Premise of research. The Devonian was a crucial period in plant history for both vegetative and reproductive evolution. We focus on reproductive structures during this time period and ask whether a simple difference among lineages in how their sporangia were borne on fertile axes, either laterally or terminally, may explain large-scale patterns of reproductive evolution over the Devonian.

Methodology. We used continuous and discrete characters to quantify three basic aspects of Silurian and Devonian taxa: sporangium size, supporting axis diameter, and overall organization and architecture of their reproductive structures. We then used standard statistical tests to determine whether lineages with lateral sporangia and those with terminal sporangia exhibited different patterns in these traits over time.

Pivotal results. Taxa with lateral sporangia (mainly lycophytes) produced relatively large sporangia on reproductive structures that mainly varied in how densely these sporangia were packed around an axis. Taxa with terminal sporangia (mainly euphyllophytes) produced smaller sporangia borne on a wider range of branching architectures, reflecting either the elaboration of or the simplification of an ancestral fertile branching system.

Conclusions. A simple difference in sporangium position among major Devonian tracheophyte lineages appears to have had important consequences for their reproductive evolution, because it influenced the pathways of character change that these lineages were most likely to follow and therefore controlled the range of reproductive morphologies that they were likely to evolve.