The stratigraphic framework of the Lower Miocene series within the Sivas Basin (central Turkey) is discussed on the basis of hyaline larger foraminifera from a transgressive shallow-marine sequence (Karacaoren Formation). A biometric study of Aquitanian to lower Burdigalian taxa (genera Miogypsina, Miogypsinoides, Nephrolepidina, Eulepidina, Spiroclypeus and Operculina) provides new insight into their taxonomy, stratigraphy and paleobiogeography in the Tethys. The miogypsinid taxa can be arranged in the Miogypsinoides bantamensis, Miogypsina gunteri, M. tani and M. globulina parts of the main miogypsinid lineage of Drooger (1963, 1993), the majority of which are described for the first time from Turkey. Moreover, a new Miogypsinoides taxon, M. sivasensis n. sp., from the lower Aquitanian is also introduced. Nephrolepidina, the most common lepidocyclinid taxon is associated with typical Aquitanian and early Burdigalian miogypsinids. Based on biometric data Nephrolepidina does not serve a high-resolution biostratigraphic perspective since all Aquitanian assemblages are referable to N. morgani or a transitional stage from N. morgani to N. tournoueri (N. ex. interc. morgani-tournoueri (type 1), although stratigraphically lowest samples have more primitive features. The second distinct Nephrolepidina assemblage, identified in association with early Burdigalian M. globulina, is referred to N. ex. interc. morgani-tournoueri (type 2), which has surprisingly more primitive features than those described from Aquitanian. Eulepidina, identified only from the early Burdigalian in association with M. globulina, is assigned to E. aff.formosa, showing a clear influence of Indo-Pacific fauna. Nummulitids are rare, only Spiroclypeus blanckenhorni could be found in reasonable quantity in the lower Aquitanian deposits in association with M. glutted and N. morgani. Simultaneously, its stratigraphical range is extended to the early Aquitanian. Assemblages from the basal part of the Karacaoren Formation suggest that the transition from continental to marine sedimentation cannot be explained by a single regional transgression, but requires at least three such marine flooding events, confined to a time interval between early and late Aquitanian.