The Enigmatic Paleozoic plants Spermopteris and Phasmatocycas reconsidered

Abstract

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.

Investigating the Genetic Diversity, Population Differentiation and Population Dynamics of Cycas segmentifida (Cycadaceae) Endemic to Southwest China by Multiple Molecular Markers

“…Cycads are dioecious and long-lived plants. They have experienced a long process of evolution (Gao and Thomas, 1989; Axsmith et al, 2003) and should own high genetic diversity. However, a recent research proposed that the extant cycads evolved recently (Nagalingum et al, 2011).…”

Genetic diversity, genetic structure and demographic history of Cycas simplicipinna(Cycadaceae) assessed by DNA sequences and SSR markers

“…Fossil evidence shows that cycads originated approximately 275–300 million years ago [10,11]. Molecular evidence also shows that cycads originated much earlier than flowering plants [12,13], which originated approximately 125 million years ago [14,15].…”

A low diversity, seasonal tropical landscape dominated by conifers and peltasperms: Early Permian Abo Formation, New Mexico

“…These features can be seen clearly on the specimen, even though details of venation cannot be determined. Axsmith et al (2003) have reexamined specimens of this genus and have concluded that its cycad affinities are problematic. Phasmatocycas has been associated with Taeniopteris sterile foliage, which has not been identified in our collections from New Mexico.…”