Characterizing the branching architecture of drepanophycalean lycophytes (Lycopsida): an exceptional specimen from the Early Devonian Hunsrück Slate, southwest Germany, and its paleobiological implications

An exceptionally large and well-preserved drepanophycalean lycophyte specimen from the Early Emsian Hunsrück Slate exhibits branching morphology and architecture that suggest previously unrecognized diversity of drepanophycalean lycophytes in the German Early Devonian. The specimen shows long prostrate axes giving rise to both rooting axes and erect leafy, unbranched axes via K-branching. Rhizomatous growth along the substrate promoted rapid vegetative expansion of the plant forming monoclonal patches, potential adaptations to colonizing an unstable deltaic environment. K-branches including dormant buds may have allowed resurrection of the shoot system following burial by sediment or damage to growing shoot apices. Cutbank erosion may have uprooted parts of the plants, which were then transported into the open marine areas and eventually buried in the Hunsrück Slate depositional environment. We hypothesize that remains of Late Silurian/Early Devonian plants (and possibly Prototaxites) may have drifted for many kilometers and were large enough to raft small organisms over considerable distances. Thus, the Late Silurian/Early Devonian may have witnessed the earliest passive oceanic dispersal by rafting of terrestrial invertebrates and therefore marks a crucial time in Earth’s history with respect to this important paleobiogeographical factor.