ONE CONE, TWO GENDERS: COULD LATE PALEOZOIC BISEXUAL CORDAITEANS BE ANGIOSPERM ANCESTORS?

Angiosperm flowers can be viewed as condensed bisexual cones, in which the upper cone scales (carpels) produce seeds, intermediate scales (stamens) produce pollen, and lower scales (sepals and petals) are sterile. Extinct cordaitean gymnosperms have compound cones with a primary axis bearing secondary fertile shoots. A new cordaitean cone has secondary fertile shoots organized like flowers: the upper fertile scale bears a single, basally attached seed; intermediate scales bear pollen sacs; and the lower scales are sterile.

The new cone comes from localities in the Kalo Formation (Pennsylvanian) of Iowa. Four specimens have attached seeds, identified as Nucellangiumbased on features of the integument. Like angiosperm flowers, the new cone matured from base to tip (pollen first, seed last), so that specimens with mature seeds lack intact, filled pollen sacs. We interpret it as bisexual due to the extraordinary amount of cordaitean pollen found between cone scales, and fragmented pollen sacs with cordaitean pollen attached to male scales.

Today, angiosperms are the only seed plants with bisexual cones (flowers): understanding the origin of angiosperm bisexuality is critical for identifying their ancestry. Phylogenies based on morphology link angiosperms to bennetitaleans (Mesozoic gymnosperms with bisexual cones) and gnetaleans (Recent gymnosperms whose ancestors had bisexual cones). Molecular phylogenies suggest that angiosperms are either the sister group of all living seed plants, or of cycads, and link gnetaleans to conifers. Pennsylvanian seed plants with bisexual cones support molecular clock data placing the divergence of angiosperms and other seed plants in the Late Paleozoic.

Cordaiteans, as the sister group of conifers, have not been considered likely angiosperm ancestors: discovery of a bisexual cordaitean cone may change this. The flower-like organization of bisexual cordaitean cones suggests that the A-, B-, C- and E-gene families, which control floral development and the order of sterile and fertile organs in angiosperm flowers, appeared in the Late Paleozoic. The appearance of bisexual cones in cordaiteans, a wind-pollinated group with simple seeds and no pollen tubes, suggests that flower-like organization preceded closed carpels, double fertilization and a doubled seed integument.