# Bayan Shireh Formation

The Bayan Shireh Formation (also known as Baynshiree/Baynshire, Baynshirenskaya Svita or Baysheen 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 Bayan Shireh 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 300 m (980 ft) 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 Bayan Shireh 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 Bayan Shireh 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 Bayan Shireh 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 Bayan Shireh 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 Bayan Shireh 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 Bayan Shireh 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. 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 Bayan Shireh, 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 Bayan Shireh 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.
| 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. |

### Crocodylomorphs

Crocodylomorphs reported from the Bayan Shireh Formation :
| Genus | Species | Location | Material | Notes |
| Paralligator | P. gradilifrons | Sheeregeen Gashoon | "Complete skull with fragmentary postcrania." | A paralligatorid. |
| Paralligator | 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 Bayan Shireh 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 Bayan Shireh Formation :
| Genus | Species | Location | Material | Notes |
| 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. |

### Mammals

Deltatheroidans reported from the Bayan Shireh Formation :
| Genus | Species | Location | Stratigraphic Position | Material | Notes | Images |
| Tsagandelta | T. dashzevegi | Tsagan Tsonj | | "Partial left dentary." | A deltatheroidan. | |

### Pterosaurs

Pterosaurs reported from the Bayan Shireh Formation :
| Genus | Species | Location | Stratigraphic Position | Material | Notes | Images |
| Azhdarchidae indet. | Indeterminate | Bayshin Tsav | | "Middle cervical vertebra." | An azhdarchid. | |
| Azhdarchidae indet. | Indeterminate | Burkhant | | "Four cervical vertebrae." | An azhdarchid. | |

### Turtles

Turtles reported from the Bayan Shireh Formation :
| Genus | Species | Location | Stratigraphic Position | Material | Notes |
| 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-Carettochelys | Indeterminate | Uryl’b Usu Locality, Dornogovi Province | Lower Beds | Shell fragments | A stem-carettochelyid, originally assigned to the species "Anosteira" shuwalovi |

### Dinosaurs
#### Ornithischians
##### Ankylosaurs

Ankylosaurs reported from the Bayan Shireh Formation :
| Genus | Species | Location | Stratigraphic Position | Material | Notes |
| Amtosaurus | A. magnus | Amtgai | | "Partial braincase." | An ankylosaurid now regarded as nomen dubium. |
| Maleevus | M. disparoserratus | Sheeregeen Gashoon | Upper 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 Bayan Shireh Formation :
| Genus | Species | Location | Stratigraphic Position | Material | Notes | Images |
| Graciliceratops | G. mongoliensis | Sheeregeen Gashoon | Upper 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 Bayan Shireh Formation :
| Genus | Species | Location | Stratigraphic Position | Material | Notes |
| 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 Bayan Shireh 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 Bayan Shireh Formation :
| Genus | Species | Location | Material | Notes |
| 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 footprints." | Sauropod tracks. |
| Unnamed titanosaur | Indeterminate | Bor Guvé | "Vertebrae." | A titanosaur. |

#### Theropods
##### Dromaeosaurids

Dromaeosaurids reported from the Bayan Shireh Formation :
| Genus | Species | Location | Material | Notes |
| 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

Ornithomimosaurs reported from the Bayan Shireh Formation :
| Genus | Species | Location | Material | Notes |
| "Gallimimus" | "G. mongoliensis" | Bayshin Tsav | "Complete skull with postcrania only lacking the caudal region." | An informally named ornithomimid that actually differs from Gallimimus. |
| Garudimimus | G. brevipes | Bayshin Tsav | "Skull with fragmented postcrania." | An ornithomimosaur. |
| Undescribed ornithomimosaur | Indeterminate | Amtgai | "Nearly 1 m (3.3 ft) long ulna." | A large ornithomimosaur. |
| Undescribed ornithomimosaurs | Indeterminate | Baynshire, Bayshin Tsav, Khongil Tsav, Sheeregeen Gashoon, Shine Us Khuduk | "More than five specimens and a bonebed containing mainly postcranial elements." | Multiple ornithomimosaurian taxa. |

##### Oviraptorosaurs

Oviraptorosaurs reported from the Bayan Shireh Formation :
| Genus | Species | Location | Material | Notes |
| Caenagnathoidea indet. | Indeterminate | Baynshire | "Nests including elongatoolithid eggs and associated juveniles." | A large caenagnathoid nesting site. |
| Macroelongatoolithus indet. | Indeterminate | Baynshire | "Eggs within a large nest." | Eggs probably laid by a large oviraptorosaur. |
| Unnamed caenagnathid | Indeterminate | Tsagan Teg | "Partial lower jaw similar to Gigantoraptor." | A large caenagnathid. |

##### Therizinosaurs

Therizinosaurs reported from the Bayan Shireh Formation :
| Genus | Species | Location | Stratigraphic Position | Material | Notes |
| Dendroolithidae spp. | Indeterminate | Baynshire | | "Various nests and egg clutches." | Eggs probably laid by a therizinosaur |
| Enigmosaurus | E. mongoliensis | Khara Khutul | | "Incompletely preserved pelvis." | A therizinosauroid. |
| Erlikosaurus | E. andrewsi | Bayshin Tsav | Upper beds | "Skull, right pes, and left humerus." | A therizinosaurid. |
| Segnosaurus | S. galbinensis | Amtgai, Bayshin Tsav, Khara Khutul, Ulribe Khuduk | | "Multiple specimens including the lower jaws and partial postcrania." | A large therizinosaurid. |
| Undescribed therizinosaurs | Indeterminate | Bayshin Tsav, Khara Khutul, Shine Us Khuduk | | "Cranial and postcranial elements from multiple specimens." | Some elements may represent already named taxa. |
| Unnamed therizinosaur | Indeterminate | Ulribe Khuduk | | "Forelimbs with two digits, partial pelvis and vertebrae." | A two-fingered therizinosaur. |

##### Troodontids

Troodontids reported from the Bayan Shireh Formation :
| Genus | Species | Location | Stratigraphic Position | Material | Notes | Images |
| Undescribed troodontid | Indeterminate | Bayshin Tsav | | "Partial vertebrae, forelimbs and hindlimbs." | A troodontid. | |

##### Tyrannosauroids

Tyrannosauroids reported from the Bayan Shireh Formation :
| Genus | Species | Location | Stratigraphic Position | Material | Notes | Images |
| Alectrosaurus? | A. olseni? | Bayshin Tsav | | "Two specimens, fragmentary skull and postcrania." | A tyrannosauroid also present in the Iren Dabasu Formation | |
| Tyrannosauroidea indet. | Indeterminate | Tsagan Teg | | "An isolated nearly complete left frontal." | A tyrannosauroid. | |

##### Other Theropods

| Genus | Species | Location | Stratigraphic Position | Material | Notes | Images |
| Deinonychosauria indet. | Indeterminate | Shine Us Khuduk | | "Isolated remains including a pedal phalanx II-2." | The pedal phalanx II-2 is similar to Achillobator. | |
| Theropoda indet. | Indeterminate | Ulribe Khuduk | | "Nest with eggs." | Eggs probably laid by a theropod. | |