Trawetsia princetonensis gen. et sp. nov. (Blechnaceae): A Permineralized Fern from the Middle Eocene Princeton Chert

Trawetsia princetonensis Smith, Stockey, Nishida & Rothwell gen. et sp. nov. is described from several permineralized rhizomes, stipes, and higher-order frond members of a blechnoid fern that have been identified from the Middle Eocene Princeton Chert of British Columbia. Rhizomes bear stipes arising from all sides of the stem as well as diarch adventitious roots. The dictyostelic rhizomes are up to 16 mm across and have five to seven amphiphloic cauline bundles that are capped internally and externally by sclerenchyma. Ground tissue, where well preserved, is transitional from arenchymatous to parenchymatous, with abundant small sclerotic nests found throughout the pith and cortex. Multicellular scales are borne on the rhizome but are sparse and do not imbricate. At least three orders of frond material are preserved: stipes, primary pinnae, and laminar pinnules. Stipes have an adaxial groove, two adaxial hippocampiform bundles, and seven or fewer small, circular bundles arranged in an abaxial U-shaped arc. In distal sections of the rachis, the vascular bundles are reduced to three or fewer. Ground tissue of the stipe has sclerenchyma nests near the base but distally becomes completely aerenchymatous, further evidence that the Princeton Chert preserves an ancient wetland environment. This fossil material expands our knowledge of fern diversity from the Princeton Chert locality, which also includes osmundaceous, dennstaedtioid, and dryopterid ferns, and it contributes to the understanding of the timing and evolution of derived Filicales. While the vegetative anatomy of this fern allies it with the Blechnaceae, the lack of attached fertile fronds precludes the possible recognition of T. princetonensis as a fossil representative of a modern genus or perhaps even a living species.