Anatomically preserved marattialean plants from the Upper Permian of southwestern China: the trunk of Psaronius panxianensis sp. nov

摘要

A new species of the marattialean fern trunk Psaronius Cotta is described from the Upper Permian Cathaysian flora in the Xuanwei Formation of Guizhou Province, SW China, and named P. panxianensis sp. nov. Stems possess a helically diverging sequence of leaf traces organized in a 2/7 phyllotaxy and are characterized by stages of leaf trace development that include an early ontogenetic stage with a single vascular bundle (Stewartiopteris-type), a middle stage with two vascular bundles (Stipitopteris-type), and a late stage with three vascular bundles. Roots diverge singly or occasionally in pairs, arising alternately from the centrifugal surface of the peripheral cauline bundles (PCB) near the tip, and form both bound and free root mantles. This species is distinguished from all previously recognized species from the Euramerican and Gondwana floras in having an anchor-shaped sclerenchymatous strand internal to the PCB and a large V-shaped strand formed by the connection of two anchor-shaped strands. Psaronius panxianensis sp. nov. is important as its leaf traces show three distinct ontogenetic development stages within the trunk, with this organization interpreted as a derived condition within psaroniaceous marattialean fern evolution. In the stem abundant air space suggests that P. panxianensis lived in waterlogged substrates with aerenchyma facilitating gaseous exchange. However, the presence of marattialean fern dominated communities in the Upper Permian of SW China suggest an overall drying trend with these plants replacing earlier lycopsid and sphenopsid dominated wetland communities.