Abstract

ABSTRACT
• Premise of the study: Discovery of cupulate ovules of Doylea tetrahedrasperma within a compact, compound seed cone highlights the rich diversity of fructification morphologies, pollination biologies, post-pollination enclosure of seeds, and systematic diversity of in Early Cretaceous gymnosperms.
• Methods: Specimens were studied from anatomical sections prepared by the cellulose acetate peel technique; three-dimensional visualizations were rendered using AVIZO, and phylogenetic analyses of morphological characters were performed using TNT.
• Key results: Doylea tetrahedrasperma has bract/fertile short shoot complexes that are helically arranged within a compact seed cone of compound structure. Individual complexes diverge from the axis as a single unit, and separate distally into a free bract tip and two sporophylls of an axillary fertile shoot. Each sporophyll bears a single, abaxial seed, recurved toward the cone axis, that is enveloped after pollinaton by scale tissue, forming a closed cupule. Ovules are pollinated by bissacate grains that are captured by micropylar pollination horns, and enter the pollen chamber through a narrow micropyle. A phylogenetic analysis of seed plants helps to clarify the systematic affinities of this plant.
• Conclusions: The unique combination of characters shown by Doylea tetrahedrasperma includes the presence of cupulate seeds borne in conifer-like compound seed cones, an ovuliferous scale analogue that is also structurally equivalent to the ovulate stalk of Ginkgo biloba, gymnospermous pollination, and nearly complete enclosure of mature seeds. These features characterize the Doyleales ord. nov., clearly distinguish it from the seed fern order Umkomasiales, and allow for recognition of another recently described Early Cretaceous seed plant as a second species of Doylea. A morphological phylogenetic analysis of seed plants highlights systematic relationships of the Doyleales ord. nov., and emphasizes the explosive phylogenetic diversification of gymnosperms that was underway during the time interval when flowering plants may have originated and/or first began to radiate.