[en] We investigate the influence of material dispersion on the mechanism of self-shortening of femtosecond pulses interacting with fused silica plates having thicknesses an order of magnitude smaller than the dispersion length for the initial 72-fs pulse. The femtosecond pulse self-shortening is observed at large values of the B integral, when small-scale self-focusing develops in the central part of the pulse; this self-focusing plays the role of an optical shutter selecting the unperturbed radiation of the pulse leading edge. A study of the femtosecond pulse self-shortening in 1- and 3-mm-thick fused silica samples reveals that, under the experimental conditions in use, the self-phase modulation enhances the role of dispersion in the formation of a shortened pulse. With an increase in the sample thickness, the shortened pulse width increases as a result of the dispersion spread of the initial pulse in time. At the same time, this shortened pulse remains close to the transform-limited one, independent of the sample thickness. (paper)