The medullosan pteridosperms are a common group of early seed plants that extend from the Lower Carboniferous to the Permian of North America and Europe. The earliest occurrence of the genus Medullosa is from the Missippian Fayetteville and Imo Formations of Arkansas, which are equivalent to the uppermost Lower Carboniferous (Namurian A) in Europe. Four specimens of permineralized stems with diverging leaf traces have been discovered, and these represent a previously undescribed species with features that are characteristic of vine-like growth architecture. Specimens range up to 40 cm long and 2-3 cm in diameter. The number of vascular segments ranges from two to eight, with relatively equal development of centripetal and centrifugal secondary xylem. In cross sections the vascular segments vary from circular to oval to tangentially elongated, as they anastomose from level to level. The stelar zone is surrounded by a thin zone of periderm and primary cortex. Cross sections reveal that the primary cortex is subdivided into leaf bases in differing stages of divergence, and this indicates helical phyllotaxis. Within the cortex, vascular bundles, resin canals and sclerenchyma form the typical Myeloxylon histological configuration for species of Medullosa. Leaf traces to a single frond are produced over an extended vertical distance along the stem, and are relatively large at the level of separation from the stele. Traces subdivide further in the leaf base. In cross sections of the stem, traces from each protoxylem sympodium consistently diverge in two directions at the same level. This produces two emission areas in each cylinder of secondary vascular tissues. These stems represent the earliest occurrence of the genus Medullosa, and are stratigraphically equivalent to the earliest records of the family Medullosaceae.

Key words: Medullosa, Namurian A, North America, seed fern