Structural and physiological constraints on arborescent lycopsid establishment and growth

The absence of fossil documentation of arborescent lycopsid life-history stages between preserved embryos and mature trees requires interpolation of establishment and juvenile growth. Here, a fossil specimen preserving both proximal and distal vascular anatomy of a single lycopsid tree was studied to compare proximal and distal trunk hydraulic conductivity. Proximal hydraulic conductivity was less than half that of the distal trunk, suggesting the proximal stem could not have been the conduit connecting the mature rooting system and distal shoot. Adding this constraint to other emerging evidence, we demonstrate that existing ontogenetic models could not have applied to arborescent lycopsid ontogeny. We propose a new model that, despite its unusual nature, agrees with all available evidence: sequentially larger stigmarian rooting axes must have been serially produced as the unipolar plant body grew upward and expanded its primary body during early ontogeny, thereby bypassing the proximal shoot. Previous ideas regarding arborescent lycopsid growth relied on highly unusual physiologies and uniquely high growth rates that would then imply extreme geobiological impact on the Earth system. The growth model advanced here accounts for their unique anatomy with much more typical plant biology.