The seed fern Lyginopteris is described from Upper Mississippian (middle Chesterian - Namurian A) shales in northwestern Arkansas. The pyritized stem is 29 cm long and slightly compressed; the diameter is about 11 x 5 mm. Seven leaf bases diverge at intervals of 2 to 5.5 cm. A Lyginopteris-type rachis with a paired vascular bundle is also preserved. The cauline primary bundles are mesarch and positioned at the margin of the pith. They form a eustele with five sympodia. The secondary xylem includes numerous rays. The vascular cambium and phloem are incompletely preserved. The inner cortex consists of thin-walled parenchyma cells. The outer cortex shows the characteristic Dictyoxylon structure of Lyginopteris. Coprolites produced by plant tissue decomposers were observed in different tissues of the stem. The study of the leaf trace divergence necessitated the use of a deformation model to help reconstructing the original position of the cauline bundles in the compressed stele. Phyllotaxis of the specimen approaches 2/5. The cauline bundles divide radially and the leaf traces diverge from the stele at very low angle, extending through five internodes before entering the leaf bases. They follow a slightly dextrorse coiling pattern upward. The leaf traces become bilobed but do not divide; in the leaf bases they acquire a butterfly shape. A characteristic feature of the specimen is the presence of a sclerotic bundle adaxial to the diverging leaf trace. The bundles accompany each leaf trace from its emergence in the cortex and along its whole trajectory to the leaf base. In contrast to previously described Lyginopteris stems, there are no capitate epidermal glands. These features suggest that the specimen could belong to a new species. This new occurrence of Lyginopteris confirms the North American distribution of the genus and emphasizes that it was not restricted to Western Europe.

Key words: coprolites, Lyginopteris, Mississippian, North America, seed fern