Evolution and Phylogeny of Gnetophytes: Evidence from the Anatomically Preserved Seed Cone Protoephedrites eamesii gen. et sp. nov. and the Seeds of Several Bennettitalean Species
International journal of plant sciences
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... one with characters that have been hypothesized as transitional to the origin of crown group gnetophytes has been discovered in Lower Cretaceous deposits on Vancouver Island, British Columbia, Canada. This cone, described as Protoephedrites eamesii gen. et sp. nov., provides the first anatomically preserved fossil evidence for the evolution of gnetophyte cones with seeds. The cone is a compound shoot system consisting of a primary axis with nodes that bear bracts and axillary fertile shoots in an opposite/ decussate arrangement. The secondary fertile shoot axis produces one or two pairs of diminutive bracteoles and a pair of erect ovules in an opposite/decussate pattern. In contrast to crown group gnetophytes, bracteoles subtend rather than surround and enclose the ovules. Ovules have a single multiseriate integument that is adnate to the nucellus in the basal region and free distally. The micropylar tube is short with a thick integument, but it displays distinctive cells of the inner integumentary epidermis that are characteristic of gnetophyte seeds. Each seed produces a large pollen chamber with a uniseriate wall. Paired erect ovules conform to the pleisomorphic morphology previously hypothesized for Ephedra seed cones, thus supporting the proposal that ancestral gnetophyte seeds are borne terminally on oppositely arranged sporophylls. The combination of gnetophyte synapomorphies and putative pleisomorphic characters displayed by Protoephedrites broadens the known range of morphologies for the most ancient gnetophytes. Detailed comparisons to several species of Bennettitales confirm that there are fundamental structural differences separating the seeds of Gnetales from those of Bennettitales, support the hypothesis that the outer seed envelope evolved within the gnetophyte clade, and suggest that Bennettitales are not as closely related to Gnetales as hypothesized by some authors.