Lochkovian plants from the Xitun Formation of Yunnan, China, and their palaeophytogeographical significance

Abstract

AbstractA megafossil plant assemblage containing three zosterophyll plants (Gen. nov. A, aff.Huiasp. and a unnamed spike) is described from the Lower Devonian Xitun Formation (Lochkovian) of Qujing, Yunnan Province, China, providing new data on the diversity of plant types during the Lochkovian, a time poorly represented by fossil vascular plants. Gen. nov. A has a character combination of naked axes, a diagnostic branching pattern (i.e. K-type branching as well as small lateral branches scattered along the axes) and lateral stalked sporangia. Aff.Huiasp. has ovate and stalked sporangia which are arranged in loose spirals and are reflexed adaxially. Combining the plants previously reported from the Xitun Formation (i.e.Xitunia spinitheca,Zosterophyllum shengfengenseandZ.minorstachyum), the Xitun flora is dominated by zosterophylls and exhibits varied vegetative and fertile morphologies. Unlike the coeval plant assemblages in many localities of Laurussia and western Gondwana, which are mainly composed of plants with isotomous branching and terminal sporangia, the Lochkovian flora of South China is dominated by zosterophylls, documenting the existence of a distinct Northwest Gondwanan phytogeographic unit in the Lochkovian and a strong effect of phytogeographical isolation.