Lower Carboniferous Gastropods of Dombar Hills (Visean-Serpukhovian Boundary Beds, Western Kazakhstan)

An Early Carboniferous gastropod assemblage from the Visean/Serpukhovian boundary deposits of the Dombar Hills (western slope of the South Urals) is described. The assemblage is interpreted as having inhabited relatively deep-water environments of carbonate deposition on the passive continental margin of Laurussia. The lenses of crinoid limestones with abundant ammonoid shells are interpreted as deposits of mud mounds. The gastropod assemblage is distinguished by a set of specific features: some families, which are common for most Early Carboniferous assemblages, are absent, while most of the species present here are not found in shallow-water deposits of carbonate platforms. The described assemblage has a number of similarities with the older Erdbach assemblage. Several species from the described assemblage are also found in limestones of the same age in the Verkhnyaya Kardailovka deposited in the relatively deep-water carbonate ramp of the Magnitogorsk island arc. All taxa described here are recorded for the first time from the Urals. Of these, one genus Squamoworthenia gen. nov. and two species (Hammatospira cancellata sp. nov. and Stuckenbergispiradombarensis sp. nov.) are new. The other twelve species were previously described from Britain, Belgium, Germany, Ukraine and North America: Straparollus (Straparollus) dionysii Montfort, 1810, Sinuitina (Sinuitina) gratiosa (Koninck, 1883), Hesperiella thomsoni (Koninck, 1883), Agnesia prosseri Hyde, 1953, Ptychomphalina subconoidea (Koninck, 1883), Lunulazona lirata (Phillips, 1836), Glabrocingulum minutum (Zernetskaja, 1983), Dictyotomaria cauchyana (Koninck, 1843), Squamoworthenia duponti, (Holzapfel, 1889), Platyceras vetustum (Sowerby, 1829), Auriptygma naticoides (Holzapfel, 1899), and Soleniscus ventricosus (Koninck, 1881).