Leaves and megasporophylls of Cycadophytes from the Lower Cretaceous of Beishan area, Gansu Province, Northwest China, and their evolutionary significance

Abstract

Cycadophytes comprise ancient lineages of seed plants, and include Cycadales and Bennettitales. In this study, Phasmatocycas mazongshanensis sp. nov., represented by megasporophylls, is reported from the Lower Cretaceous of the Beishan area in Gansu Province, Northwest China. Based on the strip-shaped megasporophylls and ovules in two symmetrical rows, Phasmatocycas mazongshanensis is supposed to be closely related to Paleozoic primitive Cycadales. In addition, fossil leaves Pterophyllum beishanensis sp. nov. and Nilssoniopteris binggouensis are studied on leaf morphological and epidermal features. Syndetocheilic stomatal apparatus, thickened guard cells and curved anticlinal walls, all three features were observed on the epidermis, indicating that the leaves should be assigned to Bennettitales. The female reproductive organs of cycadophytes can be classified into four lineages: Phasmatocycas mode, Williamsonia mode, Cycas mode, and Zamia mode. Moreover, Phasmatocycas mazongshanensis is considered to be the lastest fossil record of Paleozoic-grade Cycadales. The reproduction of Phasmatocycas has previously been restricted to the Permian, but the new species, Phasmatocycas mazongshanensis, have been discovered from the Lower Cretaceous demonstrating its resurgence after a long absence from the stratigraphic record.