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PALAEONTOLOGY
Experiments with simple chordate animals show how decay may make the resulting fossils seem less evolved. The consequence is to distort evidence of the evolution of the earliest vertebrates and their precursors.
Just as human corpses become more difficult to identify as information is lost through decay, so too do the fossils of our marine ancestors from back in the Cambrian. For example, a remarkable diversity of soft-bodied, fishlike fossils, dating to about 525 million years ago, have been described from Chengjiang in China. Uncertainties about the nature of these creatures, however, have fuelled controversies about their place in the early evolution of chordates, the group that includes all vertebrates and some closely related invertebrates.
On page 797 of this issue, Sansom and colleagues1 describe laboratory observations of the decay of two living forms similar to these earliest chordates: the lancelet Branchiostoma (a fish-like invertebrate that has a stiffened structure called the notochord) and the larva of the more familiar lamprey. They find that decomposition of these two creatures always occurs in more or less the same sequence. Features of the head, for example, tend to be lost before those of the trunk, including the notochord and muscle blocks.
Even more strikingly, the appearance of the carcass is transformed by decay until it resembles the much simpler morphology of an ancestral (stem) chordate. Attributes tend to disappear in the opposite order to that in which they evolved, so that only the more ancestral morphology remains. As a result, the corresponding fossil falls in a misleadingly low position on the chordate...