Non-random decay of chordate characters causes bias in fossil interpretation

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Abstract

Exceptional preservation of soft-bodied Cambrian chordates provides our only direct information on the origin of vertebrates1,2. Fossil chordates from this interval offer crucial insights into how the distinctive body plan of vertebrates evolved, but reading this pre-biomineralization fossil record is fraught with difficulties, leading to controversial and contradictory interpretations3,4. The cause of these difficulties is taphonomic: we lack data on when and how important characters change as they decompose, resulting in a lack of constraint on anatomical interpretation and a failure to distinguish phylogenetic absence of characters from loss through decay3. Here we show, from experimental decay of amphioxus and ammocoetes, that loss of chordate characters during decay is non-random: the more phylogenetically informative are the most labile, whereas plesiomorphic characters are decay resistant. The taphonomic loss of synapomorphies and relatively higher preservation potential of chordate plesiomorphies will thus result in bias towards wrongly placing fossils on the chordate stem. Application of these data to Cathaymyrus (Cambrian period of China) and Metaspriggina (Cambrian period of Canada) highlights the difficulties: these fossils cannot be placed reliably in the chordate or vertebrate stem because they could represent the decayed remains of any non-biomineralized, total-group chordate. Preliminary data suggest that this decay filter also affects other groups of organisms and that ‘stem-ward slippage’ may be a widespread but currently unrecognized bias in our understanding of the early evolution of a number of phyla.

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Figure 1: Phylogeny and anatomy of chordates.
Figure 2: Results of experimental decay.
Figure 3: Morphological decay stages of Branchiostoma (left) and larval Lampetra (right) and the phylogenetic position of each stage if interpreted as a fossil.

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Acknowledgements

This work was funded by UK Natural Environment Research Council grant NE/E015336/1. Specimens were provided by H. Escriva (Banyuls-sur-Mer, France) and B. Morland and S. Morland (Bellflask Ecological Survey), with practical assistance from C. Koch, C. Pratt and P. Griffiths. We thank D. Briggs for his feedback on the submitted manuscript.

Author Contributions S.E.G. and M.A.P. conceived and designed the research program; R.S.S., S.E.G. and M.A.P. designed the experimental setup; R.S.S. conducted the experimental work and analysed the data; and R.S.S., S.E.G. and M.A.P. wrote the paper.

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Correspondence to Mark A. Purnell.

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Sansom, R., Gabbott, S. & Purnell, M. Non-random decay of chordate characters causes bias in fossil interpretation. Nature 463, 797–800 (2010) doi:10.1038/nature08745

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