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

<i>Equisetum</i>, 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, <i>Equisetum shandongensis sp. nov</i>. 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 <i>Equisetum</i> 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 <i>Equisetum</i> in the Lower Cretaceous. The biogeographic distribution of <i>Equisetum</i> has been evaluated based on the phylogenetic and biogeographic analyses, which lead to the speculation that the subgenus <i>Equisetum</i> derived from the Laurasian clade no later than the Early Cretaceous.

<i>Equisetum</i>（木贼属）作为楔叶纲（Sphenopsida）的现存唯一代表，被视为"活化石"，拥有令人瞩目的物种丰富度及可追溯至泥盆纪（Devonian）的漫长化石历史。它是理解维管植物（vascular plants）演化的关键分类群（taxon）。

本文描述了采自中国东部山东省莱阳盆地莱阳组早白垩世（Early Cretaceous）的一个新种——<i>Equisetum shandongensis</i> sp. nov.（山东木贼）。该物种是莱阳盆地木贼类（horsetails）的首次记录，其化石包含具明显叶鞘的较完整茎干、具螺旋排列五边形孢子囊柄（sporangiophores）的孢子穗（sporangiate spikes）、地下根状茎（rhizomes）及块茎（tubers）。

宏观形态与表皮特征（macromorphological and epidermal characters）的存在支持了这些化石与<i>Equisetum</i>的密切亲缘关系。该新种加深了我们对早白垩世<i>Equisetum</i>多样性与分布范围的认识。

基于系统发育（phylogenetic）与生物地理分析（biogeographic analyses），我们对<i>Equisetum</i>的生物地理分布进行了评估，推测<i>Equisetum</i>亚属不晚于早白垩世起源于劳亚大陆分支（Laurasian clade）。