Exceptional Phosphatic Preservation in the Lower Cambrian Comley Lagerstätte of England
Thomas H. P. Harvey1†, Philip Wilby2, Mark Williams1, Sarah E. Gabbott1, Dieter Waloszek3 and David J. Siveter1
1 Department of Geology, University of Leicester, England, UK
2 British Geological Survey, Keyworth, Nottingham, England, UK
3 Section for Biosystematic Documentation, University of Ulm, Germany
The exceptional preservation of embryos and small, non-mineralizing metazoans in early diagenetic phosphate provides a unique window onto evolution across the Proterozoic–Phanerozoic transition. However, a poor understanding of the underlying taphonomic mechanisms limits the palaeobiological inferences that can be drawn. In particular, it is not known whether the conspicuously narrow range in taxonomy and body size of the preserved organisms is governed by intrinsic biological properties, or whether it reflects conditions of preservation in a shared ecological or environmental setting. Furthermore, there is disagreement as to whether the various assemblages represent the products of a common taphonomic pathway.
We contribute new empirical data to the debate through a detailed analysis of the Lower Cambrian Comley lagerstätte of England, which has yielded exceptionally preserved soft parts of phosphatocopines and thus represents one of the earliest known occurrences of phosphatized micro-arthropods. Our re-excavation of a classic locality, combined with petrographic and energy-dispersive X-ray analysis, has allowed a new, high-resolution reconstruction of the Comley sequence, while extensive new collections of microfossils have revealed a preservational spectrum with implications for the depositional and post-burial environment. Through comparison with broadly contemporaneous assemblages, we find that the shallow depositional setting of the Comley sequence is comparable to that of the embryo-bearing facies of the Doushantuo Formation (Neoproterozoic, China), although the style of phosphatization is closer to that of the arthropod-dominated, quiet-water Orsten deposits from the middle to late Cambrian of Sweden. We question whether the apparent diversity of depositional settings is genuine or artefactual, and explore the implications for understanding the history of this phosphatization window and of the life that it reveals.