Article: Head structure in upper stem-group euarthropods

Continuing debate over the evolution and morphology of the arthropod head has led to considerable interest in the relevance of the evidence from the fossil record. However, dispute over homology and even presence of appendages and sclerites in Cambrian arthropods has resulted in widely differing views of their significance. The head structures of several important taxa, Fuxianhuia, Canadaspis, Odaraia, Chengjiangocaris and Branchiocaris are redescribed, revealing the essential similarity between them. In particular, all possessed an anterior sclerite, probably followed by a large posterior, ventral sclerite that is likely to be homologous to the hypostome of trilobites. The presence of a similar feature in Sanctacaris is also possible, but less well-supported. An anterior sclerite, usually bearing eyes, as in Fuxianhuia, appears to be a widespread feature of basal arthropods. Whether or not this sclerite represents an original articulating protocerebral segment on its own is, however, open to debate.