Leech cocoon fossil is remarkable discovery

By Alyce Taylor 6 December 2012
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An ancient soft-bodied organism discovered in a cocoon could open doors for fossil research.

SCIENTISTS BELIEVE THE discovery of a 200-million-year-old soft bodied organism encased in a leech cocoon could begin a series of new fossil discoveries.

The ancient Vorticella-like organism, a microscopic aquatic animal commonly known as a bell animal, was perfectly preserved and highly unsual, due to its soft-bodied nature.

“To find something so incredibly small, and so rare, fossilised is quite an achievement,” says associate professor Kate Trinajstic a chemist at Curtin University, WA.

The specimen was collected by Dr Benjamin Bomfleur, of Kansas University’s Biodiversity Institute, during a geological expedition in Antarctica.

Benjamin procured expert help to identify the microfossil. “By then, we realised that we had made a truly extraordinary find,” he says.

Cocoon discovery could open up “treasure trove” of fossils

It has always been difficult for palaeontologists to find fossils of animals without hard features, such as bones or shells. Typically, the only way soft-bodied organisms are preserved is by becoming fossilised inside a protective casing, such as amber or asphalt.

This discovery, however, has opened up the possibility of clitellate cocoons as a hub for soft-bodied fossil research.

Earthworms and leeches produce clitellate cocoons which are extremely durable, allowing them to conserve material trapped inside for millions of years. It can take up to seven days of secretion for the cocoon’s membrane to harden into a shell, trapping other organisms inside.

“We believe that leech cocoon fossils may, as bizarre as it may seem, form a real treasure trove in the search for fossil microbes, similar to amber,” says Benjamin.

Slow rate of evolution leaves a “living fossil”

The specimen’s age, microscopic size and casing material are not the only surprising features of the fossil.

“That it is almost an identical form to a current living organism suggests that it has been quite conservative in its evolution. People often expect organisms to change over such a large timescale,” says associate professor John Paterson, a palaeontologist at the University of New England in Armidale, NSW.

“It is an incredible continuation of lineage,” agrees Kate. “To have animals that are still alive from that time shows that some animals have, in evolutionary terms, hit the nail on the head.”

John hopes the finding will lead to a new wave of fossil discoveries, which will help to build our understanding of the Earth’s history.

“What this demonstrated is that if you start looking, maybe marvels of other organisms are just sitting there ready to be found,” says John. “It is a case of using the right equipment and techniques, but also a matter of just looking.”

The paper was published this week in the US journal PNAS.