Additional observations on Rhynchosperma quinnii (Medullosaceae): a permineralized ovule from the Chesterian (Upper Mississippian) Fayetteville Formation of Arkansas

New ovules from the Fayetteville Formation (Upper Mississippian) of Arkansas expand our knowledge of the morphology and anatomy of Rhynchosperma and suggest it was produced by a medullosan seed fern. Rhynchosperma has been described as radially symmetrical with a two-layered integument and vascularization in the integument only. The apical portion of the integument is ribbed; the nucellus is fused to the integument and apically differentiated into a dome-shaped pollen chamber. The vascular system is incompletely known and apparently restricted to the base of the integument. The new specimens are like Rhynchosperma in external shape, size, number of ribs, and numerous histological features. However, new data reveal that the nucellus is vascularized by a sheath of tracheids, the integument is vascularized by discrete bundles, the pollen chamber has a nucellar beak, and the nucellus is attached to the integument for a variable distance from the base. In addition, the integument is tripartite with an elaborate apical region; ribs formed by the integument are more pronounced at the apex; and internally open, hollow lobes form a stellate micropylar canal. The presence of a tripartite integument, the nature of the vascular system, the nucellus-integument attachment, the pollen chamber structure, symmetry, and the association with medullosan vegetative remains suggest medullosan affinity for these ovules and strengthens the evidence for the origin of the family before the end of the Lower Carboniferous.