  We investigate the ultrafast dynamics of Cd$_2$Os$_2$O$_7$, a prototype
material showing a Lifshitz-type transition as a function of temperature. In
the paramagnetic metallic state, the photo-reflectivity shows a sub-picosecond
relaxation, followed by a featureless small offset. In the antiferromagnetic
state slightly below $T_N$, however, the photo-reflectivity resurges over
hundreds of picoseconds, which goes beyond the usual realm of the
effective-temperature model. Our observations are consistent with the Lifshitz
phase transition of Cd$_2$Os$_2$O$_7$ driven by the evolution of the local
magnetic moment.
