The Madygen Formation (Russian: Madygen Svita) is a Middle–Late Triassic (Ladinian–Carnian) geologic formation and lagerstätte in the Batken and Osh Regions of western Kyrgyzstan, with minor outcrops in neighboring Tajikistan and Uzbekistan. The conglomerates, sandstones and mudstones of the thick formation were deposited in terrestrial lacustrine, alluvial, fluvial and deltaic environments.
The formation, extending across the Fergana Valley and Fergana Range, is unique for Central Asia, as it represents one of the few known continental deposits and the Madygen Formation is renowned for the preservation of more than 20,000 fossil insects, making it one of the richest Triassic lagerstätten in the world. Other vertebrate fossils as fish, amphibians, reptiles and synapsids have been recovered from the formation too, as well as minor fossil flora.
The lake sediments of the Lagerstätte provided fossil cartilaginous fishes and their egg capsules and unusual Triassic reptiles like Sharovipteryx and Longisquama. The wide diversity of insect fossils was first discovered in the 1960s and first described by Russian paleontologist Aleksandr Sharov, with a notable example being Gigatitan.
Description
The Madygen Formation is a thick succession of predominantly siliciclastic rocks accumulated in a tectonically induced basin, covering parts of the Fergana Range and Fergana Valley of Kyrgyzstan with minor outcrops in Tajikistan and Uzbekistan. The Middle-Late Triassic layers rest on top of Paleozoic basement with local Permo-Triassic molasse sediments. They are likewise overlain by latest-Triassic to Early Jurassic sediments, some of which host their own arthropod faunas.
The formation consists of a wide and multicolored variety of siliciclastic rocks: mudstones, sandstones, conglomerates, fanglomerates, and rare coal layers. This diversity of sediment types reflects the a complex set of depositional environments through time and space, including alluvial fans, sandflats, swamps, back-swamp areas, and littoral to profundal lake zones. The fluvio-lacustrine deposits of the Madygen Formation belong to one of only a few occurrences of continental Triassic beds in Central Asia.
Isotope analyses of fish teeth confirm that the lakes and rivers represented by the formation were entirely freshwater and deep inland, about 600 km from the nearest coast. Though fossils can be found throughout the entire formation, the most extensive fossil deposits occur in the northern area, known as Dzaylyaucho (also spelled Dzhayloucho or Dzhailoucho), which is dominated by brown and grey claystones and mudstones. The abundance of burrows and absence of darker sediments in this area indicate that it was a well-oxygenated lacustrine environment, such as a large oxbow lake.
The environment represented by the Madygen Formation was positioned on the Cimmerian microcontinent, a slab of crust that collided with Laurasia during the Cimmerian orogeny in later Mesozoic times. This orogeny led to the disappearance of the Paleo-Tethys Ocean. The Madygen Formation was emplaced at warm temperate latitudes (34° to 40° north) in a rift basin surrounded by old, eroded mountains, the ancestral Tien Shan Mountains. The climate was likely humid and moist year-round, as drying features such as mudcracks and evaporite deposits are absent.
Petroleum geology The formation grades from bottom to top from alluvial to fluvial into a thick succession of lacustrine mudstones, followed by an alluvial package, on top of which lacustrine, fluvial, deltaic and alluvial layers were deposited.
The hydrocarbon potential of samples of the Madygen Formation ranges from poor to excellent. The sediments containing more than 0.5% Total Organic Carbon (TOC) may be regarded as sources of gaseous hydrocarbons rather than of oil. The Hydrogen Index (HI) of outcrop samples reaches 100 and the maximum recorded maturity (Ro) is 0.8.
Paleontological significance During the 1960s, Russian paleontologists recovered an unusually rich fossil content in the type strata of the Madygen Formation, including abundant macrophytes, more than 20,000 insect remains and unique small reptiles with well preserved soft tissue. Spirorbis-like polychaete worm tubes, crustaceans (ostracods, kazacharthrans, conchostracans, malacostraca), freshwater Bivalves and gastropods are known from shallow to deeper lake environments. Non-aquatic insects are among the most common fossil remains of the Madygen Formation, with half a thousand or so species having been uncovered from these sediments. This makes the Madygen Formation one of the most prolific insect fossil sites In the world. These include representatives of both extinct and extant orders, including the Ephemeroptera, Odonata, Notoptera, Blattodea, Titanoptera, Ensifera, Caelifera, Rhynchota, Auchenorrhyncha, Stenorrhyncha, Coleoptera, Hymenoptera, Trichoptera and Diptera. Traces of insect larvae are preserved in near-shore lake deposits.
Fish remains mostly represent endemic genera assigned to the actinopterygian families Evenkiidae (Oshia), Palaeoniscidae (Ferganiscus, Sixtelia) and Megaperleidus and Alvinia. The actinopterygian Saurichthys and the dipnoan Asiatoceratodus are cosmopolitan taxa also recorded in the Madygen Formation. Two distinctive elasmobranch egg capsule types, i.e. Palaeoxyris, indicating a small Lissodus- or Lonchidion-like hybodont shark and an indeterminate capsule type, imply the presence of two different elasmobranch species which used the freshwater environments of the Madygen Formation as spawning grounds. Tetrapods (and stem-tetrapods) are known from the mostly larval urodelan (Triassurus), a small procynosuchid cynodont (Madysaurus), a chroniosuchid reptiliomorph (Madygenerpeton), an early drepanosaurid reptile (Kyrgyzsaurus), a gliding archosauromorph (Sharovipteryx) and the enigmatic diapsid Longisquama.
Paleobiota
Amphibians Genus Species Material Notes Images Madygenerpeton M. pustulatus Skull and osteoderms. A chroniosuchid reptiliomorph with aquatic adaptations. It was first named in 2010 on the basis of a nearly complete skull and associated osteoderms. It seems that this tetrapod could've been semi-aquatic, due to the increased flexibility of the trunk region. Triassurus T. sixtelae plentiful remains, mostly of larval individuals The earliest known member of the caudata (salamanders and their kin). Most of the known fossils include larval remains (the neural arches of the vertebrae were still paired and no vertebral centers show any degree of ossification).
Reptiles Reptiles are the most abundant of the tetrapod paleofauna at Madygen, with three described genera. Genus Species Material Notes Images Kyrgyzsaurus K. bukhanchenkoi A single specimen preserving the front half of a skeleton and scale impressions. Potentially the oldest known member of the drepanosaurs, a diverse clade of Triassic reptiles that evolved arboreal and fossorial lifestyles. It was also the first member of the group to be described from Asia. Longisquama L. insignis A specimen preserving the front half of a skeleton and "plumes", and at least five additional "plume" fragments. A neodiapsid reptile possessing unusual large scaly growths ("plumes") on its back. This animal has gone through a confusing taxonomic history, with some authors suggesting a placement within Archosauromorpha, or as a basal diapsid. Sharovipteryx S. mirabilis A single skeleton with impressions of gliding membranes, split across a slab and counterslab. A gliding archosauromorph, and the type genus of the family Sharovipterygidae. Originally named as Podopteryx, a genus name which was preoccupied by a damselfly.
Synapsids Genus Species Material Notes Images Madysaurus M. sharovi A single partial skeleton. A small procynosuchid cynodont, and the only synapsid known from the Madygen Formation. The holotype lacked a tail, and was only around in length, meaning it was smaller than most of its vertebrate, and even some of its arthropodan contemporaries.
Cartilaginous fishes Possible xenacanth denticles, egg cases, as well as hybodont fossils have also been reported from the formation. The high amount of juvenile individuals found suggest the freshwater systems at the Madygen Formation served as a spawning grounds and nursery's for these prehistoric elasmobranchs. Genus Species Material Notes Images Fayolia F. sharoviEgg capsules An egg capsule likely belonging to a xenacanthid. The egg is elongate and tapers towards both ends, and surrounded by helically twisted collarettes, with one end (the beak) having a tendril. Palaeoxyris P. alternaEgg capsulesThe egg cases of hybodonts that appear frequently throughout the fossil record. They comprise a beak, a body and a pedicle. They display a conspicuous right-handed spiral of collarettes around the body, and in some cases, the pedicle, resulting in a rhomboidal pattern when flattened during fossilisation. The eggs were most likely produced by the contemporary genus Lonchidion, due to the number of preserved juvenile individuals. Lonchidion L. ferganensis Teeth, denticles, and egg capsules A hybodontid elasmobranch that lived from the Lower Triassic to the Upper Cretaceous. The fossils found at Madygen (which mainly come from juvenile individuals) suggest this animal spawned within freshwater areas. The majority of the teeth from the Madygen Formation originated from areas full of abundant bivalve fossils, suggesting the young hybodonts frequented these areas due to the rich food supply.
Bony fishes The following fish fossils were found in the formation: Genus Species Material Notes ImagesAlviniaA. serrataPartial skeletons.A small perleidid.AsiatoceratodusA. sharoviPartial skeletons.A medium-sized dipnoan (lungfish), around in length.FerganiscusF. osteolepisNearly complete skeletons.A small and abundant palaeoniscid, in length.MegaperleidusM. lissolepisPartial skeletons.A medium-sized perleidid.OshiaO. ferganicaPartial skeletons, scales.A medium-sized evenkiid scanilepiform with predatory habits. Around in length.SaurichthysS. orientalisNearly complete skeleton, scales, other fragmentary material.A relatively small saurichthyid chondrostean, around in length.SixteliaS. asiaticaNearly complete skeletons, scales.A small and common palaeoniscid, in length.
Insects The Madygen Formation is renowned for its insect fauna, with an estimated 25,000 specimens, 500 species, 100 families, and 20 orders found in the formation. Under some estimates, it has the most diverse insect assemblage of the entire Permian-Triassic interval. From EDNA (The Fossil Insect Database), Shcherbakov (2008), and the Paleobiology Database, unless cited otherwise:
Odonata (damselflies, dragonflies and kin) The Odonata and other Odonatoptera of the Madyen Formation are rare (about 100 specimens), with moderate diversity. They were described most extensively by Pritykina (1981). †Paurophlebiidae (8 species): Cladophlebia (C. brevis, C. parvula), Neritophlebia (N. elegans, N. longa, N. vicina), Nonymophlebia (N. venosa), Paurophlebia (P. angusta, P. lepida) †Triadophlebiidae (6 species): Triadophlebia (T. distincta, T. honesta, T. madygenica, T. magna, T. minuta, T. modica) †Zygophlebiidae (4 species): Cyrtophlebia (C. sinuosa), Mixophlebia (M. mixta), Zygophlebia (Z. ramosa), Zygophlebiella (Z. curta) †Kennedyidae (4 species): Kennedya (K. carpenteri, K. ferganensis, K. gracilis, K. madygensis) †Voltzialestidae (3 species): Terskeja (T. paula, T. pumilio, T. tenuis) †Batkeniidae (2 species): Batkenia (B. pusilla), Paratriassoneura (P. primitiva) †Mitophlebiidae (1 species): Mitophlebia (M. enormis) †Protomyrmeleontidae (1 species): Ferganagrion (F. kirghiziensis) †Triadotypidae (1 species): Reisia (R. sogdiana) †Triassolestidae (1 species): Triassolestodes (T. asiaticus) †Xamenophlebiidae (1 species): Xamenophlebia (X. ornata)
Blattodea (roaches)