THE PHYSIOLOGY OF ARBORESCENT LYCOPSID CORTEX: INSIGHTS FROM COMPARATIVE ANATOMY AND TAPHONOMY

Recent research has shown that many long-held hypotheses regarding arborescent lycopsid paleobiology are mutually contradictory. For example, high photosynthetic rates via a unique metabolic pathway and a short lifespan are at odds with the absence of internode growth and habitation in waterlogged, anoxic peat swamps that would have been nutrient-limited. Central to all questions of arborescent lycopsid physiology is their secondary cortex or periderm (i.e. bark), which could be abundant in Middle Pennsylvanian coals of Euramerica. In the absence of significant wood production, periderm is thought to have provided the structural support for these giant trees. However, sampling of coal ball peels suggests both the periderm and wood were weak tissues based on taphonomic and anatomical evidence, and periderm must have fissured during life due to geometric constraints on its growth. These checks on the structural potential of periderm suggest that this tissue was not the principal supporting tissue for these trees. Rather, arborescent lycopsids of all genera must have been dependent on primary tissues for structural support, constraining their potential ontogenies and life history. The most likely candidate for being a primary source of mechanical support may be the outer primary cortex, given that this tissue usually exhibits a higher preservational quality and less taphonomic deformation than periderm, while also being decay-resistant and composed of thick-walled cells.