TOMESCU, ALEXANDRU MIHAIL FLORIAN. Department of Environmental and Plant Biology, Ohio University, Athens, OH 45701. - The seed fern Lyginopteris in the Mississippian of North America.
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