AXSMITH, BRIAN J1*, RUDOLPH SERBET2, MICHAEL KRINGS2, THOMAS N TAYLOR2, and EDITH L TAYLOR2. 1Department of Biological Sciences (LSCB 124), University of South Alabama, Mobile, Alabama 36688; 2Department of Ecology and Evolutionary Biology and Natural History Museum and Biodiversity Research Center, University of Kansas, Lawrence, Kansas 66045. - New intepretations of the enigmatic Paleozoic plants Spermopteris and Phasmatocycas.
Despite being somewhat overshadowed by relatively recent reports of
indisputable cycad remains from the Permian of China, the North
American Upper Carboniferous and Lower Permian taxa
Spermopteris and Phasmatocycas, respectively, have been
of considerable interest, due primarily to their suggested status as
basal cycads with links to the “pteridosperms.” According to an
influential theory, cycad megasporophylls were derived from those of a
Spermopteris-like ancestor via Phasmatocycas. This
putative transformation entailed the phylogenetic shift of the ovules
from the abaxial lamina surface of Spermopteris to the
megasporophyll midrib in Phasmatocycas and extant Cycas.
We have initiated a restudy of these fossils based on the original
Spermopteris coriacea specimens from the Upper Carboniferous
(Virgilian) Lawrence Shale described by Cridland and Morris, which are
more complete and informative than previously realized, and newly
collected specimens of Phasmatocycas kansana from the Lower
Permian (Leonardian) Wellington Formation. We have found that the
ovules of Spermopteris are attached to the midrib rather than
the leaf lamina. Based on this and other characters, we propose that
Spermopteris and Phasmatocycas megasporophylls are
nearly identical, differing only in venation density and the height of
the lamina attachment on the midrib. We also provide evidence that the
apical cleft of the ovules of both forms is an original feature rather
than a preservational artifact as previously suggested. Comparisons
with Spermopteris reveal that the abaxial “flange” of
Phasmatocycas is an artifact produced by compression of an
originally inflated midrib. The implications of these new
interpretations for the supposed cycadalean affinity of
Spermopteris and Phasmatocycas will be discussed.
Key words: Phasmatocyas kansana, Spermopteris coriacea, Carboniferous, cycads, Permian, pteridosperms