FOSSILIZATION CAUSES ANIMALS TO APPEAR ERRONEOUSLY PRIMITIVE BY DISTORTING PHYLOGENY

Fossils are vital for calibrating rates of molecular evolution through geological time and reconstruction of historic morphological transitions. This can only be done, however, through the robust placement of extinct organisms in phylogenetic frameworks. Here, we demonstrate that the inevitable bias of the fossil record to preserve just hard, skeletal morphology systemically distorts phylogeny. Removal of soft part characters from 78 modern vertebrate and invertebrate morphological datasets resulted in significant changes to phylogenetic signal; it caused individual taxa to drift from their original position, predominately downward, toward the root of their respective trees. This last bias could systematically inflate evolutionary rates inferred from molecular data because first fossil occurrences will not be recognised as such. Stem-ward slippage, whereby fundamental taphonomic biases cause fossils to be interpreted as erroneously primitive, is therefore a ubiquitous problem for all biologists attempting to infer macroevolutionary rates or sequences.