Two anachoropterid fern rachises from the in situ volcanic ash of the Whetstone Horizon (Kladno Formation, Pennsylvanian), Radnice Basin, Czech Republic

Two partly anatomically preserved axes parallelly situated on a slab collected from an in situ volcanic ash bed called “Bělka” of the Whetstone Horizon, Kladno Formation (Pennsylvanian), Czech Republic, were studied in detail. Anatomically, both axes possess a C-shaped xylem strand with protoxylem tracheids situated on the convex side, demonstrating an inversicatenalean-type anachoropterid affiliation. They are further suggested to belong to one biological species, as they share a number of similar characteristics and common structures. Systematically, one of the two studied axes retains a primitive form of foliar anatomy with the oldest known anachoropterid plant (Anachoropteris sp.) in having two rather short lateral arms compared to the long median region. Although lacking foliage information, both axes likely belong to the rachis part of Discosoropteris zlatkokvacekii Pšenička, Zhou, Boyce, Votočková Frojdová, Bek and Wang, a fern species that was recently established based on the same slab where the two studied axes were preserved. Such a combination may further indicate the presence of a new family in the late Paleozoic anachoropterid plants. In addition, selected anatomically preserved ferns from the Whetstone Horizon were reviewed, which promotes a better understanding of the anatomical variability of fern species.