An upright psaroniaceous stump and two surrounding pecopteroids from the early Permian Wuda Tuff Flora

Abstract

An upright Psaronius stump was discovered in the upper part of Taiyuan Formation (Asselian) of Wuda Coalfield, Inner Mongolia, China. The stump is 47 cm high and truncated at the upper part of the ash layer in which it is preserved. At the height of 38 cm, the stem possesses a stelar configuration comprised of 3~4 cycles in 5 orthostichies. The Wuda Psaronius stump rapidly widens at the base until it attains a stable axial diameter; a root mantle is present in the proximal zone of primary body expansion, but not distal to that zone. This architecture is further demonstrated by a prostrate Psaronius trunk almost 7 m long with a stable stem diameter and two rows of frond scars visible over most of its length. This developmental mode is seen among extant leptosporangiate tree ferns but differs from that previously documented among North American Pennsylvanian marattialean tree ferns whereby the primary body of Psaronius stems increases continually in width towards the apex and is buttressed by a thick root mantle that compensates for the obconical nature of the stem. Two types of pecopteroid fronds associated with the Psaronius stump are identified as Pecopteris cf. polymorpha and Pecopteris orientalis based on their pinnule morphology. The Pecopteris orientalis frond possesses a penultimate rachis with stewartiopteroid anatomy and is more likely to be the leaf of the Psaronius stump based on a recently reported Psaronius specimen with stem and frond in connection from the same locality.