TheYarenskianGorizont ("Yarenskian Horizon") is a majorbiostratigraphic unit inRussia corresponding to UpperOlenekian-age terrestrial sediments. It lies above theUstmylian Gorizont of theVetlugian Supergorizont, and is located below a shortunconformity overlain by theDonguz Gorizont. Some paleontologists elevate the Yarenskian to a supergorizont and subdivide it into two smaller units: the olderFedorovskian Gorizont (Fedorovkian Horizon) and the youngerGamskian Gorizont (Gamian Horizon). Others prefer to consider the Fedorovskian and Gamskian to be subgorizonts or members of the Yarenskian. Vertebrateindex fossils of the Fedorovskian include thelungfishGnathorhiza, thetrematosauridInflectosaurus, and theprocolophonidBurtensia. In the Gamskian, these index fossils are replaced byCeratodus,Trematosaurus, andKapes, respectively. The Yarenskian as a whole is characterized by thecapitosaur amphibianParotosuchus, and has also been known as theParotosuchus fauna.[1][2] A few geographical influence on the faunas can be observed;erythrosuchids andrhytidosteids are more common in southern exposures while procolophonids and putative "rauisuchids" are more common in the north.[2][3]
The Yarenskian encompasses several svitas (equivalent togeological formations). The most fossiliferous and best-exposed is thePetropavlovskaya Svita (anglicized asPetropavlovka Formation), a Fedorovskian-age assemblage in the Cis-Ural region (near theUral River inOrenburg Oblast). Another productive unit is the Gamskian-ageLipovskaya Formation, which is found near theDon River inVolgograd Oblast. TheCaspian Depression includes coastal sediments equivalent to the Yarenskian (such as the Bogdo orBogdinskaya Svita),[2] though there is some controversy over how the stratigraphy of the area should be defined.[4] Thin exposures of Yarenskian units are also known further north in theVyatka,Vychegda,Luza, andPechora basins.[2]
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