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ABSTRACT-
Articulated scleritomes of the chancelloriids Archiasterella fletchergryllus new species and Chancelloria cf. eros Walcott, 1920 are described from the Early Cambrian (Branchian) Sekwi Formation, Mackenzie Mountains, Northwest Territories, Canada. Early diagenetic, microbially mediated lithification has resulted in unusual three-dimensional preservation of the body surface, which potentially allows consideration of the evolutionary affinities of these enigmatic organisms. Sclerites are mounted on short stalks of the integument, connected to the undersurface of the central disc, and are external to the body surface.
INTRODUCTION
THE CAMBRIAN radiation (e.g., Briggs and Fortey, 1989; Gould, 1989; Conway Morris, 1993a, 2000; Briggs et al., 1994; Budd and Jensen, 2000) marks the appearance in the fossil record of a number of problematic groups whose evolutionary position within the Metazoa is debated. The Chancelloriidae are one such problematic group. They were originally described as heteractinellid sponges (Walcott, 1920) based on superficial similarities of skeletal elements, but this view was later challenged (e.g., Goryansky, 1973). Bengtson and Missarzhevsky (1981) showed that unlike sponge spicules, chancelloriid sclerites are hollow. They moved the Chancelloriidae to the order Coeloscleritophora: a group possessing hollow skeletal elements and containing a number of problematic groups, including the wiwaxiids, halkieriids, sachitids, and siphogonuchitids. Over the last two decades, various members of the Coeloscleritophora have been compared to sponges (Butterfield and Nicholas, 1996; Conway Morris and Chapman, 1997), annelids (Conway Morris and Peel, 1995), mollusks (Bengtson, 1992; Runnegar, 2000), ascidians (Mehl, 1996), and brachiopods (e.g., Conway Morris, 1998), and the group is likely polyphyletic as other authors have suggested (e.g., Conway Morris and Chapman, 1997).
Recent collecting from the Early Cambrian (Branchian) Sekwi Formation of the Selwyn Basin, Mackenzie Mountains, Northwest Territories, Canada (Fig. 1), as part of a broader investigation of the Early Cambrian biosphere, has produced new and well-preserved chancelloriid scleritomes and an associated fauna of trilobites, archeocyathids, hyolithids, and brachiopods. The new material described here uniquely preserves the three-dimensional relief of the chancelloriid body surface and may add to previous discussions of chancelloriid affinities by Bengtson and Missarzhevsky (1981), Butterfield and Nicholas (1996), Mehl (1996), Bengtson and Hou (2001), Janussen et al. (2002), and others.
Because chancelloriids are often found as isolated sclerites, some of which have been illustrated with poor figures, the taxonomy of the group...