A novel Late Devonian (Frasnian) woody Cladoxylopsid from China

An investigation of the fossil plant Polypetalophyton wufengensis Geng ex Hilton, Geng et Kenrick provides new data on an early woody member of the Late Paleozoic group Cladoxylopsida. The morphology and anatomy of the lateral branch system have been studied in detail. Branches were produced in an irregular helix, but ultimate appendages were borne in a pinnate arrangement on a frondlike system. Vegetative pinnules were dichotomous and planar (Sphenopteridium-type), whereas fertile pinnules were decussately branched and bore fusiform sporangia terminally. Anatomically, Polypetalophyton is unusual because it has a stele that consists of several primary vascular segments, each of which consists of tracheids and parenchyma surrounded by wood comprising radially aligned tracheids with ray-like slits. Polypetalophyton is clearly most closely related to the extinct Cladoxylopsida, which is a poorly understood group of considerable phylogenetic significance. Comparisons are made with Pseudosporochnales and Iridopteridales, and it is argued that Polypetalophyton resembles most closely the former. Polypetalophyton differs from the handful of well-known members of this group in possessing radially aligned xylem and in lacking sclereid nests in the stem cortex. It is also unique in its plagiotropic pinnate frondlike branching system, which we interpret to be the functional equivalent of a leaf or frond. Polypetalophyton represents the first evidence of Sphenopteridium-type foliage occurring within Cladoxylopsids and demonstrates that this kind of foliage has evolved independently within at least three distinct plant clades: Zygopterids, Cladoxylopsids, and seed plants. The name Polypetalophyton wufengensis is validated through the designation of a type specimen.