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^1-6. Among different hypotheses^7-11, one proposes that the front appendages, known as chelicerae, of chelicerates evolved from the short great appendages (SGAs) of Cambrian megacheirans^12-16 and that book gills originated from their trunk limbs⁷. Although taxa such as Mollisonia plenovenatrix⁷ and Megachelicerax cousteaui¹⁶ 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 sclerotized hypostome, pincer-like SGAs and biramous trunk appendages with overlapping exite flaps. Its pincer-like SGAs represent a bridge structure between the appearance of multisegmented SGAs and true chelicerae, and its trunk appendages support a megacheiran origin of book gills. Phylogenetic analyses consistently confirm the monophyly of Cheliceromorpha¹⁷, with Urokodia being the earliest-branching upper stem-group chelicerate that links lower stem-group megacheirans to crownward forms such as Mollisonia and Megachelicerax.