A palaeontological solution to the arthropod head problem

The composition of the arthropod head has been one of the most controversial topics in zoology, with a large number of theories being proposed to account for it over the last century(1). Although fossils have been recognized as being of potential importance in resolving the issue(2,3), a lack of consensus over their systematics(4,5) has obscured their contribution. Here, I show that a group of previously problematic Cambrian arthropods from the Burgess Shale and Chengjiang faunas form a clade close to crown- group euarthropods, the group containing myriapods, chelicerates, insects and crustaceans(6). They are characterized by modified or even absent endopods, and two pre- oral appendages. Comparison with reconstructions of the crown- group euarthropod ground plan(6) and recent investigations into onychophorans(7,8) demonstrates that these two appendages are the first antenna (of extant crustaceans) and a more anterior appendage associated with an ocular segment. The latter appendage has been reduced in all crown- group euarthropods. Its most likely relic is as a component of the labrum(9). These fossils thus tie together results from disparate living groups (onychophorans and euarthropods).