An early Cambrian euarthropod with radiodont-like raptorial appendages

Kylinxia zhangi is a transitional fossil that is an evolutionary 'missing link' between radiodonts (also known as anomalocaridids) and true arthropods, providing insights into the origin and early evolution of Arthropoda. Resolving the early evolution of euarthropods is one of the most challenging problems in metazoan evolution(1,2). Exceptionally preserved fossils from the Cambrian period have contributed important palaeontological data to deciphering this evolutionary process(3,4). Phylogenetic studies have resolved Radiodonta (also known as anomalocaridids) as the closest group to all euarthropods that have frontalmost appendages on the second head segment (Deuteropoda)(5-9). However, the interrelationships among major Cambrian euarthropod groups remain disputed(1,2,4,7), which impedes our understanding of the evolutionary gap between Radiodonta and Deuteropoda. Here we describe Kylinxia zhangi gen. et. sp. nov., a euarthropod from the early Cambrian Chengjiang biota of China. Kylinxia possesses not only deuteropod characteristics such as a fused head shield, a fully arthrodized trunk and jointed endopodites, but also five eyes (as in Opabinia) as well as radiodont-like raptorial frontalmost appendages. Our phylogenetic reconstruction recovers Kylinxia as a transitional taxon that bridges Radiodonta and Deuteropoda. The most basal deuteropods are retrieved as a paraphyletic lineage that features plesiomorphic raptorial frontalmost appendages and includes Kylinxia, megacheirans, panchelicerates, 'great-appendage' bivalved euarthropods and isoxyids. This phylogenetic topology supports the idea that the radiodont and megacheiran frontalmost appendages are homologous, that the chelicerae of Chelicerata originated from megacheiran great appendages and that the sensorial antennae in Mandibulata derived from ancestral raptorial forms. Kylinxia thus provides important insights into the phylogenetic relationships among early euarthropods, the evolutionary transformations and disparity of frontalmost appendages, and the origin of crucial evolutionary innovations in this clade.