Exites in Cambrian arthropods and homology of arthropod limb branches

The common ancestor of all living arthropods had biramous postantennal appendages, with an endopodite and exopodite branching off the limb base. This study uses microtomographic imaging of the Cambrian arthropod Leanchoilia to reveal a previously undetected exite at the base of most appendages, suggesting a deeper origin for exites in arthropod phylogeny. The last common ancestor of all living arthropods had biramous postantennal appendages, with an endopodite and exopodite branching off the limb base. Morphological evidence for homology of these rami between crustaceans and chelicerates has, however, been challenged by data from clonal composition and from knockout of leg patterning genes. Cambrian arthropod fossils have been cited as providing support for competing hypotheses about biramy but have shed little light on additional lateral outgrowths, known as exites. Here we draw on microtomographic imaging of the Cambrian great-appendage arthropod Leanchoilia to reveal a previously undetected exite at the base of most appendages, composed of overlapping lamellae. A morphologically similar, and we infer homologous, exite is documented in the same position in members of the trilobite-allied Artiopoda. This early Cambrian exite morphology supplements an emerging picture from gene expression that exites may have a deeper origin in arthropod phylogeny than has been appreciated.

Automated summary

This study reveals the presence of exites at the base of most appendages in the Cambrian arthropod Leanchoilia, suggesting a deeper origin for exites in arthropod phylogeny. The discovery of a previously undetected exite at the base of appendages sheds new light on the evolutionary history of arthropods.