Decay Processes and Chordate Phylogeny

I have just read a Nature paper reporting some experimental work studying the pattern of decay in two soft-bodied species, Lampetra and Branchiostoma, which are thought to be the best proxies of the early chordates (chordates are the group of animals that includes the vertebrates and those invertebrates that are their closest relatives).

The authors, Sansom et al. (2010), note that our understanding of the early evolution of the chordates is very sparse, in large part because the early chordates were entirely soft-bodied and are only rarely preserved, and in part because the interpretation of those soft-bodied fossils we do have is complex. It is especially hard to distinguish the earliest true chordates from their close, non-chordate relatives (called the "stem chordates"). They suggest that this might be rectified by better understanding of the sequence in which features of early chordates decay. In particular, we need to know whether the characteristics which characterise the true chordates decay relatively fast upon the death of their bearer, as if this is true, the partially-decayed true chordates will be misinterpreted as stem chordates, which they now resemble (Sansom et al. 2010).

And, indeed, this is exactly what Sansom et al. found was the case. In their experiments, which tracked the order in which features of the two species decayed, those which were lost first were those which were most informative about the relationships between early chordates. As a result, the relative abundance of stem chordates in comparison with true chordates in the fossil record may be the result of the incomplete preservation of the crucial characteristics which would enable researchers to identify their real relationships.

I think this paper is fascinating. At the same time, though, if it is true that the characteristics which are most informative about early chordate evolution are those which decay first, it is difficult to see how we will every sort the true chordates from their stem chordate relatives, barring finds of even more exceptionally preserved fossils than we already have from the relevant period. Despite this, knowing more about taphonomy (the processes of decay and destruction that affect dead organisms) can only inform our reconstructions of the evolutionary history of life, even if some parts of that history can never be fully resolved.



References

Sansom, R., Gabbott, S., & Purnell, M. (2010). Non-random decay of chordate characters causes bias in fossil interpretation Nature, 463 (7282), 797-800 DOI: 10.1038/nature08745