Omphalophloios wagneri sp. nov., a new sub-arborescent lycopsid from the middle Moscovian (Middle Pennsylvansian) of the Illinois Basin, USA

Described is a new species of sub-arborescent lycopsid, Omphalophloios wagneri sp. nov. from roof shale of the middle Moscovian (Bolsovian/Atokan) (Middle Pennsylvanian) Lower Block Coal in the Illinois Basin, Indiana, USA. Available material represents mostly fragments of 16 to > 60 mm wide fertile axes that bear densely spaced sporangia, which are flattened due to compaction into disc-like shape, 2.5-3.1 mm in diameter. The outer sporangial cell wall layer displays a gradual transition from isometric cells in the central parts of adaxial and abaxial sporangial valves, to a narrow belt of elongated cells in the equatorial area, interpreted as a dehiscence zone. Micro- and megasporangia alternate in irregular patches, a single sporangium being attached adaxially to an extended sporophyll base. Cyperites-like sporophyll distal laminae are ~ 25 mm long at fertile shoot apices but become progressively longer (> 110 mm) down the shoots. Based on size distribution of stems, fertile shoots are estimated to bifurcate at least five times, which suggests that O. wagneri formed a relatively dense crown. The entire habit of the plant is difficult to assess from existing material; however, the tree was probably small and interpreted as colonizing low lying to slightly raised peat swamps. Omphalophloios wagneri differs from all other adpression-based species of the genus in having the smallest sporangia and several orders of fertile-axis bifurcations. In situ megaspores of the Zonalesporites brassertii type also are different from all other Omphalophloios megaspores.