Madygen Formation

The Madygen Formation is a Middle–Late Triassic 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, 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 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 may be regarded as sources of gaseous hydrocarbons rather than of oil. The Hydrogen Index of outcrop samples reaches 100 and the maximum recorded maturity 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, 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, Palaeoniscidae 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 are known from the mostly larval urodelan, a small procynosuchid cynodont, a chroniosuchid reptiliomorph, an early drepanosaurid reptile, a gliding archosauromorph and the enigmatic diapsid Longisquama.

Paleobiota

Amphibians

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 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

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. sharovi	Egg 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 having a tendril.
Palaeoxyris	P. alterna	Egg capsules	The 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 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	Images
Alvinia	A. serrata	Partial skeletons.	A small perleidid.
Asiatoceratodus	A. sharovi	Partial skeletons.	A medium-sized dipnoan, around in length.
Ferganiscus	F. osteolepis	Nearly complete skeletons.	A small and abundant palaeoniscid, in length.
Megaperleidus	M. lissolepis	Partial skeletons.	A medium-sized perleidid.
Oshia	O. ferganica	Partial skeletons, scales.	A medium-sized evenkiid scanilepiform with predatory habits. Around in length.
Saurichthys	S. orientalis	Nearly complete skeleton, scales, other fragmentary material.	A relatively small saurichthyid chondrostean, around in length.
Sixtelia	S. asiatica	Nearly complete skeletons, scales.	A small and common palaeoniscid, in length.