Parapeytoia yunnanensis: The organism that connects the upper stem-group euarthropods

Abstract: The study of euarthropod evolution in the Cambrian is challenging due to controversies surrounding the identification of various anatomical features. Radiodontans are supported to be a monophyletic group to the lower stem of euarthropods. Radiodontans have complex bodies made up of powerful frontal appendages called “long” great appendages and an oral opening. Similarly, megacheirans are also a group of Cambrian marine organisms, however, with an uncertain placement in the arthropod family tree. They are characterized by their “short” great appendages. Megacheirans are thought to either be stem-group chelicerates or the basal upper stem-group euarthropods. Parapeytoia yunnanensis, which shows morphological aspects of both radiodontans and megacheirans, could be the “missing link” between the megacheirans and the radiodontans, and shed light on what stem-group the megacheirans belong to.  P. yunnanensis frontal appendages share great similarity between the frontal appendages of megacheirans suggesting that P. yunnanensis might have been a stem-group chelicerate. However, the gnathobases of P. yunnanensis resemble closely that of radiodontans. Furthermore, P. yunnanensis oral morphology is deeply rooted in stem-group euarthropods. Therefore, it is indicated that megacheirans were stem-group euarthropods, and not chelicerates. Completing a transition in the euarthropod stem-group with P. yunnanensis as a transition from radiodontans to megacheirans.