YUGUANGIA ORDINATA GEN. ET SP. NOV., A NEW LYCOPSID FROM THE MIDDLE DEVONIAN (LATE GIVETIAN) OF YUNNAN, CHINA, AND ITS PHYLOGENETIC IMPLICATIONS

摘要

A new heterosporous and ligulate lycopsid, Yuguangia ordinata gen. et sp. nov., is described from the Haikou Formation (late Givetian) of Yunnan Province, southwestern China. Its slender stems branch isotomously. Vegetative leaves are arranged in pseudowhorls. Compact bisporangiate strobili terminate distal branches and have megasporangia located proximally and microsporangia distally. Sporophylls are differentiated into horizontal pedicel, upturned lamina, and downward "heel," and they are pseudowhorlly arranged, with about eight per gyre. Each sporophyll bears adaxially one mega- or microsporangium. Megasporangia and micro-sporangia are obovoid, round to elliptical in shape. Each megasporangium contains four or more Trileites-type megaspores. Microspores are comparable, to some extent, to those of the dispersed spore genus Acinosporites. A ligule is located at the distal end of the horizontal pedicel of a sporophyll. Distal axes have an exarch primary xylem strand, phloem, and a two-zoned cortex. This study provides a new temporal framework for ligule acquisition and early diversification of heterosporous lycopsid lineages and implies that homosporous, ligulate lycopsid lineages (i.e., Leclercqia of Protolepidodendrales) and heterosporous, ligulate lycopsid lineages (i.e., Selaginellales and Isoetales) had diverged by the late Middle Devonian.