Preservation and phylogeny of Cambrian ecdysozoans tested by experimental decay of Priapulus.

Robert S Sansom

doi:10.1038/srep32817

Abstract

The exceptionally preserved Cambrian fossil record provides unique insight into the early evolutionary history of animals. Understanding of the mechanisms of exceptional soft tissue preservation frames all interpretations of the fauna and its evolutionary significance. This is especially true for recent interpretations of preserved nervous tissues in fossil ecdysozoans. However, models of soft tissue preservation lack empirical support from actualistic studies. Here experimental decay of the priapulid Priapulus reveal consistent bias towards rapid loss of internal non-cuticular anatomy compared with recalcitrant cuticular anatomy. This is consistent with models of Burgess Shale-type preservation and indicates that internal tissues are unlikely to be preserved with fidelity if organically preserved. This pattern, along with extreme body margin distortion, is consistent with onychophoran decay, and is therefore resolved as general for early ecdysozoans. Application of these patterns to phylogenetic data finds scalidophoran taxa to be very sensitive to taphonomically informed character coding, but not panarthropodan taxa. Priapulid decay also have unexpected relevance for interpretation of myomeres in fossil chordates. The decay data presented serve not only as a test of models of preservation but also a framework with which to interpret ecdysozoan fossil anatomies, and the subsequent evolutionary inferences drawn from them.

Highlights

The exceptionally preserved Cambrian fossil record provides unique insight into the early evolutionary history of animals
The fossil record has been vital in interpreting the nature and timing of the event, as well as the circumstances that led to the construction of modern body plans and ecosystems, and by extension, the very nature of evolutionary processes
By consequence cuticular characters, are demonstrated to be resistant to decay whilst non-cuticular anatomy and characters are quick to decay

Summary

Introduction

The exceptionally preserved Cambrian fossil record provides unique insight into the early evolutionary history of animals. Understanding of the mechanisms of exceptional soft tissue preservation frames all interpretations of the fauna and its evolutionary significance. This is consistent with models of Burgess Shale-type preservation and indicates that internal tissues are unlikely to be preserved with fidelity if organically preserved This pattern, along with extreme body margin distortion, is consistent with onychophoran decay, and is resolved as general for early ecdysozoans. The priapulids are widely considered as representative of the morphological condition of the early ecdysozoans and are closest available proxy for the primitive condition for the group[13] As such, they are the best model system for experimental investigation of mechanisms of ecdysozoan preservation and interpretations of their fossil record. Two main routes of preservation of soft tissues occur in Burgess Shale-type www.nature.com/scientificreports/

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Conclusion