A transitional, early Cambrian species bridges radiodonts and upper stem-group euarthropods.

Euarthropods are the planet's most species-rich phylum. They possess a well-differentiated head in which specialized appendages coordinate feeding and sensory functions, and head organization is thus pivotal to understanding their evolution. Radiodonts (Anomalocaris and relatives) are considered to play a critical role in understanding how the functionally integrated euarthropod head evolved. These stem-group euarthropods bear ventrally directed frontal appendages, a radially arranged oral cone, and segmental lateral swimming flaps. In contrast, both transitional upper stem-group and fossil crown-group euarthropods possess dorsally directed great appendages, a distinct hypostome, and biramous limbs. An evolutionary gap remains unresolved by available fossil data and current interpretations of the branching sequence of taxa in the euarthropod stem group. Here we describe Oreinorema bergstroemi gen. et sp. nov. from the early Cambrian Chengjiang biota. High-resolution microtomography reveals that Oreinorema combines radiodont-like characters-including stalked eyes associated with an anterior sclerite, frontal appendages with paired endites, and segmental lateral flaps throughout the body-with features characteristic of upper stem-group euarthropods: the frontal appendages are dorsally directed and are borne by a distinct post-ocular segment, a sclerotized hypostome covers a posteriorly facing mouth opening (instead of a radiodont-style oral cone or similar feeding apparatus), and neck and trunk tergites are sclerotized. Phylogenetic analyses recover Oreinorema as a transitional form between Radiodonta and more crownward stem-group euarthropods, clarify homologies in the euarthropod head, and illuminate early steps toward the crown-group euarthropod body plan.