Complex sentence: Heterospory evolved during the Devonian period from isospory independently in several plant groups; the clubmosses, the arborescent horsetails, and progymnosperms as part of the process of evolution of the timing of sex differentiation.
Split into simpler sentences: Heterospory evolved during the Devonian period from isospory independently in several plant groups: the clubmosses, the arborescent horsetails, and progymnosperms. This occurred as part of the process of evolution of the timing of sex differentiation.