Morphology and wall ultrastructure of a unique megaspore, Flabellisporites zhaotongensis Sui, McLoughlin et Feng sp. nov., from the upper Permian of Southwest China

Abstract

A new lycopsid megaspore species with intricate ornamentation, Flabellisporites zhaotongensis Sui, McLoughlin et Feng sp. nov., is described from the Lopingian Xuanwei Formation of Yunnan Province, Southwest China, using scanning and transmission electron microscopy. This trilete megaspore is characterized by elevated membraneous flanges adorned with multifurcate spines, the longest of which are discrete ribbon-like appendages that basally converge to form a membrane around the equator; and evenly spaced single or bifurcate spines with sharp tips covering the remaining surfaces. Ultrastructural analysis reveals that the megaspore wall consists of four layers: (1) a thin and dense foot layer; (2) an inner spongy layer with small circular or elongate sporopollenin grains with parallel arrangement; (3) an outer, more porous, spongy layer consisting of larger and better interconnected sporopollenin units, that form a grid-like structure in certain cases; and (4) an electron-dense outermost layer forming the sculptural elements. Both morphological and ultrastructural characteristics indicate that the new megaspore species belongs to Isoetales. Our discovery represents the oldest occurrence of Flabellisporites, indicating the origin of this morpho-lineage before the end-Permian mass extinction, and enriches the known diversity of isoetalean lycopsids in the late Permian Cathaysian Flora.