Reconstructing the Callianthus plant–An early aquatic angiosperm from the Lower Cretaceous of China

Abstract

The Lower Cretaceous Yixian Formation in northeastern China has shed unique light on the early evolution of angiosperms, including Chaoyangia, Archaefructus, Sinocarpus, Callianthus, Liaoningcarpus, Baicarpus, Nothodichocarpum and Neofructus. Among them, Callianthus was represented only by its reproductive organs, while its vegetative parts and whole plant remained unknown. This hinders our understanding of this early angiosperm in the Early Cretaceous. Herein we report new materials of Callianthus dilae Wang and Zheng 2009, emended, including a young plant, roots, articulated stems, leaves and reproductive organs from the Formation. Based on new materials and the specimens documented previously, we reconstruct Callianthus as a plant with straight, decussately branching stems, flaccid leaves with sparse meshes and an intramarginal vein, and bisexual pedicellate reproductive organs with two whorls of slightly differentiated foliar appendages. Morphological analysis indicates that Callianthus was an aquatic plant, as Archaefructus, suggesting that angiosperms might have explored aquatic habitats by the end of Barremian in the Early Cretaceous.