BITS AND PIECES FROM THE CAMBRIAN EXPLOSION

When processed carefully, unoxidized Early and Middle Cambrian shales regularly yield a diversity of small, carbonaceous, metazoan remains, including chaetae/setae, jaw structures, thecae, fragmentary cuticle, and (inevitable) problematica. In isolation, these bits and pieces contribute little to the resolution of Cambrian palaeobiology, but increased sampling is beginning to reveal both general trends and particular insights. To date, Burgess Shale-type micro-metazoan assemblages have been recovered from the Early Cambrian Mahto Fm. (Jasper National Park, Canada), the Early and Middle Cambrian Mount Cap Formation (NWT, Canada), the early Middle Cambrian Kaili Fm. (South China), the Middle Cambrian Burgess Shale Fm. (Yoho National Park, Canada), the Middle Cambrian Bright Angel Shale (Grand Canyon, USA [Strother 2004]), the Middle Cambrian Hess River Fm. (NWT, Canada), and the late Middle Cambrian Pika Fm (Jasper National Park, Canada). Despite the wide temporal and geographic range represented, these biotas exhibit some conspicuous similarities: Wiwaxia sclerites, for example, appear to be globally distributed through this interval, while the repeated co-occurrence of various isolated elements invites reconstruction as multi-element scleritomes and whole organisms. Direct evidence of such relationships is found in the preservation of intact or partially intact scleritomes, which include articulated jaw apparatuses, articulated series of Wiwaxia sclerites and, in one instance, articulated Wiwaxia sclerites in direct association with an articulated jaw apparatus. The Hess River biota also preserves rhabdopleurid thecae, and some strikingly intricate complexes of branching structures of unknown affiliation. The enhanced preservation of articulated and delicate elements in these micro-metazoan assemblages is largely a consequence of increasing mechanical strength at small size. As such, these novel fossil data hold particular promise for reconstructing early metazoan architecture, and thereby the course of the Cambrian explosion.