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.木贼属（Equisetum）是楔叶纲（Sphenopsida）现存的唯一代表类群，被视作‘活化石’，具有丰富的物种多样性，其化石记录可追溯至泥盆纪，拥有极为悠久的演化历史。该类群是解析维管植物（vascular plants）演化历程的关键分类单元。本文记述了采自中国东部山东省莱阳盆地莱阳组早白垩世地层的一新种——山东木贼（Equisetum shandongensis sp. nov.，新种）。该种是莱阳盆地首次发现的木贼类（horsetails）化石记录，其化石保存有相对完整的茎干（具清晰的叶鞘）、螺旋排列的五角形孢子囊柄孢子叶穗、地下根状茎与块茎。宏观形态特征与表皮特征均证实，本次发现的化石与木贼属具有紧密的亲缘关系。这一新种的发现，深化了我们对早白垩世早期木贼属多样性与分布范围的认知。研究团队基于系统发育与生物地理学分析，对木贼属的生物地理分布格局进行了评估，推测木贼亚属（subgenus Equisetum）不晚于早白垩世便从劳亚大陆支系（Laurasian clade）分化而来。