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Knowledge about the earliest stages of vertebrate evolution draws heavily on information from 500-million-year-old fossils of early chordates — an animal group that includes vertebrates and their closest relatives. Lacking bones, teeth or shells, these ancient soft-bodied animals left behind a sparse record, and there are few well-preserved examples. This has made it hard to interpret their anatomy and evolutionary significance. Now, Mark Purnell and his colleagues at the University of Leicester, UK, have learned more about the way such organisms decay and how this may affect their proper placement in the evolutionary tree (see page 797). Purnell tells Nature more.

What did you use as a proxy for such ancient animals?

We studied the two modern-day chordates thought to most closely resemble their early relatives. Amphioxus are finless fish-like creatures; ammocoetes are larvae of lampreys, jawless fish that resemble eels. The amphioxus came from the Mediterranean, the ammocoetes from Yorkshire, UK.

How did you do the experiment?

We wanted to record how particular anatomical characteristics of chordates change or disappear during decomposition. So we sealed the specimens in clear plastic containers filled with sea water or fresh water, depending on where they had lived, and placed them in temperature-controlled cabinets. Then we left them to rot. To say it was unsavoury and unpleasantly pungent is an understatement.

How did you record the decay?

We photographed and dissected the organisms to see which characteristics endured. We expected the animals to lose features over time, but what surprised us was that the decay followed a very clear pattern. Those characteristics most useful for correct phylogenetic ordering were the first to disappear. In other words, the characteristics that decayed first were the ones that were unique to amphioxus or to ammocoetes.

Do your findings have implications for the field of evolutionary biology?

Yes. They confirm that we can't assume any fossil has been unaffected by decay. Fossils that seem to have only decay-resistant characteristics and seem to be primitive must be viewed carefully. It could be that they're actually the badly decomposed remains of a more complex organism. Fossil soft-bodied animals are undoubtedly important, but we may have to accept that there are uncertainties about the anatomy and placement of some.