EXCEPTIONAL (AND UNEXCEPTIONAL) PRESERVATION OF SMALL CARBONACEOUS FOSSILS (SCFS): EMERGING TRENDS IN CAMBRIAN TAPHONOMY AND PALEOBIOLOGY

Small carbonaceous fossils (SCFs) are, by definition, larger or more delicate than standard palynomorphs, so have mostly been overlooked. However, our large-scale survey of Cambrian mudrocks, focussing on the Western Canada Sedimentary Basin, reveals that SCFs are surprisingly widespread – although, as in other taphonomic modes, there is a broad spectrum of preservation. “Exceptional” specimens offer unrivalled insights into the anatomy, autecology, and phylogeny of a suite of Cambrian organisms, but are only locally abundant. However, even poorly preserved SCFs help to illuminate broad-scale distributions in space and time. Among crustacean SCFs in the Earlie/Deadwood succession of SW Saskatchewan, for example, arrays of setae that exhibit both sub-micron-scale preservation and a high degree of articulation are restricted to just one of 135 processed samples. In contrast, crustacean mandibles (jaws) are more widespread, occurring in 20 samples – presumably reflecting their greater intrinsic preservation potential. The most informative mandible specimens preserve an intact gnathal edge. However, the range extends to single, disarticulated teeth that are robust enough to withstand regular palynological processing, which has the advantage of recovering taphonomically degraded and/or highly dispersed specimens, and has recently revealed a second occurrence of Cambrian copepods, in the Nolichucky Shale of Tennessee. Aside from crustaceans, comparable taphonomic spectra are emerging for various other SCF taxa, including Wiwaxia (which ranges from sclerite-fragment palynomorphs to articulated arrays of sclerites and jaws) and priapulids (from tooth-fragment palynomorphs to articulated oral cones). The preservational controls are likely to be numerous. Certainly, thermal maturity and weathering of the host rocks constrain what is recoverable, with the greater fragility of heated and weathered SCFs resulting in low-diversity assemblages. A more fundamental control, however, is traceable to early diagenesis – with clear analogies to macroscopic “Burgess Shale-type” preservation. Even so, it is clear that most SCFs require a fundamentally less demanding set of preservational conditions, and represent a powerful complementary dataset for reconstructing the Cambrian “explosion”.