DISTINGUISHING ABSENCE FROM LOSS: PHYLOGENETIC ANALYSIS OF EXCEPTIONALLY PRESERVED FOSSILS REQUIRES VERY FEW ERRORS TO BE MADE

Exceptional preservation of soft bodied fossils provides unique insight into many important evolutionary events, foremost being the Cambrian explosion. Use of these fossils in evolutionary contexts requires robust placement in phylogenetic frameworks. The complex taphonomic processes of decay and preservation underlying the formation of these fossils make it difficult, however, to distinguish phylogenetic absence from taphonomic loss. Is a particular morphological character (e.g. appendage, tentacle or nerve cord) missing from a fossil because it was never there, or just happened to not be preserved? Here, we demonstrate that the introduction of only a few miscoded absences to simulated and real phylogenetic datasets (0s rather than ?s) causes taxa to drift down trees, toward the root. This low threshold for errors may cause fossil organisms to be erroneously interpreted as more primitive than they were in life. It therefore presents a problem for all evolutionary studies that attempt to use exceptionally preserved soft bodied fossils to reconstruct rates of evolution (molecular clocks) or unlock sequences of morphological change (evolutionary transitions). Stemward slippage, whereby taphonomic processes cause organisms to appear artificially primitive, therefore appears to be a ubiquitous and problematic phenomenon, even when no decay biases exist.