Silicified cupulate seed-bearing structures from the Early Cretaceous of eastern Inner Mongolia, China: rethinking the corystosperm concept

Abstract

The group referred to informally as the corystosperms, described initially based on compression fossils from the Triassic of Gondwana, have long been considered critical extinct plants for understanding seed plant phylogeny, the evolution of seed plant reproductive structures and the relationships of angiosperms. Here we describe a new genus and species of corystosperm seed-bearing structure, Jarudia zhoui gen. et sp. nov., based on abundant silicified material collected from the newly discovered chert in the Early Cretaceous Huolinhe Formation of eastern Inner Mongolia, north-eastern China. Jarudia zhoui is a lax seed cone consisting of a flexible central axis bearing deciduous, helically arranged, lateral seed-bearing units. Individual seed-bearing units consist of an elongate bract partially fused to an unbranched cupule stalk that bears a single, reflexed cupule apically. Each cupule is formed by the strongly reflexed cupule stalk and one median and two lateral flaps. The cupule stalk supplied by two vascular bundles and three unvascularized flaps partially enclose two three-angled seeds. Jarudia zhoui bears a striking resemblance to Doylea tetrahedrasperma from the Early Cretaceous of Canada and similar plants from the Early Cretaceous of Mongolia. There are also strong similarities with ovulate structures of Umkomasia from the Triassic of Gondwana in the structure and anatomy of individual cupules, their axial nature, and the architecture of the entire seed-bearing structure that has two orders of branching. New information from Jarudia zhoui, together with information on other corystosperm ovulate organs from the Northern Hemisphere, significantly expands our understanding of this key group of extinct plants, suggests that the cupules of the Early Cretaceous and Triassic corystosperms are homologous, and raises critical questions about the definition and phylogenetic circumscription of the corystosperms, including how Early Cretaceous and Triassic corystosperms are related to each other and to other groups of seed plants, including angiosperms.

Keywords:

• silicified
• corystosperm
• cupule
• anatomy
• homology
• seed plant

Acknowledgements

This paper is dedicated to Professor Zhiyan Zhou on the occasion of his ninetieth birthday. We thank Bole Zhang, Chong Dong, Suxin Yin, Shusheng Hu, Hui Jiang, Qijia Li, Wenfang Zhang and Fengwei Lu for assistance with fieldwork in Inner Mongolia, China, Zhe-Xi Luo, April Neander and Craig Brodersen for assistance with micro-CT scanning, Else Marie Friis, Xin Xu and S. Donskaya for assistance in processing micro-CT data, and Chao Tan for the plant reconstruction. We also thank Peta Hayes and Paul Kenrick for access to the Thomas specimens of Umkomasia at the Natural History Museum, London, and also to Marion Bamford, and especially John and Heidi Anderson, for access to the collections of Umkomasia from the Upper Triassic Molteno Formation of South Africa, which are housed at the University of the Witswatersrand. We are also grateful to the editors and two anonymous reviewers for their helpful comments. Figures 1A, 1B, 1F, 2A, 2D, 3B, 4C, 5C, 6C are reproduced with thanks from Shi et al. (2021a, figures 1a, 3a, 3b, 2a, 2g, 2f, 2b, 2d, 2h respectively); Figures 1C, 3A, 3D, 4E, 5B, 7F, 8A are reproduced with thanks from Shi et al. (2021a, Extended Data figures 2g, 3a, 3d, 2b, 2c, 2l, 2i respectively). This work was supported by the Youth Innovation Promotion Association, Chinese Academy of Sciences (2017359, Y2021082), the US National Science Foundation grant DEB-1748286, the Strategic Priority Research Program of the Chinese Academy of Sciences (XDB26000000), the National Natural Science Foundation of China (41790454), the Field Museum, and the Oak Spring Garden Foundation.

Associate Editor: Paul Kenrick