The NMR spin-lattice proton relaxation dispersion in the smectic mesophase of two liquid crystals, 4cyano-4'-8-alkylbiphenyl and 4,4'-bis-heptyloxyazoxybenzene, are studied over several decades of Larmor frequencies. The results show that the order fluctuation of the local smectic director contribution to T1(nu(L)) undergoes a transition between two power regimes: from T1(nu(L)) proportional, variantnu(1)(L) to nu(1/2)(L) on going from low to high Larmor frequencies. We explain this behavior by assuming, in the smectic mesophases, short coherence length nematiclike cooperative molecular reorientations.