily composed by varicoloured claystones and sandstones 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, 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 in the upper part. The Baynshire and Burkhant localities are mainly composed by mudstone, siltstone, 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. Although it is considered to be partially lacustrine, largescale cross-stratification in many of the sandstone layers at the Baynshire locality seem to indicate that a large meandering fluvial system was present. It has been implied that during the late Bayan Shirehnian times, large rivers with direct connections to the sea 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 by sandstone, indicating again, the presence of rivers but also lakes and other water bodies.
Based on comparisons with other formations, Jerzykiewicz and Russell suggested that the Bayanshiree paleofauna seems to correspond best with the Turonian through early Campanian stages of the Late Cretaceous, about 93 to 80 million years ago. However, examination of the magnetostratigraphy of the formation indicates that the entire Bayanshiree lies within the Cretaceous Long Normal, which lasted only until the end of the Santonian stage, giving a possible Cenomanian through Late Santonian age, between 98 and 83 million years ago. In 2012, Averianov and Sues re-examined many formations from the Gobi Desert and using biostratigraphic occurrences and previous dating, the Bayanshiree Formation was considered to be Cenomanian to Santonian 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 Turonian to late Santonian). Calcite 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 has been long suggested by most authors, mainly based on the highly similar vertebrate assemblages. Translated paper However, Van Itterbeeck et al. 2005 argued against this correlation concluding that instead, the Iren Dabasu Formation was coeval with the younger Nemegt Formation 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, therizinosaurs and turtles 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 come from this locality. Also, niche partitioning has been reported within the therizinosaurids Erlikosaurus and Segnosaurus, and the ankylosaurs Talarurus and Tsagantegia. Mammals however, are extremely uncommon; Tsagandelta is the only mammal described so far. Besides vertebrate fossils, abundant fossil fruits have been collected from the Bor Guvé and Khara Khutul localities and they are especially abundant at Bor Guvé. Although they resemble Abelmoschus esculentus their taxonomic position remains unclear and further examination is required.
Crocodylomorphs Crocodylomorphs reported from the Bayanshiree Formation Genus Species Location Stratigraphic Position Material Notes ImagesParalligatorP. gradilifrons Sheeregeen Gashoon "Complete skull with fragmentary postcrania." A paralligatorid. P. major Sheeregeen Gashoon "Fragmentary crania." A paralligatorid. Unnamed crocodylomorph ichnotaxon Indeterminate Shine Us Khuduk "Swim tracks." Tracks made underwater by a swimming individual.
Fish Sharks reported from the Bayanshiree Formation Genus Species Location Stratigraphic Position Material Notes Images Hybodus H. kansaiensis Bayshin Tsav "Teeth and postcrania." A hybodontid. Osteichthyes spp. Indeterminate "Unspecified material." Bony fish.
Flora Flora reported from the Bayanshiree Formation Genus Species Location Stratigraphic Position Material Notes Images Bothrocaryum B. gobience Khara Khutul "Unspecified material." A cornacean. Nyssoidea N. mongolica Khara Khutul "Unspecified material." A cornacean. Angiosperm spp. Indeterminate Bor Guvé, Khara Khutul "Fossil fruits." Very abundant at Bor Guvé.TaxodiumT. spKhara KhutulA relative of the Bald Cypress
Mammals Deltatheroidans reported from the Bayanshiree Formation Genus Species Location Stratigraphic Position Material Notes Images Tsagandelta T. dashzevegi Tsagan Tsonj "Partial left dentary." A deltatheroidan. Bayshinoryctes B. shuvalovi "Dental remains" A stem placental Ravjaa R. ishiii "Lower jaw bones" A zhelestid eutherian
Pterosaurs Pterosaurs reported from the Bayanshiree Formation Genus Species Location Stratigraphic Position Material Notes Images Tsogtopteryx T. mongoliensis Bayshin Tsav "Middle cervical vertebra." An azhdarchid. Gobiazhdarcho G. tsogtbaatari Burkhant "Four cervical vertebrae." An azhdarchid.
Turtles Turtles reported from the Bayanshiree Formation Genus Species Location Stratigraphic Position Material Notes Images Charitonyx C. tajanikolaevae Khara Khutul "Fragmented postcrania." A nanhsiungchelyid. Gobiapalone G. orlovi Baynshire, Burkhant, Khongil Tsav "Skull and postcrania." A trionychid. Hanbogdemys H. orientalis Khara Khutul "Fragmented cervical remains." A nanhsiungchelyid. Kharakhutulia K. kalandadzei Khara Khutul Lower beds "Fragmented postcrania from multiple specimens." A nanhsiungchelyid. Kirgizemys sp. Indeterminate Khara Khutul "Fragmented postcrania." A nanhsiungchelyid. Kizylkumemys K. schultzi Khara Khutul Lower beds "Fragmented postcrania." A nanhsiungchelyid. Lindholmemys L. martinsoni Sheeregeen Gashoon, Usheen Khuduk Upper beds "Fragmentary shell." A lindholmemydid. "Trionyx" T. baynshirensis Baynshire "Fragmentary shell." A trionychine.Pan-CarettochelysIndeterminateUryl'b Usu Locality, Dornogovi ProvinceLower BedsShell fragmentsA stem-carettochelyid, originally assigned to the species "Anosteira" shuwalovi
Dinosaurs
Ornithischians Ankylosaurs Ankylosaurs reported from the Bayanshiree Formation Genus Species Location Stratigraphic Position Material Notes Images Amtosaurus A. magnus Amtgai "Partial braincase." An ankylosaurid now regarded as nomen dubium. Maleevus M. disparoserratus Sheeregeen GashoonUpper beds "Partial maxillae and a referred braincase." An ankylosaurid now regarded as nomen dubium. Talarurus T. plicatospineus Bayshin Tsav, Baynshire, Shine Us Khuduk "Multiple specimens including partial to nearly complete skulls and postcranial remains." An ankylosaurid. Tsagantegia T. longicranialis Tsagan Teg "Skull lacking lower jaws." An ankylosaurid.
Ceratopsians Ceratopsians reported from the Bayanshiree Formation Genus Species Location Stratigraphic Position Material Notes Images Graciliceratops G. mongoliensis Sheeregeen GashoonUpper beds "Fragmented skull and postcrania." A neoceratopsian originally identified as Microceratus. Unnamed neoceratopsian Indeterminate Bayshin Tsav "Left maxilla and teeth." A neoceratopsian.
Hadrosaurs Hadrosaurs reported from the Bayanshiree Formation Genus Species Location Stratigraphic Position Material Notes Images Amblydactylus spp. Indeterminate Baynshire "Five three-toed footprints." Ornithopod tracks. Gobihadros G. mongoliensis Bayshin Tsav, Baynshire, Char Teeg, Khongil Tsav, Khoorai Tsav Upper beds "Multiple specimens with a virtually complete skull and postcranial remains". A hadrosauroid. Hadrosauroidea indet. Indeterminate Shine Us Khuduk "Isolated surangular". A hadrosauroid. Ovaloolithidae indet. Indeterminate Mogoyn Ulagiyn Khaets "Eggs, shells and egg clutches." Eggs probably laid by a hadrosaur. Spheroolithus sp. Indeterminate Sheeregeen Gashoon "Egg clutch." Eggs probably laid by a hadrosaur. Undescribed Hadrosauroidea Indeterminate Amtgai, Bayshin Tsav, Bayshin Tsav IV, Khoorai Tsav "Multiple postcranial remains including an articulated skeleton." Hadrosauroid remains. Undescribed hadrosauroids Indeterminate Baynshire "Remains of at least two juvenile hadrosauroids." Juvenile hadrosauroids in a caenagnathoid nesting site. Unnamed hadrosauroid Indeterminate Baynshire, Char Teeg Upper beds "Partial right dentary and sparse postcranial remains." A hadrosauroid slightly smaller than Gobihadros. Unnamed hadrosauroid Indeterminate Bayshin Tsav "Nearly complete skeleton from a juvenile individual." A hadrosauroid distinct from Gobihadros. Pachycephalosaurs Pachycephalosaurs reported from the Bayanshiree Formation Genus Species Location Stratigraphic Position Material Notes Images Amtocephale A. gobiensis Amtgai "Nearly complete frontoparietal dome." A primitive pachycephalosaurid.
Sauropods Sauropods reported from the Bayanshiree Formation Genus Species Location Stratigraphic Position Material Notes Images Erketu E. ellisoni Bor Guvé "Vertebrae and postcrania." A somphospondylan. Sauropoda indet. Indeterminate Amtgai "Partial vertebrae and pelvic elements." A sauropod. Unnamed sauropod Indeterminate Shine Us Khuduk "Articulated skeleton." A sauropod. Unnamed sauropod ichnogenus Indeterminate Khavirgiin Dzo "Four consecutive hind prints. Some Prints are Giant, at 106 cm in length. Sauropod tracks. Titanosauria indet. IndeterminateBor Guvé "Vertebrae." Femur. Specimen was large already and was still subadult. A titanosaur.
Theropods Dromaeosaurids Dromaeosaurids reported from the Bayanshiree Formation Genus Species Location Stratigraphic Position Material Notes Images Achillobator A. giganticus Burkhant "Maxilla and fragmentary postcrania." A giant dromaeosaurid. Dromaeosauridae indet. Indeterminate Burkhant "Claw and large postcranial remains." A dromaeosaurid. Unnamed velociraptorine Indeterminate Shine Us Khuduk, Tel Ulan Chaltsai "Fragmentary crania and postcrania from two specimens." A dromaeosaurid.
Ornithomimosaurs