The Enigmatic Paleozoic plants Spermopteris and Phasmatocycas reconsidered

Ovule-bearing leaves from the Paleozoic of North America assigned to Spermopteris and Phasmatocycas have been interpreted as primitive cycad megasporophylls. According to this hypothesis, Cycas megasporophylls were derived from a Spermopteris-like ancestor via Phasmatocycas and various other taeniop...

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Bibliographic Details
Published in:American journal of botany 2003-11, Vol.90 (11), p.1585-1595
Main Authors: Axsmith, Brian J., Serbet, Rudolph, Krings, Michael, Taylor, Thomas N., Taylor, Edith L., Mamay, Sergius H.
Format: Article
Language:English
Subjects:
Botany
Carboniferous
cycads
Flowers & plants
Fossils
Holotypes
Leaves
Natural history
Ovules
Paleobotany
Permian
Phasmatocyas
Phylogenetics
Plant fertility
Plant reproduction
Plants
pteridosperms
Specimens
Spermopteris
Taeniopteris
Taxa
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Summary:Ovule-bearing leaves from the Paleozoic of North America assigned to Spermopteris and Phasmatocycas have been interpreted as primitive cycad megasporophylls. According to this hypothesis, Cycas megasporophylls were derived from a Spermopteris-like ancestor via Phasmatocycas and various other taeniopterid forms. This putative transformation entailed the phyletic shift of ovule attachment from the abaxial lamina surface of Spermopteris to the leaf midrib in Phasmatocyas. However, reexamination of the original Spermopteris specimens from the Lawrence Shale of Kansas has shown that the ovules are attached to the leaf midrib. Therefore, Spermopteris and Phasmatocyas differ only in a few details of lamina morphology. The apical cleft of the ovules of both forms is interpreted as an original feature rather than a preservational artifact; however, the abaxial flange is probably a result of compression of a terete midrib. Spermopteris is typified by sterile specimens of Taeniopteris coriacea from Europe, which are of uncertain affinity to the fertile leaves. Therefore, we propose that the ovule-bearing leaves now known as Spermopteris coriacea be named as a new species of Phasmatocycas (Phasmatocycas bridwellii sp. nov.). The new concept of Phasmatocyas is less cycad-like than previously thought, and the phylogenetic position of the genus is unclear.
ISSN:0002-9122
1537-2197
DOI:10.3732/ajb.90.11.1585