Eocladoxylon (Protopteridium) minutum (Halle) Koidzumi from the Middle Devonian of Yunnan, China: An Early Rhacophyton‐Like Plant?

Abstract

New specimens of Eocladoxylon (Protopteridium) minutum (Halle) Koidzumi collected from the Middle Devonian Xichong Formation, Yunnan province, China, demonstrate three orders of branching. Sterile branching systems have a pinnate two‐dimensional branching pattern with alternate insertion of lower orders. Up to several times deeply dissected, overall wedge‐shaped, flattened appendages are inserted alternately on the third order of branching. Second and third orders of branching have an opposite pair of similar appendages inserted at the base, perpendicular to the plane of the remaining branching system. Fertile appendages, consisting of a dichotomous axial system and terminal pairs of sporangia attached closely in groups of eight, replace the sterile appendages in fertile branches. Anatomy of a probable second‐order axis demonstrates a bipolar clepsydroid vascular system. The sterile branching system and anatomy bear some similarity to the Late Devonian fernlike plant Rhacophyton. Eocladoxylon is neither a cladoxylopsid nor a progymnosperm, as was previously believed. The potential for an ancestor/descendant relationship between Iridopteridales and Rhacophyton via Eocladoxylon is briefly discussed in the context of the geographical isolation of the Middle Devonian flora of South China.