A new lobopodian from the middle Cambrian of Utah: did swimming body flaps convergently evolve in stem-group arthropods?

Arthropods are ubiquitous in all modern habitats and yet their origin remains poorly documented. It is widely thought that their segmented and arthrodized body evolved from the annulated vermiform body of a lobopodian ancestor c. 540 Ma. This major transformation included the evolution of sclerotized and articulated appendages from annulated non-jointed limbs or lobopods. However, this scenario is complicated by the presence in many stem-group arthropods of body flaps of various origins, characteristics and functions. We describe the new lobopodian Utahnax vannieri gen. et sp. nov. from Drumian strata of the House Range in Utah. Known from an incomplete specimen, this taxon features a vermiform, annulated body flanked by pairs of swimming flaps, except in the caudal region, and a digestive system with putative glands. Extensions of the body cavity into the body flaps of Utahnax suggest that they are modified lobopods similar to the ventral body flaps of radiodonts; as such, they are not homologous to the dorsal body flaps of the stem-group arthropods Opabinia, Pambdelurion, and possibly Kerygmachela. We discuss the details of the limb anatomy of these three taxa and consider an alternative interpretation for Kerygmachela. Despite morphological similarities in radiodonts, Utahnax, and possibly Kerygmachela, our parsimony and Bayesian inference phylogenetic analyses suggest that ventral body flaps have evolved convergently in those taxa, probably accompanying a shift towards a predominantly swimming predatory lifestyle. Ecological competition with radiodonts is proposed as an explanation for the scarcity of non-radiodont swimming lobopodians in Cambrian Lagerstätten.