Fitema

a subdivision of the Precambrian geologic time scale. The term fitema was introduced in 1966 by the Soviet geologist B. M. Keller. A fitema corresponds to a stratigraphic system in the later geologic history of the earth but has not been completely substantiated on the basis of biostratigraphy; fitemy are identified mainly on the basis of phytoliths, that is, structures of blue-green algae. The detailed zonal subdivision characteristic of stratigraphic systems is yet to be carried out for fitemy. Radioisotope dating indicates that the time required for the formation of a fitema corresponds to an era in the Phanerozoic history of the earth (see). In all other features, a fitema resembles a stratigraphic system. A typical example of fitemy is the subdivisions of the Riphean, namely, the Lower, Middle, and Upper Riphean.