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A whole-plant concept for an early Devonian basal lycophyte – implications for taxonomy and morphological evolution
A whole-plant concept was developed for the Early Devonian (Lochkovian – Pragian, ca. 410 Ma) drepanophycalean lycophyte Sengelia radicans gen. et sp. nov. from the Beartooth Butte Formation of Wyoming, based on 1131 fossil fragments. The concept, developed using data on morphology, taphonomy, sedimentology, and encrusting aquatic invertebrates, provides a complete picture of the morphology, growth habit, and environment of this early lycophyte. Sengelia radicans had a mat-forming habit, with prostrate stems and positively gravitropic root-bearing axes produced at K-branching points. The rooting axes were subsurface foraging organs, anchoring the plant and producing lateral roots (among the oldest true roots known to date) at different depths. Sengelia was fully terrestrial, occupying wetlands (river floodplains, deltas) characterized by frequent, possibly seasonal, high water stands and more severe episodic floods that buried plant populations. In the context of lycophyte phylogeny, the morphology of Sengelia radicans raises questions about the evolutionary origins of roots and organ homology. Production of roots on rooting axes suggests that lycophyte roots did not evolve from apical branches of axial organs, as previously hypothesized, and instead represent a unique organ distinct from stems and axes. Rooting system architecture in Sengelia suggests uncoupling in the evolution of root identity and positive gravitropism, in the lycophytes. More broadly, the morphology of Sengelia bolsters a hypothesis explaining diversity in lycophyte body plans as a result of heterotopic and heterochronic changes in the expression of a branching-associated developmental pathway for bipolar organ identity, evidence for which is present in nearly every lycophyte lineage.