DUNN, MICHAEL T.*, GAR W. ROTHWELL, and GENE MAPES. Department of Environmental and Plant Biology, Ohio University, Athens Ohio. - Upper Mississippian ovules with characteristics of two unique morphogenera: Stephanospermum and Rhynchosperma.
Two new ovules from the Fayetteville Formation blur the generic
boundaries of Rhynchosperma and Stephanospermum and
provide a possible link with Medullosa. The genus
Rhynchosperma Taylor and Eggert was erected for permineralized
seeds that are externally indistinguishable from the seed cast
morphogenus Rhynchogonium. Described from four specimens from
the upper Chesterian Fayetteville Formation of Arkansas,
Rhynchosperma is radially symmetrical with a two layered
integument. The apical portion of the integument is ribbed and
non-vascularized; the nucellus is fused to the integument and apically
differentiated into a dome shaped pollen receiving structure. The
vascular system is poorly preserved and apparently restricted to the
integument. Nothing is known about the plant that produced those
ovules. In contrast, the trigonocarpalean genus Stephanospermum
is widely regarded as belonging to medullosan seed ferns. As in all
other trigonocarps, Stephanospermum is radially symetrical,
with a stalked nucellus attached to the integument only at the base,
and has a three parted integument. Seven species ranging from the
Westphalian D to the Stephanian A-B boundary are placed in
Stephanospermum, typically characterized by the presence of a
sheath of tracheids in the nucellus and usually, a micropylar crown.
The new Fayetteville specimens are similar to Rhynchosperma in
external shape, two layered integument, number of ribs, and the
presence of a dome shaped pollen receiving structure. However, they
are more like Stephanospermum in vascular architecture and
nucellar attachment in that the nucellus is vascularized by a sheath
of tracheids, the integument is vascularized by discrete bundles, and
the nucellus is attached to the integument at the base. A stalked
nucellus has not been observed. The discovery of the Fayetteville
specimens with characters of two unique Carboniferous ovules provides
exciting insights into the evolution and phylogeny of upper Paleozoic
seed plants.
Key words: integument, Medullosaceae, nucellus, Rhynchosperma, Stephanospermum