Abstract
The Permian aged osmundalean fern Palaeosmunda yunnanense Tian et Chang has been re-investigated based on the original specimens and demonstrated to represent a new genus within the extinct family Guaireaceae that we name Tiania yunnanense (Tian et Chang) gen. et. comb. nov. The stem of T. yunnanense is small, c. 30 mm in diameter, and comprises an ectophloic siphonostele without leaf gaps. Pith is bilayered and consists of longitudinally elongated cells. Cortex is not differentiated into an inner parenchymatous and outer sclerenchymatous zone characteristic of members of the Osmundaceae, and possesses longitudinally elongated cells and large secretory cells. Leaf traces are numerous (> 100) and adaxially curved, with two endarch protoxylem strands upon departing the stele, increasing to more than four protoxylem strands as the trace passes the cortex, all distributed along the adaxial side of the leaf trace. Leaf traces have more-or-less incurved lateral tips, and are encircled by a vascular bundle sheath. A mass of thick-walled, longitudinally elongated cells occur in the adaxial concavity of the leaf trace. Adventitious roots arise singly from the abaxial and lateral margin of individual departing leaf traces, and extend in a sinuous manner horizontally and longitudinally. Abundant tyloses-like contents occur in cells of the cortex and represent the first account of these within fossil Osmundales, but unlike other plant groups in which they occur within xylem, fibre-tracheids and lactifers, in T. yunnanense they occur in longitudinally elongated cortex cells where they presumably impeded fungal hyphae growth. Radially aligned ‘secondary’ parenchyma within the stem appears to represent a wound reaction in the cortex in the living plant. Due to its siphonostele without leaf gaps, Tiania is interpreted as an evolutionary intermediate between the protostelic thamnopterids and the more advanced dictyostelic osmundaleans. Biogeographic implications of Tiania are discussed from which it is concluded that the osmundalean diversity from the Late Permian of South China is unusually high and that these plants are likely to play an increasingly important role in our understanding of the early evolutionary history and systematic relationships of the Osmundales.