Using Decay Experiments to Constrain Interpretations of Soft-Bodied Fossils: Application to the Emergence of the Chordata  

Many of the early and important episodes of metazoan evolution occur prior to the advent of biomineralization.  Consequently, our knowledge of these crucial stages in novel body plan origins is limited to the narrow windows provided by soft-tissue preservation.  Often rare, collapsed and partially decomposed, soft-bodied fossils are difficult to interpret unequivocally, principally because of a lack of understanding of the sequence of loss and transformation of key anatomical characters during decay.  Nowhere is this more true than the Cambrian fossil record, especially that of chordates.  In order to provide rigorous taphonomic constraints upon the interpretation of putative soft-bodied chordate fossils, we undertook laboratory experiments to investigate the decay of chordate characters, using adult Branchiostoma (Amphioxus) and larval Lampetra (ammocoete) as extant proxies. 

Key characters undergo morphological change, and eventually loss, in a non-random sequence.   A marked correlation was observed between the relative order of loss of characters, and the phlyogenetic level at which characters are informative (e.g. Deuterostoma, Chordata, Cephalochordata or Vertebrata). Different classes of characters thus have different preservation potentials which, if not taken into account, will lead to a systematic bias in the identification of fossil chordates.  Recognition of this potential bias, in light of the new taphonomic data, sheds new light upon interpretation of the anatomy and affinity of purported Cambrian chordates from the Burgess Shale and Chengjiang biota such as Metaspriggina, Pikaia, Cathaymyrus and Haikouichthys.  Exposing the biases due to decay will enable reevaluation of the emergence of the phylum Chordata, and bring to light the potential role of preservation bias in our understanding of other evolutionary episodes.