Epidermal anatomy of Pterophyllum ptilum (Cycadophyta: Bennettitales) from the Upper Triassic of Sichuan Province, Southwest China

Abstract

Pterophyllum ptilum Harris 1932 is a distinctive fossil plant species that occurred exclusively in the Late Triassic but in a broad geographic distribution ranging from North Europe to East Asia. Thus, this species has long been considered significant in stratigraphic correlation of non-marine sequences. However, previous studies of P. ptilum have been mostly focused on the morphological characteristics of its foliages, a complete biological feature, particularly the epidermal anatomy, remains poorly understood. Here, we show detailed epidermal anatomies of P. ptilum based on well-preserved cuticular specimens from the Upper Triassic of Sichuan Province, Southwest China. Pterophyllum ptilum is characterised by petiolate leaves with small lateral leaflets, which possess contracted bases and bicuspid apices with five to eight parallel veins and syndetocheilic type stomata. In the leaflets, the stomata are only present on the abaxial surface, whereas the stomata are present on both the abaxial and adaxial surfaces of the rachis. Both the adaxial and abaxial leaflet surfaces can be divided into costal and intercostal fields. The epidermal cells in the costal fields are elongated rectangular but shortened rectangular in the intercostal fields. The stomata are only present in the intercostal fields of the abaxial leaflet surface. On the rachis, the epidermal cells are all elongated rectangular and arranged tightly in vertical files. Based on a thorough comparison of fossil records and observation of extant cycads, we propose that the bicuspid leaflet apices of P. ptilum may resulted from herbivorous arthropod apical feeding damage. Fossil records reveal that P. ptilum were primarily preserved in coastal plain, deltaic, and tidal flat environments in the South China Block; the absence of papillae and hairs in the present specimens may suggest a relatively more humid local environment.