A new permineralized seed fern stem from the Fayetteville Formation (middle Chesterian/upper Mississippian) of Arkansas has a combination of features including narrow diameter, eustele, secondary xylem with multiseriate, elliptical bordered pits on the tracheids, and leaf traces consisting of a few large bundles, and thus conforms to the concept of lyginopterid seed ferns. However, the specimen is unlike all previously reported lyginopterids in that it possesses a unique mode of leaf trace production. The stem is approximately 36 cm long and 2.5 by 1.5 cm wide, has five cryptic sympodia, and no sclerenchyma bundles in the pith region. Secondary xylem is thick with relatively small tracheids, and numerous thick rays. Phloem is well preserved. Secondary tissues are surrounded by a thin periderm. Sclerenchyma bundles are dispersed within the parenchyma of the inner cortex, and sclerenchyma bands form a “Dictyoxylon” type outer cortex. Leaf traces diverge from the stele in a 2/5's phyllotaxy. These traces are accompanied by concentric secondary xylem, and increase in size as they extend through the wood of the stem. Trace shape changes from isodiametric, to tangentially elongate, to a squat ‘C’ shape as it nears the periphery of the secondary xylem and enters a leaf base. In the leaf base the trace divides into three discrete elongate bundles, each surrounded by secondary xylem. Buds are produced in the axil of several leaves. The discovery of this new lyginopterid stem adds to the growing list of unique taxa described from the Fayetteville Formation and further solidifies it’s reputation as one of the most important upper Mississippian plant fossil sites in North America.

Key words: fossil, lyginopterid, Paleozoic, seed fern