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(American Journal of Botany. 2009;96:296-322.) doi: 10.3732/ajb.0800209 © 2009 Botanical Society of America, Inc. |
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Special Invited Papers |
2 Department of Environmental and Plant Biology, Ohio University, Athens, Ohio 45701 USA 3 Plant Biology and L.H. Bailey Hortorium, 412 Mann Library, Cornell University, Ithaca, New York 14853 USA 4 Department of Biological Sciences, University of Alberta, Edmonton, AB, T6G 2E9, Canada
ABSTRACT
Bennettitales is an extinct group of seed plants with reproductive structures that are similar in some respects to both Gnetales and angiosperms, but systematic relationships among the three clades remain controversial. This study summarizes characters of bennettitalean plants and presents new evidence for the structure of cones and seeds that help clarify relationships of Bennettitales to flowering plants, Gnetales, and other potential angiosperm sister groups. Bennettitales have simple mono- or bisporangiate cones. Seeds are borne terminally on sporophylls. They have a unique structure that includes a nucellus with a solid apex, no pollen chamber, and a single integument, and they are clearly not enclosed by a cupule or other specialized structures. Such features differ substantially from Gnetales, flowering plants, and the seed fern Caytonia, providing no compelling evidence for the origin of the angiospermous carpel. Cladistic tests were performed to assess the strength of the "anthophyte hypothesis" and possible relationships of Bennettitales, Gnetales, and Caytonia to flowering plants. Our results do not support the anthophyte hypothesis for the origin of angiosperms by a transformation of fertile organs that were already aggregated into a cone or flower-like structure. However, the anthophyte topology of the seed plant tree continues to be supported by morphological analyses of living and extinct taxa.
Key Words: anthophytes • Bennettitales • Caytonia • Cycadeoidea • Erdtmanithecales • Gnetales • Williamsonia
Received for publication 25 June 2008. Accepted for publication 20 November 2008.
FOOTNOTES
1 While the contents of this work are solely the responsibility of the authors, we are indebted to several individuals, including J. Hilton, K. Nixon, and D. Sokoloff for fruitful discussions and helpful recommendations for improvements to the presentation. Figures 1 and 2 were drafted by M. Rothman. This study was supported in part by the National Science Foundation (Grant No. EF-0629819 to G.W.R. and R.A.S.) and NSERC grant A-6908 to R.A.S.
5 Author for correspondence (e-mail: rothwell{at}ohio.edu), phone: 740-593-1129
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