Understanding the genealogical relationships among the arachnid orders is an onerous task, but fossils have aided in anchoring some branches of the arachnid tree of life. The discovery of Palaeozoic fossils with characters found in both extant spiders and other arachnids provided evidence for a series of extinctions of what was thought to be a grade, Uraraneida, that led to modern spiders. Here, we report two extraordinarily well-preserved Mesozoic members of Uraraneida with a segmented abdomen, multi-articulate spinnerets with well-defined spigots, modified male palps, spider-like chelicerae and a uropygid-like telson. The new fossils, belonging to the species Chimerarachne yingi, were analysed phylogenetically in a large data matrix of extant and extinct arachnids under a diverse regime of analytical conditions, most of which resulted in placing Uraraneida as the sister clade of Araneae (spiders). The phylogenetic placement of this arachnid fossil extends the presence of spinnerets and modified palps more basally in the arachnid tree than was previously thought. Ecologically, the new fossil extends the record of Uraraneida 170 million years towards the present, thus showing that uraraneids and spiders co-existed for a large fraction of their evolutionary history.
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Financial support was provided by the Strategic Priority Research Program (B) (XDB18000000) and the programme Macroevolutionary Processes and Palaeoenvironments of Major Historical Biota (XDPB05) of the Chinese Academy of Sciences, the National Natural Science Foundation of China (41688103 and 91514302) and US National Science Foundation grants 1457300 (to G.H.) and 1457539 (to G.G.) (Collaborative Research: Phylogeny and diversification of the orb weaving spiders (Araneae)). We are grateful to J. Sun for reconstruction and S. Wu for micro computed tomography assistance. Published by a grant from the Wetmore Colles fund.