Macrofossil evidence unveiling evolution and ecology of early Ephedraceae

Abstract

The Early Cretaceous Yixian Formation from western Liaoning in northeastern China is well known for its extremely high diversity of plant macrofossils including important basal members of the Gnetales. The Gnetales from the Yixian Formation currently play a significant role in our present understanding of evolution and adaptation within the group. In the present paper the ephedroid macrofossil genus Prognetella Krassilov et Bugdaeva previously known from the Early Cretaceous of Transbaikalia in southern Russia is revised based on additional specimens from the Yixian Formation. Baicarpus Gang Han et al., a recently described genus interpreted as an angiosperm is reduced to synonymy and reinterpreted as a member of the Gnetales, and placed within the genus Prognetella. Reproductive shoots of Prognetella consist of nodes and internodes, and each of these constitutes a lax compound female spike; foliar bracts are paired at each node, and each foliar bract usually encloses a reproductive unit; the paired female reproductive units together with their subtending bracts and the supporting peduncle constitute a diaspore. The diaspore was abscised from the lower node and disseminated when the plant ripened. A cladistic analysis was conducted based on morphological characters of reproductive organs to understand the evolution of early Ephedraceae. The result suggests that Prognetella is transitional between the genera Siphonospermum Rydin et Friis and Chengia Yang et al. and further illustrates the reduction and sterilization hypothesis previously proposed within the Ephedraceae. In addition, this study discovered a new and unusual dispersal mechanism within the basal members of the Ephedraceae.