3D morphology of the Cambrian bivalved arthropod Sunella informs about head segmentation, arthrodization, and arthropodization

A head region with specialised appendages, sclerotization, and segmentation of the trunk (arthrodization), and its appendages (arthropodization) represent three key innovations in arthropod evolution. Two scenarios have been proposed for acquisition of these innovations either in a synchronous or a sequential mode. Here we describe a new species Sunella dimorphismus sp. nov., from the Chengjiang biota (ca. 518 Ma), which displays a bivalved carapace, raptorial frontal appendages, and an arthrodized trunk with a series of biramous arthropodized appendages revealed by new specimens and new observations with assistance of computed tomography. Phylogenetic analyses placed Sunella as the earliest diverging deuteropods besides Erratus. Ancestral state reconstructions refine our understanding of the gain and loss of key characters in the euarthropod. The results demonstrate that trunk limb arthropodization preceded trunk arthrodization, with both prior to the evolution of a six-segmented functional head, while the trunk arthrodization is found to be lost in isoxyiids.