Taphonomic bias in exceptionally preserved biotas

Abstract Exceptionally preserved fossil biotas provide crucial data on early animal evolution. Fossil anatomy allows for reconstruction of the animal stem lineages, informing the stepwise process of crown group character acquisition. However, a confounding factor to these evolutionary analyses is information loss during fossil formation. Here we identify that the Ordovician Fezouata Shale has a clear taphonomic difference when compared to the Cambrian Burgess Shale and Chengjiang Biota. In the Fezouata Shale, soft cellular structures are most commonly associated with partially mineralized and sclerotized tissues, which may be protecting the soft tissue. Also, entirely soft non-cuticularized organisms are absent from the Fezouata Shale. Conversely, the Cambrian sites commonly preserve entirely soft cellular bodies and a higher diversity of tissue types per genus. The Burgess and Chengjiang biotas are remarkably similar, preserving near identical proportions of average tissue types per genus. However, the Burgess shale has almost double the proportion of genera that are entirely soft as compared to the Chengjiang Biota, indicating that the classic Burgess Shale was the acme for soft tissue preservation. Constraining these biases aids the differentiation of evolutionary and taphonomic absences, which is vital to incorporating anatomical data into a coherent framework of character acquisition during the earliest evolution of animals.