Abstract The relative nucleus density (RND) model of dynamically recrystallized grain size [Sakai, T., Jonas, J.J. 1984. Dynamic recrystallization: mechanical and microctructutal consideration. Acta metallurgica, 32, 198–209] was applied to experimentally and to naturally deformed marbles that have undergone dynamic recrystallization. The model shows that a relationship between initial grain size ( D 0 ) and stable dynamically recrystallized grain size ( D S ) for a given value of temperature-corrected strain-rate ( Z ) controls grain size evolution during dynamic recrystallization. New microstructural mechanism maps (MM-maps) for experimentally and naturally deformed marbles (based on previously published data) were defined in log grain size–log Z space and show two distinct regions of grain reduction and grain coarsening. The boundary between these two regions corresponds to an equation relating dynamically recrystallized grain size and temperature corrected strain rate, as proposed in this work. The new MM-map was used to trace semi-quantitatively microstructural and grain size evolution in naturally deformed marbles that underwent dynamic recrystallization at different thermal conditions. The boundary between grain coarsening and grain reduction does not necessarily coincide with the boundary between rotation and migration recrystallization mechanisms. Assessment of available natural data shows that the boundary condition D 0 = 2 D S between grain-coarsening and grain-reduction introduced by Sakai and Jonas [Sakai, T., Jonas, J.J. 1984. Dynamic recrystallization: mechanical and microctructutal consideration. Acta metallurgica, 32, 198–209] is not required for naturally deformed marble.