Earliest evolution of stipules among vascular plants documented in the late Paleozoic stem group of Marattiales.

backgroundStipules are specialized appendages borne at the base of a leaf petiole and may perform a variety of functions including sheltering delicate growing tissues from environmental exposure, facilitating vegetative propagation and dispersal, and providing climbing hooks or protective spines. While stipules are widespread in extant angiosperms and a few fern groups, their origins in geological history remain poorly understood. This study critically reconsiders the absence of stipules in the ancestry of Marattiales.

resultsBased on extraordinary collections from the early Permian Wuda Tuff Flora, we report, for the first time, aphlebia fossils organically attached to psaroniaceous petioles. The psaroniaceous aphlebiae are homologous to marattiaceous stipules, as evidenced by numerous shared characteristics. Functionally, psaroniaceous stipules appear to shelter juvenile fronds and the stem apex, with a continued role in mature fronds. Furthermore, their continued and potentially indeterminate growth, along with their fully laminated structure, suggests a possible role in vegetative propagation after detachment from the parent frond. However, no direct fossil evidence of stipules acting as vegetative propagules is currently available.

conclusionsOur discovery provides unprecedented view of stipules in psaroniaceous tree ferns. The discovery of psaroniaceous stipules is significant, as it represents the earliest known stipule in the plant kingdom and underscores their multifunctional roles in plant development.