Abstract
A new species of the seed fern Lyginopteris is described from a nodule in Upper Mississippian (middle Chesterian-Namurian A) shales of the Fayetteville Formation in northwestern Arkansas. The pyritized stem is 29 cm long and slightly compressed, with a diameter of about 11×5 mm, and shows seven diverging leaf bases. The primary xylem of the specimen is eustelic. Foliar bundles extend through five internodes before entering the rachis bases; they do not divide along their trajectory through the stem. Phyllotaxis of the specimen approaches 2/5. Longitudinally oriented sclerenchyma strands accompany the diverging foliar vascular bundles adaxially in the cortex, and are incorporated in the outer cortex above the level of foliar rachis divergence. In contrast to previously described Lyginopteris species, this species lacks capitate epidermal glands. A Lyginopteris type rachis with a paired vascular bundle is preserved in the same nodule. Coprolites probably produced by oribatid mites are present in different tissues of the stem. The study of the leaf trace divergence necessitated the use of a deformation model to help reconstruct the original position of the cauline bundles in the compressed stele. This occurrence of Lyginopteris confirms earlier reports of the genus from North America and emphasizes that it was not restricted to Western Europe.
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