Data from: The grass was greener: repeated evolution of specialized morphologies and habitat shifts in ghost spiders following grassland expansion in South America

While grasslands, one of Earth’s major biomes, are known for their close evolutionary ties with ungulate grazers, these habitats are also paramount to the origins and diversification of other animals. Within the primarily South American spider subfamily Amaurobioidinae (Anyphaenidae), several species are found living in the continent’s grasslands, with some displaying putative morphological adaptations to dwelling unnoticed in the grass blades. Here, a dated molecular phylogeny provides the backbone for analyses revealing the ecological and morphological processes behind these spiders’ grassland adaptations. The multiple switches from Patagonian forests to open habitats coincide with the expansion of South America’s grasslands during the Miocene, while the specialized morphology of several grass-dwelling spiders originated at least three independent times and is best described as the result of different selective regimes operating on macroevolutionary timescales. Although grass-adapted lineages evolved towards different peaks in adaptive landscape, they all share one characteristic: an anterior narrowing of the prosoma allowing spiders to extend the first two pairs of legs, thus maintaining a slender resting posture in the grass blade. By combining phylogenetic, morphological, and biogeographic perspectives we disentangle multiple factors determining the evolution of a clade of terrestrial invertebrate predators alongside their biomes.

草原作为地球主要生物群系（biome）之一，素来以与有蹄类植食动物（ungulate grazers）紧密的演化关联为人熟知，但这类生境对于其他动物的起源与物种多样化同样至关重要。在隶属于暗蛛科（Anyphaenidae）的南美蜘蛛Amaurobioidinae亚科中，已有多个物种被记录栖息于该大陆的草原生境中，其中部分物种展现出推测可使其隐匿于草叶间的形态适应性特征。本研究依托定年分子系统发育树，构建了解析这类蜘蛛草原适应性背后生态与形态演化过程的核心分析框架。多次从巴塔哥尼亚森林向开阔生境的生境转换事件，与中新世时期南美草原的扩张阶段相吻合；而数种专性栖息于草原的蜘蛛所特有的形态特征，至少独立起源过三次，这类特征可被更精准地描述为宏观演化尺度下不同选择压力作用的产物。尽管适应草原生活的演化支在适应性景观（adaptive landscape）中朝向不同的适应峰演化，但它们均具备一项共同特征：头胸部（prosoma）前部收窄，使得蜘蛛能够伸展前两对步足，从而在草叶间保持纤细的静息姿态。本研究结合系统发育学、形态学与生物地理学视角，厘清了陆生捕食性无脊椎动物演化支（clade）与其伴生生物群系协同演化的多项决定性因素。