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 ImagesKiyatritonK. leshchinskiyi Atlas, trunk vertebrae, fragmentary dentaries, femora, humerus, and nearly complete iliumA crown-group salamanderKuzbassiaK. sola Shestakovo 1Two atlantal centra and a trunk vertebral centrumA karaurid

Mammaliamorphs +Mammaliamorphs reported from the Ilek Formation Genus Species Locality Material Notes ImagesAcinacodusA. tagaricusA. V. Lopatin; E. N. Maschenko & A. O. Averianov (2010). "A new genus of triconodont mammals from the Early Cretaceous of Western Siberia". Doklady Biological Sciences. 433 (1): 282–285. doi:10.1134/S0012496610040137.Right dentary fragmentAn amphidontid mammalBaidabatyrB. clivosusAlexander Averianov; Alexey Lopatin; Pavel Skutschas; Stepan Ivantsov; Elizaveta Boitsova; Ivan Kuzmin (2017). "An enigmatic multituberculate mammal from the Early Cretaceous of Siberia, Russia". Journal of Vertebrate Paleontology. 37 (2): e1293070. doi:10.1080/02724634.2017.1293070. Bol'shoi Kemchug 3Upper premolarA multituberculateGobiconodonG. borissiakiTrofimov, B. A. (1978). "The first triconodonts (Mammalia, Triconodonta) from Mongolia". Doklady Akademii Nauk SSSR. 243 (1): 213–216.21 upper and lower jaws. A gobiconodontG. hoburensisMaschenko, E. N.; Lopatin, A. V. (1998). "First record of an Early Cretaceous triconodont mammal in Siberia". Bull. Inst. R. Sci. Nat. Belg. 68: 233–236.A fragmentary lower jaw.Kemchugia K. magnaA toothAn amphilestid mammalKiyatheriumK. cardiodensE. N. Maschenko and A. V. Lopatin. 2002. A new Early Cretaceous mammal from Western Siberia. Doklady Biological Sciences 386:475-477A maxillaA zhangheotheriid mammalSibirotheriumS. rossicum Bol'shoi Kemchug 4M1 tooth; mandibular fragmentsA docodontan mammaliaformXenocretosuchusX. sibiricusDental elementsA tritylodontid mammaliamorphYermakiaY. domitor Shestakovo 1A mandibleA tinodontid mammal

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

Lizards +Lizards reported from the Ilek Formation Genus Species Locality Material Notes ImagesParamacellodidae Indeterminate Kemchug assemblage Kiya assemblage Platynota Indeterminate Kemchug assemblage Xenosauridae Indeterminate Kemchug assemblage Kiya assemblageA knob-scaled lizard

Turtles +Turtles reported from the Ilek Formation Genus Species Locality Material Notes ImagesKirgizemysA macrobaenid

Crocodylomorphs +Crocodylomorphs reported from the Ilek Formation Genus Species Locality Material Notes ImagesKyasuchus K. saeviEfimov, M. B. and Leshchinskiy, S. V. (2000). First finding of the fossil crocodile skull in Siberia [in Russian]. In: Komarov, A. V., ed., Materialy regional’noj konferencii geologov Sibiri, Dal’nego Vostoka i Severo−Vostoka Rossii. Tom II, 361–363. GalaPress, Tomsk. Shestakovo 3A partial skullA shartegosuchid crocodyliformTagarosuchusT. kulemziniFiorelli, L.E.; Calvo, J.O. (2007). "The first "protosuchian" (Archosauria: Crocodyliformes) from the Cretaceous (Santonian) of Gondwana" (PDF). Shestakovo 3Nearly complete skullA crocodyliform

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

Dinosaurs Sauropodomorphs +Sauropodomorphs reported from the Ilek Formation Genus Species Locality Material Notes ImagesLithostrotia IndeterminateCaudal vertebraePotentially three distinct taxa of titanosaurSibirotitanS. astrosacralis Shestakovo 1 Shestakovo 3Vertebrae, sacrum, and pedal elementsA somphospondylan sauropodEuhelopodidaeIndeterminateShestakovo 3Partial fibula

Ornithischians +Ornithischians reported from the Ilek Formation Genus Species Locality Material Notes ImagesOrnithischia Indeterminate Shestakovo 1 Bol'shoi Kemchug 3Isolated teethOriginally identified as two distinct "hypsilophodontid" taxaPsittacosaurusP. sibiricusAverianov, Alexander O.; Voronkevich, Alexei V.; Leshchinskiy, Sergei V.; Fayngertz, Alexei V. (2006). "A ceratopsian dinosaur Psittacosaurus sibiricus from the Early Cretaceous of West Siberia, Russia and its phylogenetic relationships". Journal of Systematic Palaeontology. 4 (4): 359–395. doi:10.1017/s1477201906001933. ShestakovoSeveral skeletonsA ceratopsianStegosauria Indeterminate Bol'shoi Kemchug 3Isolated teeth Theropods +Theropods reported from the Ilek Formation Genus Species Locality Material Notes ImagesEvgenavis E. nobilisO'Connor, JL; Averianov, AO; Zelenkov, NV (2014). "A confuciusornithiform (Aves, Pygostylia)-like tarsometatarsus from the Early Cretaceous of Siberia and a discussion of the evolution of avian hind". Journal of Vertebrate Paleontology. 34 (3): 647–656. doi:10.1080/02724634.2013.828734.A set of limb elementsA confuciusornithiformKiyacursorK. longipes Partial skeleton including cervical and caudal vertebrae, cervical and dorsal ribs, a left scapulocoracoid, humeri, and much of both hind limbsA noasaurid theropodMystiornisM. cyriliEvgeny N. Kurochkin; Nikita V. Zelenkov; Alexandr O. Averianov; Sergei V. Leshchinskiy (2011). "A new taxon of birds (Aves) from the Early Cretaceous of Western Siberia, Russia". Journal of Systematic Palaeontology. 9 (1): 109–117. doi:10.1080/14772019.2010.522202. Isolated metatarsusAn avisaurid enantiornitheanParaves IndeterminateTeeth May belong to either Microraptorinae or TroodontidaeTyrannosauroidea IndeterminateTeeth UrbacodonU. cf. sp. Teeth A troodontid with unserrated teeth

See also List of pterosaur-bearing stratigraphic units

References