Extant primitively segmented spiders have recently diversified from an ancient lineage

Living fossils are lineages that have retained plesiomorphic traits through long time periods. It is expected that such lineages have both originated and diversified long ago. Such expectations have recently been challenged in some textbook examples of living fossils, notably in extant cycads and coelacanths. Using a phylogenetic approach, we tested the patterns of the origin and diversification of liphistiid spiders, a clade of spiders considered to be living fossils due to their retention of arachnid plesiomorphies and their exclusive grouping in Mesothelae, an ancient clade sister to all modern spiders. Facilitated by original sampling throughout their Asian range, we here provide the phylogenetic framework necessary for reconstructing liphistiid biogeographic history. All phylogenetic analyses support the monophyly of Liphistiidae and of eight genera. As the fossil evidence supports a Carboniferous Euramerican origin of Mesothelae, our dating analyses postulate a long eastward over-land dispersal towards the Asian origin of Liphistiidae during the Palaeogene (39–58 Ma). Contrary to expectations, diversification within extant liphistiid genera is relatively recent, in the Neogene and Late Palaeogene (4–24 Ma). While no over-water dispersal events are needed to explain their evolutionary history, the history of liphistiid spiders has the potential to play prominently in vicariant biogeographic studies.

活化石（living fossils）是指在漫长地质时间尺度内保留祖征（plesiomorphic traits）的生物支系。学界此前普遍认为，此类支系的起源与分化均发生在极为久远的地质时期。然而近期针对若干教科书级经典活化石类群的研究对这一认知提出了挑战，其中尤以现存苏铁类（cycads）与腔棘鱼（coelacanths）最为典型。本研究采用系统发育分析方法，对节板蛛科（Liphistiidae）蜘蛛的起源与分化模式展开检验：该类群因保留了蛛形纲的祖征，且仅隶属于与所有现生蜘蛛互为姐妹群的古老支系——中突蛛亚目（Mesothelae），而被视作活化石类群。依托于覆盖其整个亚洲分布区的原创性采样工作，本研究构建了重建节板蛛科生物地理历史所需的系统发育框架。所有系统发育分析结果均支持节板蛛科及其下辖8个属的单系性。鉴于化石证据表明中突蛛亚目起源于石炭纪的欧美地区，本研究的定年分析推测，节板蛛科在古近纪（39~58 Ma）经由长距离陆上东向扩散，完成了在亚洲的起源过程。与学界既有预期相悖的是，现生节板蛛科各属内的物种分化事件相对较晚，发生于新近纪与古近纪晚期（4~24 Ma）。尽管无需借助跨洋扩散事件即可解释其演化历程，但节板蛛科的演化历史有望在隔离分化生物地理学（vicariant biogeographic）研究中发挥重要作用。