  Resonance-like molecular excitations have been discovered in a series of
pyrochlore antiferromagnets, yet their origins and relationships with the
coexisting magnon excitations remain a puzzle. Here, by incorporating the
spin-lattice coupling and the consequent four-spin interactions through the
site-phonon model, we accomplish a unified description of the molecular and
magnon excitations, revealing the molecular modes as statistical approximations
of the dispersive magnon excitations. Our work demonstrates a general approach
to understand exotic spin dynamics in magnetoelastic systems.
