A century of seed ferns: A symposium to celebrate paradigm shifts in the understanding of seed plant
Doyle, James A. .
Seed ferns and the origin of angiosperms.
“Seed ferns” are a paraphyletic mixture of stem relatives of extant (crown group) seed plants and extinct lines within the crown group, so understanding of their relationships requires analysis of seed plant phylogeny as a whole. Because molecular phylogenetic analyses indicate that Gnetales are related to conifers rather than angiosperms and no other living gymnosperm group is directly related to angiosperms, studies on the origin of angiosperms must focus on extinct taxa, especially seed ferns. Some workers have homologized the seed fern cupule with the angiosperm carpel, but angiosperm ovules have basically two integuments rather than one, as in other seed plants. This has led others to homologize the cupule with the angiosperm outer integument. However, morphological and phylogenetic arguments indicate that cupules are not homologous in different seed fern taxa. Cupules of hydraspermans and lyginopterids may correspond to fertile appendages of “progymnosperms,” but those of more derived groups 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). Developmental genetic studies confirm that the outer integument of angiosperms (unlike the inner integument) is a foliar structure, with the nucellus plus inner integument (original seed plant ovule) borne on its adaxial side. Analysis of a revised morphological data set for seed plants with living taxa constrained into the arrangement found in molecular analyses links angiosperms with glossopterids and Caytonia, supporting the cupule-bitegmic ovule homology and implying that siphonogamy arose independently in angiosperms and derived gymnosperms. Origin of the carpel poses more problems: it could correspond to the cupule-bearing leaf of glossopterids, but no comparable structure is known in Caytonia. New data on unknown characters of derived seed ferns are needed to test these hypotheses.
1 - University of California, Evolution and Ecology, One Shields Ave, Davis, California, 95616, USA
Presentation Type: Symposium
Location: Ballroom 1 (Cliff Lodge)
Date: Tuesday, August 3rd, 2004
Time: 11:30 AM