NEW CHANCELLORIIDS FROM THE EARLY CAMBRIAN SEKWI FORMATION WITH A COMMENT ON CHANCELLORIID AFFINITIES

Journal of Paleontology, Sep 2005 by Randell, Robert D, Lieberman, Bruce S, Hasiotis, Stephen T, Pope, Michael C

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 is difficult. For example Chancelloria sp. of Mehl, 1998 (pi. 7, fig. 9) is very similar to Mlrriella hainanensis Jiang and Huang, 1986 (pi. I, fig. 11); similarly, Missarzhevsky's (1989) Sissospina irregularis closely resembles Diplospinella zintchenkoi Vasil'yeva in Vasil'yeva and Sayutina, 1988. Compounding this problem, within a given scleritome there is variation in sclerite morphology that goes beyond simple variation in the number of rays. Thus, many currently accepted species might not warrant specific distinction at all, as Qian and Bengtson (1989) and Janussen et al. (2002) have recognized. A complete list of possible chancelloriid synonyms is beyond the scope of this article, but see Qian and Bengtson (1989) for useful discussion.

GEOLOGICAL SETTING

The late Early Cambrian strata in the southern Selwyn Basin, Backbone Ranges, Mackenzie Mountains, Northwest Territories, Canada (Fig. 1), record a widespread carbonate platform deposited on the passive continental margin that formed following western North American rifting (Bond et al., 1984). These roughly 700 m thick strata belong to the late Early Cambrian Sekwi Formation, which is superbly exposed above tree line in a series of easterly directed thrust sheets that telescope the late Proterozoicearly Paleozoic section within the Sekwi Mountain Γ ? 2° map sheet (Blusson, 1971a, 1971b). The type section of the Sekwi Formation and 27 other mostly complete sections were measured within this area (Fritz, 1972, 1976a, 1978, 1979; Krause, 1975; Krause and Oldershaw, 1978a). The Sekwi Formation is underlain by a thick succession of marine to continental siliciclastics with minor carbonate beds (Ingta, Backbone Ranges, and Vampire Formations) that include the Neoproterozoic-Cambrian boundary (Narbonne and Aitken, 1995; MacNaughton et al., 1997).

The stratigraphic framework for the Sekwi Formation and the overlying unnamed carbonate and shale is based on the trilobite biostratigraphy of Fritz (1972, 1976b, 1978, 1979). The late Early Cambrian strata in this area contain trilobites from the Fallotaspis, Nevadella, and Bonnia-Olenellus zones (Fritz, 1972) and are overlain (possibly unconformably) by Middle and Late Cambrian strata (Blusson, 1971a, 1971b; Fritz, 1976b, 1978, 1979). The Sekwi Formation, consisting of interbedded carbonate, shale, and sandstone (Handheld, 1968), was first recognized as a formal unit during regional mapping of the southern Selwyn Basin (Blusson, 1971a, 1971b). In addition to the important work by Fritz, Handfield (1968) described archeocyathans collected in the southern Selwyn Basin, and a Ph.D. study of the Sekwi Formation by Krause (1975) delineated the basic stratigraphie framework and platform-to-basin transition within the trilobite zones (see also Krause and Oldershaw, 1978a).