# Ilek Formation

The Ilek Formation is a Lower Cretaceous geologic formation in Western Siberia. Many different fossils have been recovered from the formation. It overlies the Late Jurassic Tyazhin Formation and underlies the Albian Kiya Formation.
The formation was described by L. A. Ragozin in 1935. It consists of sands with sandstone concretions, layers of silts, clays and marls. Age of the formation, according to a crude 1962 estimate, is Valanginian(?) - Hauterivian - Barremian. Its thickness varies greatly, reaching 746 m in Teguldet borehole. A more recent 2024 estimate based on invertebrate and vertebrate fossils placed the age of the formation at Barremian–Aptian.

## Age

The Ilek Formation conformably overlies the Upper Jurassic Tyazhin Formation. Gastropods and bivalves of Valanginian age have been recovered from the lower part of the formation, and gastropods and bivalves of Hauterivian to Barremian age have been found in the upper part, as well as Barremian ostracods. The age of the Shestakovo 1 locality is poorly constrained. Palynological samples from the Shestakovo 3 locality are characteristic of the Aptian. No angiosperm pollen has been found at this locality, indicating that it is older than the Albian. The upper layers of the formation do contain angiosperm pollen, indicating that part of the formation may extend into the Albian. The overlying Kiya Formation is upper Albian based on plant biostratigraphy.

## Fauna
### Amphibians

Amphibians reported from the Ilek Formation :
| Genus | Species | Locality | Material | Notes | Images |
| Kiyatriton | K. leshchinskiyi | Shestakovo 1 | Atlas, trunk vertebrae, fragmentary dentaries, femora, humerus, and nearly complete ilium | A crown-group salamander | |
| Kuzbassia | K. sola | Shestakovo 1 | Two atlantal centra and a trunk vertebral centrum | A karaurid | |

### Mammaliamorphs

Mammaliamorphs reported from the Ilek Formation :
| Genus | Species | Locality | Material | Notes |
| Acinacodus | A. tagaricus | | Right dentary fragment | An amphidontid mammal |
| Baidabatyr | B. clivosus | Bol'shoi Kemchug 3 | Upper premolar | A multituberculate |
| Gobiconodon | G. borissiaki | | 21 upper and lower jaws. | A gobiconodont |
| Gobiconodon | G. hoburensis | | A fragmentary lower jaw. | A gobiconodont |
| Kemchugia | K. magna | | A tooth | An amphilestid mammal |
| Kiyatherium | K. cardiodens | | A maxilla | A zhangheotheriid mammal |
| Sibirotherium | S. rossicum | Bol'shoi Kemchug 4 | M1 tooth; mandibular fragments | A docodontan mammaliaform |
| Xenocretosuchus | X. sibiricus | | Dental elements | A tritylodontid mammaliamorph |
| Yermakia | Y. domitor | Shestakovo 1 | A mandible | A tinodontid mammal |

### Choristoderes

Choristoderes reported from the Ilek Formation :
| Genus | Species | Locality | Material | Notes |
| cf. Khurendukhosaurus | K. cf. sp | Bol'shoi Kemchug 3 | Dorsal vertebra, sacral vertebra | A non-neochoristodere choristodere |
| "Shestakovo choristodere" | "Shestakovo choristodere" | Shestakovo Bol'shoi Kemchug 3 Bol'shaya Terekhtul' 2 | Fragmentary dentaries, several vertebrae, rib fragments | A possible neochoristodere |
| Choristodera | Indeterminate | Smolenskii Yar | Cervical centrum | May represent a taxon distinct from both Khurendukhosaurus and the "Shestakovo choristodere" |

### Lizards

Lizards reported from the Ilek Formation :
| Genus | Species | Locality | Notes |
| Paramacellodidae | Indeterminate | Kemchug assemblage Kiya assemblage | |
| Platynota | Indeterminate | Kemchug assemblage | |
| Xenosauridae | Indeterminate | Kemchug assemblage Kiya assemblage | A knob-scaled lizard |

### Turtles

Turtles reported from the Ilek Formation :
| Genus | Species | Locality | Material | Notes | Images |
| Kirgizemys | | | | A macrobaenid | |

### Crocodylomorphs

Crocodylomorphs reported from the Ilek Formation :
| Genus | Species | Locality | Material | Notes | Images |
| Kyasuchus | K. saevi | Shestakovo 3 | A partial skull | A shartegosuchid crocodyliform | |
| Tagarosuchus | T. kulemzini | Shestakovo 3 | Nearly complete skull | A crocodyliform | |

### Pterosaurs

Pterosaurs reported from the Ilek Formation :
| Genus | Species | Locality | Material | Notes |
| ?Ctenochasmatidae | Indeterminate | Bol'shoi Kemchug 3 | | |
| Lonchognathosaurus | L. cf. sp. | Novochernorechensk | Wing metacarpal fragment | A dsungaripterid |
| Ornithocheiridae | Indeterminate | Shestakovo 1 | | |
| Pterodactyloidea | Indeterminate | Bol'shoi Kemchug 3 | | |

### Dinosaurs
#### Sauropodomorphs

Sauropodomorphs reported from the Ilek Formation :
| Genus | Species | Locality | Material | Notes | Images |
| Lithostrotia | Indeterminate | | Caudal vertebrae | Potentially three distinct taxa of titanosaur | |
| Sibirotitan | S. astrosacralis | Shestakovo 1 Shestakovo 3 | Vertebrae, sacrum, and pedal elements | A somphospondylan sauropod | |

#### Ornithischians

Ornithischians reported from the Ilek Formation :
| Genus | Species | Locality | Material | Notes |
| Ornithischia | Indeterminate | Shestakovo 1 Bol'shoi Kemchug 3 | Isolated teeth | Originally identified as two distinct "hypsilophodontid" taxa |
| Psittacosaurus | P. sibiricus | Shestakovo | Several skeletons | A ceratopsian |
| Stegosauria | Indeterminate | Bol'shoi Kemchug 3 | Isolated teeth | |

#### Theropods

Theropods reported from the Ilek Formation :
| Genus | Species | Locality | Material | Notes |
| Evgenavis | E. nobilis | | A set of limb elements | A confuciusornithiform |
| Kiyacursor | K. longipes | Shestakovo 1 | Partial skeleton including cervical and caudal vertebrae, cervical and dorsal ribs, a left scapulocoracoid, humeri, and much of both hind limbs | A noasaurid theropod |
| Paraves | Indeterminate | | Teeth | May belong to either Microraptorinae or Troodontidae |
| Mystiornis | M. cyrili | Shestakovo 1 | Isolated metatarsus | An avisaurid enantiornithean |
| Paraves | Indeterminate | | Teeth | May belong to either Microraptorinae or Troodontidae |
| Tyrannosauroidea | Indeterminate | | Teeth | |
| Urbacodon | U. cf. sp. | | Teeth | A troodontid with unserrated teeth |