  We study the effects of magnetoelastic couplings on pyrochlore
antiferromagnets. We employ Landau theory, extending an investigation begun by
Yamashita and Ueda for the case of S=1, and semiclassical analyses to argue
that such couplings generate bond order via a spin--Peierls transition. This is
followed by, or concurrent with, a transition into one of several possible
low-temperature Neel phases, with most simply collinear, but also coplanar or
mixed spin patterns. In a collinear Neel phase, a dispersionless string-like
magnon mode dominates the resulting excitation spectrum, providing a
distinctive signature of the parent geometrically frustrated state. We comment
on the experimental situation.
