Monocyclic Psaronius from the lower Pennsylvanian of the Illinois Basin

A new species of Psaronius is described from permineralized monocyclic stems with distichous leaf scars. Diameters for the steles are 3.5–4.4 cm, up to 6.5 cm for the stem, and 8 cm for the trunk which includes inner and outer roots; the longest of 11 specimens is 32 cm. These specimens from the lower Pennsylvanian of western Illinois are the oldest known histologically preserved stems and roots for the genus. Psaronius simplicicaulis sp.nov., with its distichous leaf arrangement, is very similar to Mississippian – early Pennsylvanian Megaphyton specimens, indicating that the earliest known marattialean tree ferns were smaller in diameter than their late Pennsylvanian relatives and probably monocyclic with distichous fronds. The middle Pennsylvanian increase in the distribution and abundance of Psaronius generally coincides with increased size, polycyclic stelar anatomy, and polystichy.