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Describes a new permineralized pteridosperm morphotaxon recovered from the marine black shales of Fayetteville Formation near Fayetteville, Arkansas. General features; Description of the pith; Description of the primary xylem; Description of the secondary xylem; Description of the vascular cambium and secondary phloem.

Three specimens of a medullosan seed fern stem with attached leaf bases, from the Chesterian Series of Arkansas, USA, are the earliest unequivocal occurrence of the genus Medullosa and form the basis for a new species Medullosa steinii. The stems have numerous features characteristic of vines, including stem diameters of only 2–3 cm in diameter, numerous wide, plate-like rays, relatively wide tracheids, the presence of cortical spines and variable but numerous stelar segments. In M. steinii vascular segment numbers vary from two to eight. Primary xylem bundles consist of one to several protoxylem poles with metaxylem and abundant parenchyma; each bundle is surrounded by secondary vascular tissue. Leaf traces typically diverge from a vascular segment as pairs, with the resulting bundles separating from each other at an angle of 90–100°. Paired leaf traces are separated from each other by a wedge of secondary vascular tissue, thus producing a distinctive mode of leaf trace production for the new species. Leaf traces divide repeatedly to produce the Myeloxylon rachis base configuration at the margin of the stem cortex. Rachis bases are decurrent and are separated from one another by a discontinuous row of sclerotic bundles and from the stem by a prominent periderm. The discovery of this plant demonstrates that scrambling and climbing seed ferns were part of some plant communities by Mississippian time. [Copyright &y& Elsevier]