Sarotrocercus oblitus - small arthropod with great impact on the understanding of arthropod evoluation?

Abstract

Sarotrocercus oblitus is a small arthropod from the Cambrian Burgess Shale. It was originally described with a short head with only two appendage-bearing segments (the first appendage being limb-shaped), a short trunk of nine segments and lamellate trunk limbs. This rather "unusual" morphology inspired various authors to propose evolutionary scenarios concerning segmentation and appendages. The head of S. oblitus served also for scenarios about the evolution of the arthropod head, because it seemed to document the evolutionary step between the level of Arthropoda sensu stricto (head with one appendage-bearing segment) and that of Euarthropoda (head comprising four appendage-bearing segments). Here we report that the morphology of S. oblitus differs in several significant aspects from its original description, e.g., in the composition of the head, number of trunk segments, and appendage morphology. In consequence, many earlier assumptions based on the original description must be rejected. Although the material consists of only seven individuals, ontogenetic variation of the number of trunk segments was observed, pointing to, at least, two developmental stages. Therefore, S. oblitus is morphologically less different from other Cambrian arthropods than previously thought, but possesses a head with three appendage-bearing segments and lacks a prominent antenn(ul)a. These characters point to a position of S. oblitus inside Arthropoda s. str., deriving from the lineage towards Euarthropoda. The morphology also indicates a special life style, e.g., by the presence of large, stalked eyes, apparently in convergence to one of the Cambrian "Orsten" crustacean stem-lineage derivatives, Henningsmoenicaris scutula.