New horsetail macrofossils from the Lower Cretaceous of the Laiyang Basin, Eastern China, and biogeographic analyses

Equisetum, the sole-living representative of Sphenopsida, is considered as a 'living fossil', with impressive species richness and a long fossil history dating back to the Devonian. It is regarded as a key taxon in attempting to understand the evolution of vascular plants. Here, we describe a new species, Equisetum shandongensis sp. nov. from the Early Cretaceous of the Laiyang Formation in Laiyang Basin, Shandong Province, eastern China. This species is the first record of horsetails from the Laiyang Basin, and it consists of relatively complete stems with distinct leaf sheaths, sporangiate spikes with helically arranged pentagonal sporangiophores, underground rhizomes, and tubers. The close affinity of our fossils to Equisetum is supported by the presence of both macromorphological and epidermal characters. This new species deepens our understanding of the diversity and distribution range of the early Equisetum in the Lower Cretaceous. The biogeographic distribution of Equisetum has been evaluated based on the phylogenetic and biogeographic analyses, which lead to the speculation that the subgenus Equisetum derived from the Laurasian clade no later than the Early Cretaceous.