The functional head of the Cambrian radiodontan (stem-group Euarthropoda) Amplectobelua symbrachiata.

The functional head of A. symbrachiata must include the reduced transitional segments (and their associated structures), which have been identified in several other radiodontans. This functional view supports the idea that the integration of segments (and associated appendages) into the head region, probably driven by feeding, occurred along the euarthropod stem-lineage. However, the number of reduced transitional segments varies between different groups and it remains uncertain whether GLSs represent proximal or distal parts of appendages. Our study is the first description of appendicular structures other than the frontal appendages in the functional head region of radiodontans, revealing novel feeding structures in the morphological transition from the lower- to the upper- stem-group of Euarthropoda.