The morphology and affinities of Skania fragilis (Arthropoda) from the middle Cambrian Burgess Shale

 

Authors: Legg DA

Published in: Bulletin of Geosciences, volume 90, issue 3; pages: 509 - 518; Received 1 September 2014; Accepted in revised form 21 April 2015; Online 27 May 2015

Keywords: marrellomorph, carapace formation, Burgess Shale,

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Supplementary material

Appendix A (187 kB)

Supplementary material: character list


Appendix B (258 kB)

Supporting material for the phylogenetic analysis


 

Abstract

The enigmatic arthropod Skania fragilis, from the middle Cambrian (Series 3, Stage 5) Burgess Shale Formation (Yoho National Park, British Columbia, Canada), is redescribed based on 14 new specimens reposited at the Royal Ontario Museum. These specimens provide a clearer picture of the morphology of this taxon and help to resolve conflicting opinions regarding potential homology of particular features. Specifically, the anchor-shaped anterior, which has been compared to a similar structure in the putative Precambrian arthropod Parvancorina, is shown to represent an anterior cephalic doublure with features comparable to the mediolateral spines of marrellids, such as Marrella. Beyond a vaguely cordiform outline, no other features are shown to be shared between Skania and Parvancorina, weakening claims that crown-group arthropods were present in the Neoproterozoic. The removal of these taxa from Arthropoda is in keeping with recent molecular clock analyses, which demonstrate a Cambrian diversification of Euarthropoda. A phylogenetic analysis resolved Skania as the most basal member of Acercostraca, a clade of marrellomorphs including Vachonisia and Xylokorys, united by the presence of a cordiform dorsal shield. Similarities between these taxa and marrellids may indicate that the elongate posterior spines of Marrella and related taxa, and the dorsal shield of acercostracans have a common origin akin to the carapace anlagen of extant crustaceans.

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