Seed ferns and the origin of angiosperms

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Abstract
Doyle, J.A. (Section of Evolution and Ecology, University of California, Davis, California 95616, USA). Seed ferns and the origin of angiosperms. J. Torrey Bot. Soc. 133: 169–209. 2006.—If molecular analyses are correct in indicating that Gnetales are related to conifers and no other living gymnosperm group is directly related to angiosperms, studies on the origin of angiosperms must focus on fossil taxa, including “seed ferns.” Some authors have homologized the angiosperm carpel with the cupule of seed ferns, but because angiosperm ovules have two integuments rather than one, cupules are more likely to be homologous with the outer integument. Cupules of the earliest seed ferns may be derived from fertile appendages of “progymnosperms,” but those of later taxa appear to be modified leaves or leaflets, with ovules borne on the abaxial surface in some (peltasperms, corystosperms), the adaxial surface in others (glossopterids, Caytonia). Positional relationships and developmental genetic data sugges...