CONVERGENT EVOLUTION OF BODY FLAPS IN STEM-GROUP ARTHROPODS: INSIGHTS FROM A NEW LOBOPODIAN FROM THE MIDDLE CAMBRIAN OF UTAH

Arthropods are ubiquitous in virtually all modern ecosystems and as abundantly documented by Konservat-Lagerstaetten, already dominated the earliest animal communities populating the Cambrian seafloor. These organisms evolved from lobopodians early during the Cambrian Explosion (c. 539 to 529 Ma), through acquisition of a series of key anatomical innovations. Yet, the transition from a lobopodian body plan (i.e. a flexible annulated vermiform body bearing lobopodous limbs) to an arthropod body plan (extensively sclerotized, segmented, and articulated body and appendages) in the stem-lineage of the group remains contentious, due to the rarity of informative fossils. In particular, limb evolution remains unclear, since some stem-group representatives exhibited one or two series of lateral body flaps in addition to, or in lieu of walking limbs.

We describe a new lobopodian with lateral body flaps from the Drumian strata (Marjum Formation?) of western Utah. This taxon is known from a single incomplete specimen, c. 59 mm in length, representing approximately the posterior two-thirds of a large individual. It features a wide main body, which preserves faint annulations and a complex digestive system (internally), and the remains of eight pairs of lateral flaps. Differences in relief and coloration suggest that each of these flaps possessed a notably thicker anterior region housing an extension of the hemocoelic cavity. We regard these flaps as highly modified lobopods, rather than structures protecting such limbs dorsally as observed in Pambdelurion and Opabinia. The body flap organization of the Utah lobopodian is reminiscent to that of radiodonts (and possibly Kerygmachela), although phylogenetic analyses suggest that it evolved independently within the arthropod stem lineage. The new taxon is closely related to lobopodians with grasping frontal appendages, which coupled with the presence of lateral flaps and midgut glands, suggests that the animal was a free-swimming predator. The new fossil from Utah extends the palaeogeographic distribution of non-radiodont flap-bearing lobopodians to the northern margin of Laurentia, and evidences the persistence of such predators in the Miaolingian Epoch (Drumian Age).