A restudy of the Burgess Shale (Cambrian) arthropod Emeraldella brocki and reassessment of its affinities

Abstract

A restudy of the Burgess Shale arthropod Emeraldella brocki suggests novel interpretations of its morphology. We show that the morphology is more plesiomorphic than previously assumed, particularly regarding tagmosis. The cephalon probably only incorporates three limb-bearing postantennular segments. The trunk is not differentiated and consists of 12 tergite-bearing segments and a styliform telson. Limb structure is generally similar to that of other artiopods 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 included. Autapomorphies of this taxon 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 retrieved as the sister taxon to the stem-lineage crustacean Oelandocaris oelandica, and both constitute the sister taxon of Artiopoda. ‘Great appendage’ arthropods, traditionally included in the Arachnomorpha, are retrieved as sister to the Crustacea sensu lato + Artiopoda clade, which contradicts the arachnomorph concept.

Keywords:

• Emeraldella brocki
• Euarthropoda
• ‘arachnomorphs’
• Artiopoda

Acknowledgements

Douglas H. Erwin, Mark S. Florence and Jann Thompson (NMNH) facilitated access to the specimens. Luis A. González (Lawrence) helped with funding for M. Stein to visit the NMNH collections. Richard A. Robison, Bruce S. Lieberman (both Lawrence), Dieter Waloszek, Andreas Maas and Joachim T. Haug (all Ulm) read and commented on an early draft of the manuscript. David L. Bruton (Oslo) and John Paterson (Armidale) and associate editor Gregory D. Edgecombe are thanked for critical reviews that improved the final version of the manuscript.