Urokodia sheds light on the origin of chelicerae and book gills of Chelicerata Abstract Chelicerates are a diverse group of terrestrial and aquatic arthropods, yet their origin and early evolution remain debated 1 , 2 , 3 , 4 , 5 , 6 . Among different hypotheses 7 , 8 , 9 , 10 , 11 , one proposes that the front appendages, known as chelicerae, of chelicerates evolved from the short great appendages (SGAs) of Cambrian megacheirans 12 , 13 , 14 , 15 , 16 and that book gills originated from their trunk limbs 7 . Although taxa such as Mollisonia plenovenatrix 7 and Megachelicerax cousteaui 16 provide important clues to the early evolution of chelicerates, the morphological transition from megacheiran-like appendages to true chelicerae and book gills remains unresolved. Here we use X-ray microtomography to reveal the three-dimensional anatomy of Urokodia aequalis , an early Cambrian euarthropod from the Chengjiang biota of China. Urokodia has a seven-segmented head with a scleroti...
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