A new species of Plenasium (Osmundaceae) from the Eocene of Southeast Asia.

Osmundales (The Royal Fern order) has a rich and diverse fossil record dating back into the Permian. Of special importance are their permineralized trunks and rhizomes, whose complex anatomical structure enables precise systematic placement. A particularly interesting taxon of fossil osmundalean stems is Aurealcaulis, a genus of hitherto unknown affinity that comprises large stems with an unusual, highly dissected stele that have so far been known only from the Cretaceous and Palaeogene of North America. Here, we present the results of a systematic analysis of calcified Aurealcaulis-like osmundalean stems from the Eocene Na Duong Formation (late Bartonian-Priabonian) of northern Vietnam. The trunk fragments are up to 1 m long, simple or branching, and exceed 20 cm in diameter. Each contains a central, up to 2-cm-thick stem that is clothed in a mantle of persistent stipe bases. The pith is poorly preserved, and mostly deformed or replaced by calcite spherules. The surrounding xylem siphon is very thick (up to 20+ cells in radial thickness) and perforated by abundant, complete leaf gaps into up to 25+ narrow segments. The cortex is two-layered with a thin parenchymatic inner layer and a thicker, presumably sclerenchymatic outer layer whose preservation is poor again due to growth of calcite spherules. The stipe bases bear a heterogeneous sclerenchyma cylinder with an outer ring of thick-walled cells and lateral wings containing several scattered fibre masses. Leaf traces originate from two protoxylem poles in two adjacent xylem segments, and depart from the stele in the form of two separate masses that fuse into an endarch, C-shaped trace in the cortex—a diagnostic feature unique to Aurealcaulis. Phylogenetic network analyses combining fossil and extant taxa identify this new species as intermediate between Aurealcaulis and extant Plenasium, supporting a recent assumption that Aureacaulis may in fact be an extinct subgenus of Plenasium. Moreover, this new species is not only the youngest known fossil record of Aurealcaulis, but also the first such stem that lies within the modern range of distribution of Plenasium today. Hence, this remarkable new species provides novel insights into the evolutionary history of modern Royal Ferns in terms of biogeography, geochronology, and anatomical structure.