Parallel evolution of leaf morphology in gnetophytes

Abstract

In the present paper, an ephedroid macrofossil species from the Early Cretaceous Yixian Formation of western Liaoning of China is described as new to science: Ephedra multinervia Yang et Lin, sp. nov. This species has typical ephedroid morphology, e.g. the dichasial branching shoot system, swollen nodes, internodes having many fine longitudinal striations and opposite phyllotaxy. Ephedra multinervia has strap-shaped leaves with multiple dichotomizing veins and reduced female cones with a single pair of fertile bracts forming a cupule enclosing two inner seeds. Ephedra multinervia is similar to Ephedra archaeorhytidosperma Yang et al. and Ephedra hongtaoi Wang et Zheng in its reduced bi-ovulate female cone, but differs from the latter two species by the lengthy strap-like leaves bearing multiple parallel veins and its sessile female cones. A new evolutionary hypothesis of the gnetophytes is proposed based on a synthesis of reproductive morphology of macrofossils from the Early Cretaceous and modern representatives. A Chengia-like precursor might have given rise to the Gnetum-Welwitschia clade by diversification of leaf morphology and female reproductive organs. According to this new explanation, the Welwitschia-like strap-like leaves with multiple parallel veins in E. multinervia result from convergence.