Characterizing the Most Primitive Seed Ferns. I. A Reconstruction of Elkinsia polymorpha

Abstract

Abundant gymnospermous remains occur in association with Elkinsia polymorpha in Upper Devonian deposits near Elkins, West Virginia, and these provide an opportunity to characterize and further refine our concepts of the most ancient seed ferns. The sporophyte of E. polymorpha has helically arranged dimorphic fronds, with large numbers of either preovulate cupules or synangiate prepollen organs borne terminally on the fertile fronds. Vegetative fronds are planar, with rachides that fork up to four times, and pinnate laterals that terminate in sphenopterid pinnules. Fertile fronds branch dichotomously throughout and are three-dimensional in the distal regions. Stems display a centrarch, three-ribbed protostele with prominent sclerenchyma of the sparganum type in the outer cortex. The frond rachis is vascularized by two C-shaped bundles. The hydrasperman preovulate cupules are as characterized in earlier studies. Prepollen organs consist of six to eight radially arranged sporangia that are fused at the base and free distally. Elkinsia polymorpha is the first seed fern to be reconstructed from Devonian sediments and the first preovulate seed fern to be characterized as a plant. As such, it represents an important evolutionary unit for ongoing phylogenetic studies of the origin of seed plants.