A decade after Deuteropoda – insights into the systematics, evolution and dynamics of Cambrian euarthropods

The systematics and evolution of Cambrian euarthropods have undergone a major transformation in recent years owing to the continued discovery of new species from sites with exceptional preservation, redescription of established taxa through new material and/or imaging methodologies, and a consistent push towards broadly sampled phylogenetic datasets. These efforts have yielded impressive results, including a more comprehensive view of Cambrian euarthropod diversity and better understanding of their detailed morphology. The progress has effectively narrowed the Deuteropoda gap, namely the large accumulation of derived characters that evolved during the transition from soft-bodied lower stem-group euarthropods (lobopodians, radiodonts) into the upper stem-group euarthropods typified by deutocerebral first appendages and multisegmented heads. However, it has also caused a shift in our understanding of Cambrian euarthropod interrelationships and the higher affinities of several clades, directly impacting hypotheses on the origin and initial radiation of this phylum. Whereas the composition of lower stem-group Euarthropoda has remained stable, that of the upper stem-group has changed drastically with several clades now regarded as members of the group-crown, most notably hymenocarines, fuxianhuiids, artiopods, and (possibly) megacheirans. A notable consequence of these phylogenetic adjustments is the shortening of the upper stem-group, now consisting of forms with a raptorial first appendage including isoxyiids and Kylinxia. The resulting “short stem-lineage” scenario is consistent with the diversification dynamics of unusually large clades, notably that the crown-group will evolve exceptionally quickly. I propose that the “short stem-lineage” scenario reconciles the minimum estimates for the appearance of stem-group euarthropods based on the Fortunian trace fossil record (ca. 537 Mya), the subsequently prompt evolution of the crown-group based on the estimated origin of trilobites (ca. 535 Mya), and the predicted rapid initial diversification of the stem-group. Finally, I highlight the importance of resolving the ancestral state of the deutocerebral first appendage of Deuteropoda in order to polarize the macroevolution of Cambrian euarthropods.