INTEGRATING FOSSILS AND GENES: WHAT CAN PENNYSLVANIAN CORDAITEANS WITH BISEXUAL SEED CONES TELL US ABOUT ANGIOSPERM EVOLUTION?

The dominance of angiosperms and the relatively low diversity of extant ‘gymnosperms’ in the modern flora makes it difficult to determine the developmental origins of bisexual angiosperm flowers using only genetics. The discovery of bisexual cordaitean cones with attached Nucellangiumseeds in Pennsylvanian coal balls confirms the appearance of bisexual reproductive organs in the Late Paleozoic and may help resolve the phylogenetic uncertainties among major seed-plant clades. Previously, the earliest bisexual cones were Jurassic bennettitaleans and the trait was thought to be restricted to the glossopterid-bennettitalean-(angiosperm?) lineage.

The cordaiteans are an extinct basal seed-plant clade. The new bisexual cordaitean cone, known from five or more specimens, consists of a primary fertile shoot, two rows of secondary fertile shoots and a secondary fertile shoot at the apex. Rather than bearing only seeds or only pollen, the secondary fertile shoots have sterile basal cone scales, intermediate male scales, a set of upper sterile scales and a single apical scale bearing a basally attached, adaxial, orthotropus seed. The seed, which belongs to the dispersed genus Nucellangium, was closely surrounded by fertile and sterile scales during development. Maturation of fertile scales was acropetal (base first, apex last) and trichomes on the upper sterile scales surrounding the seed may have discouraged self-fertilization. Immature Nucellangium seeds found with micropylar extensions suggest that the fertile scale aided pollen capture. Except for the uppermost sterile scales, this organization resembles that of bisexual angiosperm flowers. If homologous, then the evolutionary path from bisexual cordaitean cones that had simple, wind-pollinated seeds with micropyles, to angiosperms would involve: 1) fusion of the ovule and fertile scale to form a bitegmatic seed; 2) fusion and reduction of the upper sterile scales to form a carpel; and 3) a change in seed orientation from orthotropus to anatropus. However, this evolutionary progression would imply that the primitive angiosperm reproductive organ was an inflorescence (compound cone) rather than a simple flower. Integrating the Nucellangium plant in phylogenetic analyses of major seed-plant lineages will be key to testing this hypothesis.