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Fossil remains of a fern were collected from the basalmost Carboniferous Tournaisian, Hardwick Formation of Queensland, Australia. The fern is described as an erect false trunk composed of intertwined branch systems and phyllophores embedded in a mesh of adventitious roots. The base of the trunk attained up to 50 cm in diameter and was composed almost totally of roots. Branching of the stems is isotomous. Each subsequent branch gives rise to a phyllophore and a geotropic adventitious root. Smaller roots arise from the bases of the phyllophores, as well as the larger roots, and all are covered with hairs and are directed horizontally. The two forms of roots are functionally complementary, the larger for support and absorption of water from the soil, the smaller for absorption in the humidity within the trunk. The repetitive branching of the stems concomitant with the increase of phyllophores and the two forms of adventitious roots comprised the oldest tree fern with false trunk known to this time. The phyllophores have no apparent phyllotaxy and many are abortive before reaching the exterior of the trunk. The cauline xylem strand, in transverse section, is a solid centrarch protostele while the phyllophore traces are simple and clepsydroid in outline. Xylem strands in the roots have four, rarely three or five, exarch protoxylem points. The combination of the very primitive anatomy with the arborescent habit is unique and the name Symplocopteris wyattii n. gen. et n. sp. is proposed. This plant is the oldest known zygopterid fern showing the clear anatomical and morphological distinction between stems, leaves and roots. |
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