A convincing hypothesis of angiosperm origins in the context of a secure understanding of seed plant phylogeny still remains to be achieved. A major impediment is the need for improved information on fossil seed plants, but there are also other challenges, particularly concerning how best to integrate the massive and increasing disparity between what is known about the genomes of living plants and what is known about the morphology and anatomy of extinct and extant taxa. In this chapter we briefly review the development of ideas concerning seed plant phylogeny and outline the status of research in this area. We also review the implications of ideas on relationships for understanding the origin and age of angiosperms.
Hypotheses of seed plant relationships
Ideas on relationships among seed plants that developed in the early twentieth century (e.g. Coulter and Chamberlain, 1917; Chamberlain, 1935) focused on gymnosperms (often excluding angiosperms completely) and recognised two groups: cycadopsids and coniferopsids. Cycadopsids included cycads, Bennettitales (Cycadeoidales) and the fossil plants grouped together at that time as seed ferns (e.g. lyginopterids, medullosans). Coniferopsids comprised conifers, cordaites and Ginkgo, and in many schemes also included Gnetales. This biphyletic interpretation of seed plant evolution gathered further support with the recognition of Devonian progymnosperms and the hypothesis that aneurophytalean progymnosperms might have given rise to the cycadopsid line, while archaeopteridalean progymnosperms might have given rise to the coniferopsid line (Beck, 1966, 1970, 1971, 1981). As developed by some authors (e.g. Doyle, 1977), this notion of two main lines of seed plant evolution interpreted angiosperms as the culmination of evolutionary elaboration in the cycadopsid line, while Gnetales were viewed in the equivalent position among coniferopsids (Figure 6.1).