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Marianne Collins's reconstruction of a colony of Odontogriphus omalus grazing on cyanobacterium. ((Copyright Caron et. al, Nature 2006))

Newly discovered features of ancient fossils from the Burgess Shale in British Columbia suggestan odd creature is the world's oldest known soft-bodied mollusc, the group that includes today's snails and squid.

Jean-Bernard Caron, invertebrate paleontology curator of the Royal Ontario Museum in Toronto, and his team looked at new specimens of Odontogriphus omalus, a half-billion-year-old toothed riddle.

The discovery, which researchers say reveals the world's oldest known soft-bodied mollusc,opens a new window on the early evolution of animals.

The 189 specimens, some as large as 12 centimetres long and four centimetres wide,were extracted from the mountains of British Columbia over the past 15 years.

The fossils show the creature had a defining feature of molluscs: a hard feeding structure called a radula.

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A complete specimen of Odontogriphus omalus that shows the overall shape of the fossil, the position of the radula feeding structure at the head end, and paired salivary glands, the darker circular structures on either side of the radula. ((Copyright Caron et. al, Nature 2006))

Odontogriphus had teeth positioned in rows on a ribbon for feeding, just as modern snails have similar features to scrape films of algae off aquarium glass, the researchers said in Thursday's issue of the journal Nature.

Fresh sets of teeth appear

As the front teeth wear out and are discarded or swallowed, the ribbon moves to expose fresh sets of teeth,Stefan Bengtson of the Swedish Museum of Natural History in Stockholm wrote in a journal commentary.

The shell-less mollusc used its radula to graze on mats ofmicrobes on the seafloor.

It had an oval body, compressed from top to bottom, with simple gill-like structures surrounding a muscular sole or "foot" on the underside, another sign of molluscs, the researchers said.

The discovery extends the roots of the mollusc lineage as far back as the Late Precambrian age, at least 560 million years ago, Caron said.

If true, molluscs and therefore many other groups of animals evolved tens of millions of years earlier than thought.