MAPES, GENE*, KEQIN SUN, MICHAEL KRINGS, GAR W. ROTHWELL, AND ROYAL H. MAPES. Department of Environmental and Plant Biology, Ohio University, Athens, OH 45701, U.S.A; China University of Geosciences, Beijing 100083, China; Department of Ecology and Evolutionary Biology, University of Kansas, Lawrence KS 66045, U.S.A.; Department of Environmental and Plant Biology, Ohio University, Athens, OH 45701, U.S.A; Department of Geological Sciences, Ohio University, Athens, OH 45701, U.S.A. - The earliest occurrence of Medullosa.
The medullosan pteridosperms are a common group of early seed plants
that extend from the Lower Carboniferous to the Permian of North
America and Europe. The earliest occurrence of the genus
Medullosa is from the Missippian Fayetteville and Imo
Formations of Arkansas, which are equivalent to the uppermost Lower
Carboniferous (Namurian A) in Europe. Four specimens of
permineralized stems with diverging leaf traces have been discovered,
and these represent a previously undescribed species with features
that are characteristic of vine-like growth architecture. Specimens
range up to 40 cm long and 2-3 cm in diameter. The number of vascular
segments ranges from two to eight, with relatively equal development
of centripetal and centrifugal secondary xylem. In cross sections the
vascular segments vary from circular to oval to tangentially
elongated, as they anastomose from level to level. The stelar zone is
surrounded by a thin zone of periderm and primary cortex. Cross
sections reveal that the primary cortex is subdivided into leaf bases
in differing stages of divergence, and this indicates helical
phyllotaxis. Within the cortex, vascular bundles, resin canals and
sclerenchyma form the typical Myeloxylon histological
configuration for species of Medullosa. Leaf traces to a
single frond are produced over an extended vertical distance along the
stem, and are relatively large at the level of separation from the
stele. Traces subdivide further in the leaf base. In cross sections
of the stem, traces from each protoxylem sympodium consistently
diverge in two directions at the same level. This produces two
emission areas in each cylinder of secondary vascular tissues. These
stems represent the earliest occurrence of the genus Medullosa,
and are stratigraphically equivalent to the earliest records of the
family Medullosaceae.
Key words: Medullosa, Namurian A, North America, seed fern