New data on the terrestrial gastropods from the Oligocene-Miocene transition in the Valley of Lakes, Central Mongolia

The Oligocene-Miocene terrestrial gastropod fauna of the Valley of Lakes, Central Mongolia, is described for the first time based on SEM images. These allowed detailed anatomical analyses and resulted in better species delimitation and taxonomic rectifications. The assemblage comprises six pupilloid species of the genera Gastrocopta, Vallonia, and Pupoides. Vallonia stworzewiczae Neubauer, sp. nov. is introduced as a new species. Two samples from the Loh Formation exposed at the Hotuliin Teeg locality have yielded excellently preserved specimens. Based on a small mammal fauna, these samples are proposed to span the interval of the Oligocene-Miocene boundary. A comparison with ecological requirements of modern congeners suggests the presence of rather open but vegetated habitats in the Valley of Lakes during sedimentation of the Loh Formation.