Trauma in the life of a Nebuloxyla, an Early Devonian basal euphyllophyte

Anatomically preserved material from Lower Devonian strata of the Battery Point Formation (Gaspé Peninsula, Quebec, Canada) offers a rare opportunity to reconstruct the sequence of events in the life of a Nebuloxyla mikmaqiana plant (early euphyllophyte) that led to wounding and the plant’s recovery after wounding. Using serial sections from cellulose acetate peels of the Nebuloxyla axis and volume renderings of the affected tissues, we show that the wound, centered around a branch and caused likely by an herbivore, resulted in removal of the extraxylary tissues of the branch starting from its very base, associated with bending of the xylem strand that supplied the branch. Downward bending of the branch xylem toward the base of the main axis led to the separation and displacement of a significant portion of it, leaving a short portion of the xylem strand at the base of the branch in its original position. The displaced strand of branch xylem, after stripping of the extraxylary tissues along with some of its own thickness, was left protruding outwards from the open wound surface. Subsequent development of wound response tissue filled the gap left by the wound around the protruding displaced xylem strand, surrounding it with wound periderm. This specimen provides evidence that plants as old as Nebuloxyla had the capacity to endure and survive traumatic events, and to deploy effective and sophisticated response mechanisms that had already evolved in the euphyllophyte clade by Emsian time. More broadly, this occurrence re-emphasizes the importance of permineralized fossils in documenting details of plant anatomy and development, even allowing glimpses into minutia of their daily life, such as interactions with their environment and immediate responses to these interactions.