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.^[1]

Ilek Formation
Stratigraphic range: Lower Cretaceous, Barremian–Aptian
Shestakovo 1 locality
Type	Geological formation
Unit of	Kiya River Basin
Underlies	Kiya Formation
Overlies	Tyazhin Formation
Thickness	Up to 746 metres (2,450 ft)
Lithology
Primary	Clay, siltstone
Other	Marl, sandstone
Location
Coordinates	55.9°N 88.0°E / 55.9; 88.0
Approximate paleocoordinates	54.2°N 84.6°E / 54.2; 84.6
Region	Western Siberia
Country	Russia
Extent	Kemerovo Oblast
Type section
Named by	L. A. Ragozin
Year defined	1935
Ilek Formation (Russia) Show map of Russia Ilek Formation (Kemerovo Oblast) Show map of Kemerovo Oblast

The formation was described by L. A. Ragozin in 1935. It consists of sands with sandstone concretions, layers of silts, clays and marls.^[2] Age of the formation, according to a crude 1962 estimate, is Valanginian(?) - Hauterivian - Barremian. Its thickness varies greatly, reaching 746 m in Teguldet borehole.^[3] A more recent 2024 estimate based on invertebrate and vertebrate fossils placed the age of the formation at Barremian–Aptian.^[4]

Age

Shestakovo 3 locality

The Ilek Formation conformably overlies the Upper Jurassic Tyazhin Formation.^[1] 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.^[5] 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.^[6] 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.^[1]

Fauna

Amphibians

Amphibians reported from the Ilek Formation

Genus	Species	Locality	Material	Notes	Images
Kiyatriton	K. leshchinskiyi[7]	Shestakovo 1	Atlas, trunk vertebrae, fragmentary dentaries, femora, humerus, and nearly complete ilium	A crown-group salamander
Kuzbassia	K. sola[8]	Shestakovo 1	Two atlantal centra and a trunk vertebral centrum	A karaurid

Mammaliamorphs

Mammaliamorphs reported from the Ilek Formation

Genus	Species	Locality	Material	Notes	Images
Acinacodus	A. tagaricus[9]		Right dentary fragment	An amphidontid mammal
Baidabatyr	B. clivosus[10]	Bol'shoi Kemchug 3	Upper premolar	A multituberculate
Gobiconodon	G. borissiaki[11]		21 upper and lower jaws.	A gobiconodont
	G. hoburensis[12]		A fragmentary lower jaw.
Kemchugia	K. magna[13]		A tooth	An amphilestid mammal
Kiyatherium	K. cardiodens[14]		A maxilla	A zhangheotheriid mammal
Sibirotherium	S. rossicum[15]	Bol'shoi Kemchug 4	M1 tooth;[15] mandibular fragments[16]	A docodontan mammaliaform
Xenocretosuchus	X. sibiricus[17]		Dental elements	A tritylodontid mammaliamorph
Yermakia	Y. domitor[13]	Shestakovo 1	A mandible	A tinodontid mammal

Choristoderes

Choristoderes reported from the Ilek Formation

Genus	Species	Locality	Material	Notes	Images
cf. Khurendukhosaurus[18]	K. cf. sp	Bol'shoi Kemchug 3	Dorsal vertebra, sacral vertebra	A non-neochoristodere choristodere
"Shestakovo choristodere"[18]		Shestakovo Bol'shoi Kemchug 3 Bol'shaya Terekhtul' 2	Fragmentary dentaries, several vertebrae, rib fragments	A possible neochoristodere
Choristodera[18]	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	Material	Notes	Images
Paramacellodidae	Indeterminate[4]	Kemchug assemblage Kiya assemblage
Platynota	Indeterminate[4]	Kemchug assemblage
Xenosauridae	Indeterminate[4]	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[19]	Shestakovo 3	A partial skull	A shartegosuchid crocodyliform
Tagarosuchus	T. kulemzini[20]	Shestakovo 3	Nearly complete skull	A crocodyliform

Pterosaurs

Pterosaurs reported from the Ilek Formation

Genus	Species	Locality	Material	Notes	Image
?Ctenochasmatidae	Indeterminate[13]	Bol'shoi Kemchug 3
Lonchognathosaurus	L. cf. sp.[21]	Novochernorechensk	Wing metacarpal fragment	A dsungaripterid
Ornithocheiridae	Indeterminate[22]	Shestakovo 1
Pterodactyloidea	Indeterminate[13]	Bol'shoi Kemchug 3

Dinosaurs

Sauropodomorphs

Sauropodomorphs reported from the Ilek Formation

Genus	Species	Locality	Material	Notes	Images
Lithostrotia	Indeterminate[23]		Caudal vertebrae	Potentially three distinct taxa of titanosaur
Sibirotitan	S. astrosacralis[5]	Shestakovo 1 Shestakovo 3[24]	Vertebrae, sacrum, and pedal elements	A somphospondylan sauropod

Ornithischians

Ornithischians reported from the Ilek Formation

Genus	Species	Locality	Material	Notes	Images
Ornithischia	Indeterminate[4]	Shestakovo 1 Bol'shoi Kemchug 3	Isolated teeth	Originally identified as two distinct "hypsilophodontid" taxa[25]
Psittacosaurus	P. sibiricus[26]	Shestakovo	Several skeletons	A ceratopsian
Stegosauria	Indeterminate[13]	Bol'shoi Kemchug 3	Isolated teeth[25]

Theropods

Theropods reported from the Ilek Formation

Genus	Species	Locality	Material	Notes	Images
Evgenavis	E. nobilis[27]		A set of limb elements	A confuciusornithiform
Kiyacursor	K. longipes[28]	Shestakovo 1	Partial skeleton including cervical and caudal vertebrae, cervical and dorsal ribs, a left scapulocoracoid, humeri, and much of both hind limbs[28]	A noasaurid theropod
Paraves	Indeterminate[29]		Teeth	May belong to either Microraptorinae or Troodontidae
Mystiornis	M. cyrili[30]	Shestakovo 1	Isolated metatarsus	An avisaurid enantiornithean
Paraves	Indeterminate[29]		Teeth	May belong to either Microraptorinae or Troodontidae
Tyrannosauroidea	Indeterminate[29]		Teeth
Urbacodon	U. cf. sp.[29]		Teeth	A troodontid with unserrated teeth

See also

• List of pterosaur-bearing stratigraphic units