Medullosan organs are widely distributed in the flood plain deposits of tropical and subtropical Carboniferous, along with those of other Paleozoic seed plants such as Cordaites. The group ranged from the Mississippian to well into the Permian, and attained their greatest diversity in the Upper Pennsylvanian. They are reconstructed as trees with an overall resemblance to an old friend, Psaronius. Secondary manoxylic wood is produced by a bifacial vascular cambium and surrounds each vascular strand.
Large rays connect parenchyma within the stele to ground tissue outside the wood.
Wood development is commonly thickest toward the center of the stem. This would make Medullosa very unusual, if not unique, among seed plants, thus complicating evolutionary relationships. Consequently, Medullosa is really monostelic, with the vascular cambium surrounding different bundles of the eustele rather than surrounding the entire collection of primary xylem as in angiosperms. Medullosa stems have a thick, sclerenchymous cortex with abundant secretory glands and ducts. The stem genus Sutcliffia is a close relative to Medullosa.