Nexus file used to run phylogenetic analysis from Burgess shale fossils shed light on the agnostid problem

Agnostids (agnostinids and eodiscinids) are a widespread and biostratigraphically important group of Cambro-Ordovician euarthropods whose evolutionary affinities have been highly controversial. Their dumbbell-shaped calcified tergum was traditionally suggested to unite them with trilobites, but agnostinids have alternatively been interpreted as stem-crustaceans, based on Orsten larval material from the Cambrian of Sweden. We describe exceptionally preserved soft tissues from mature individuals of the agnostinids Peronopsis and Ptychagnostus from the middle Cambrian (Wuliuan Stage) Burgess Shale (Walcott Quarry and Marble Canyon, British Columbia, Canada), facilitating the testing of alternative hypotheses. The digestive tract includes conspicuous ramifying cephalic diverticulae. The cephalon carries one pair of elongate spinous antennules projecting to the front, two pairs of appendages with terminal setose, oar-like exopods, and three pairs of presumably biramous appendages with endopods sporting club-shaped exites. The trunk bears five appendage pairs, at least the first two of which are similar to the posteriormost cephalic pairs. A head with six appendiferous segments contrasts strikingly with the four known in trilobites and five typical of mandibulates. Our phylogenetic analysis retrieves agnostinids as the sister group to ‘polymeroid’ trilobites, implying that crustacean-like morphologies evolved homoplastically and highlighting the variability in segmental composition of the artiopodan head. Our study supports a nektobenthic lifestyle for agnostinids and emphasizes the importance of Burgess Shale-type fossils in resolving the affinities of problematic biomineralizing taxa.