Fertile organs and in situ spores of a marattialean fern (Asterotheca) from the Early Permian of northwestern China

Abstract

The genus Asterotheca is a representative marattialean fern of reproductive organs associated with Pecopteris fronds in the Late Paleozoic. The species of Asterotheca orientalis (Schenk) Potonié has a wide distribution in Eastern Asia, and for a long time it has been regarded as an index fossil of the Early Permian floras in North China. However, the detailed structures of sporangia and in situ spores for A. orientalis were poorly understood. In this study, exceptionally well-preserved fertile specimens of A. orientalis from the Lower Permian Kaipaicileike Formation in the Tarim Basin, Xinjiang, northwestern China, were investigated based on the studies on the material using light and scanning electron microscopies. Fertile pinnules are morphologically similar to sterile ones but bear Asterotheca-type synangia. Each synangium consists of a ring of 4–6 sporangia. The spores are trilete, with circular to subcircular amb, diameter of ca. 40 μm, and granulate sculpture. Comparisons were made between A. orientalis spores and those of other related in situ spores, as well as dispersed spores based on morphological and microstructural evidence. The dispersed spore species of Cyclogranisporites minutus (Bharadwaj)share the closest similarities in both the shape and size with in situ spores of A. orientalis. The present observations on the fertile organs of A. orientalis enrich diversity of the Asterotheca in situ spores, and provide clues for further understanding on the systematics and evolution of marattialean plants fen linage.