A RESTUDY OF THE BURGESS SHALE ARTHROPOD EMERALDELLA BROCKI WITH A REASSESSMENT OF ITS AFFINITIES

Restudy of the Burgess Shale arthropod Emeraldella brocki leads to a new interpretation of its morphology that in some aspects differs markedly from previous ones. It is shown that the morphology is more conservative than previously assumed, in particular with regards to tagmosis. The cephalon only incorporated three limb-bearing postantennular limbs rather than five. The trunk was not differentiated into a thorax and limbless abdomen as previously thought, but consisted of 12 tergite- and limb-bearing segments and a styliform telson. Limb structure was generally similar to that of other 'artiopods' (trilobites and their allies) except for a tripartite exopod and a high degree of differentiation of podomere proportions along the body. A phylogenetic analysis of 20 fossil arthropod taxa based on 36 characters renders E. brocki as a basal taxon within a monophyletic group that comprises all ‘artiopods’. Synapomorphies of these taxa are a filiform antennula and a bilobate exopod that carries lamellae proximally. Trilobites are nested within a group of 'artiopods' sharing a pygidium. Agnostus pisiformis is resolved as the sister taxon to the stem-lineage Crustacean Oelandocaris oelandica and both constitute the sister taxon to the 'artiopod' clade. There is thus no support found for trilobite affinities of A. pisiformis. Great appendage arthropods, traditionally included in the Arachnomorpha together with 'artiopods' are resolved as sister to the Crustacea sensu lato + 'artiopod' clade, which contradicts the arachnomorph concept.