Rotten lampreys show bias in fossil record
1 February 2010, by Sara Coelho
Palaeontologists can usually be spotted around fossils, but a team from Leicester looked at rotting lampreys instead. Their findings suggest that the fossil record might be biased and shed new light on the evolution of the vertebrate group, to which we belong.
This unusual (and smelly) line of research revealed that the earliest known examples of vertebrates and early chordates were probably more complex than scientists have thought.
This is because palaeontologists rely on the information provided by fossils, which sometimes is not forthright. Dinosaurs can be studied by their bones, molluscs by their shells and mammals by their teeth. But when animals first appeared about 540 million years ago during the Cambrian, almost none had the hard body parts that are more likely to survive as fossils.
'Only in the most exceptional circumstances do soft tissues, such as eyes, muscles and guts, become fossilized,' says palaeontologist Dr Sarah Gabbott, from the University of Leicester. 'Yet it is precisely such remains that we rely on for understanding our earliest evolutionary relatives: half-a-billion years ago it's pretty much all our ancestors had.'
A sequence of images showing how the characteristic features of the body of the lancelet change during decay. Colours are caused by interference between the experimental equipment and the light illuminating the specimens.
Cambrian fossils are rare and the ones that exist are difficult to interpret. Are later, more evolved, features absent because they were not there yet? Or were they somehow lost during the animal's decay?
To solve the conundrum, the team collected lamprey larvae (Lampetra fluviatilis) and lancelets (Branchiostoma lanceolatum) and watched while they rotted away under controlled conditions.
Lancelets, also known as amphioxus, 'are not vertebrates, but they are thought to be the best living anatomical equivalent for early chordates,' explains palaeontologist Dr Mark Purnell, from the University of Leicester. Lampreys are vertebrates and although they can be confused with fish, they belong to a separate group with more primitive features.
As the animals rotted, Purnell and colleagues looked at how the lancelet and lampreys decayed and noted when given features disappeared. The work required them to dissect rotting corpses at different stages for about 200 days. Resistance to the stinking smell was found to be essential.
'By knowing how their anatomical features change as they rot, we are better able to correctly interpret the most ancient fossils representing the earliest ancestors in our evolutionary history,' says lead author Dr Rob Sansom.
The team found that 'the features that are most useful to place animals in the tree of life are the first to go,' says Purnell.
The lamprey's signature characteristics disappear after two stages of decay. If both lampreys and lancelets were to be preserved as fossils after four decomposition stages they could only be identified as chordates because the more precise anatomical information was lost by then.
'This means that the fossil record will be biased and animals will tend to look more primitive than they actually are,' says Sansom. 'The fossil Metspriggina, for example, has been interpreted as an early stem-vertebrate. But we don't know if some diagnostic features were lost.'
'If the decay bias is widespread, we probably should reconsider the evolutionary history of soft-bodied animals,' he adds. A solution, Purnell suggests, could be introducing the concept of 'error bars' to the classification of fossils to include the degree of uncertainty of the conclusions.
Sansom, R. S., Gabbott, S. E. & Purnell, M. A. Nature advance online publication doi:10.1038/nature08745 (2010)
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