This choice respects continued debate about relationships of these groups to other lineages of vascular plants. Zygopteris, Etapteris, Metaclepsydropsis, and Clepsydropsis. The main axis is a protostele, with rachis traces arising in a spiral. The pinnae traces arise from the rachis trace in pairs that depart from the ends of the H-arms. Ultimate pinnae have dichotomizing veins.
Fertile fronds have clusters of elongate sporangia that partially replace pinnules.
Sporangia may have one or several wall layers, making them appear transitional between eusporangiate and leptosporangiate forms. In fact, their mix of characters beyond the sporangia complicate this gradualist interpretation. Since this topology depends on a number of key homologies that are still up for debate, we have not included these plants in the overall phylogeny presented in the Virtual Paleobotany Laboratory. This hypothesis is interesting and bears careful consideration. However, such an interpretation would unseat these plants as possible ancestral fern morphologies.