Published online 6 December 2010 | Nature | doi:10.1038/news.2010.651

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Osedax worms might have had a more-rounded diet 30 million years ago.

Osedax frankpressi worms throng whale bone in Monterey Canyon off the Californian coast. 2003 MBARI

Boreholes in the fossilized bones of ancient birds are hinting that Osedax, a genus of worms that feeds on whale bones, dined on the bones of birds long before it tackled those in whales.

The research, reported online in the journal Naturwissenschaften1, targets the fossil remains of two penguin-like birds from the family Plotopteridae that were flightless and probably survived by diving for fish in marine environments. At around 30 million years old, the bones are heavily corroded with age and show scrape marks that were probably made by scavenging sharks after the birds died. Even so, boreholes can be seen in all of the bones.

The animal that produced these boreholes had a branching 'root system' that grew to a length of at least 3 millimetres. Most of the holes have a diameter of no more than 0.3 millimetres. X-ray computed tomography scans of the bones show that the holes lead into a network of cavities inside the fossil bones. All of these characteristics make the holes look very much like those that are made in whale bones by modern Osedax species.

"As soon as I saw these holes in bird bones, I knew I was seeing something that had not been seen before," recalls geobiologist Steffen Kiel of the University of Göttingen in Germany, who is lead author of the study1.

A bone from an ancient penguin-like member of the Plotopteridae is riddled with holes (see arrows) made by Osedax . G. Hundertmark/Göttingen Univ.

The discovery changes perspectives on what these worms are able to eat. To date, experiments show that Osedax species can feed on bones from elephant seals, cows and whales, but they have not been reported to eat bird, reptile or fish bones. For this reason alone, the discovery of Osedax boreholes in bird bones is remarkable because it suggests that these worms once had a wider diet.

"When I read this paper just before Thanksgiving I thought it fitting to drop a turkey to 1,800 metres in Monterey Bay, just to see if the worms would feed on its bones," says Robert Vrijenhoek at the Monterey Bay Aquarium Research Institute in Moss Landing, California, who was not involved in the study. But there is much more to the discovery than an expanded historic feeding regime.

Can of worms

Based on palaentological estimates, Osedax worms might have followed two possible evolutionary paths, explains Vrijenhoek. They could have evolved alongside whales roughly 30 million to 40 million years ago during the Paleogene period, or they could have evolved 70 million to 80 million years ago during the Cretaceous.

Until now, the discovery of Paleogene fossil whale bones with boreholes in them — and the fact that no other marine fossil had been found to contain boreholes — hinted that the worms evolved to feed on whale bones in step with the evolution of marine mammals.

By contrast, a Cretaceous evolutionary origin seemed a dim possibility, as no marine mammals existed that long ago, and modern observations have shown that the worms feed only on mammal bones. The discovery of bird bones with boreholes changes this because it suggests that the worms have not been limited to a diet of mammal bones throughout their evolution.

One problem with the theory that the worms evolved during the Cretaceous period and fed on the large marine reptiles of the time is that they would not have had much food after the Cretaceous–Tertiary extinction that took place 65 million years ago and wiped out all large reptiles.

"I've often thought that, if the worms did evolve in the Cretaceous and did feed on plesiosaurs and ichthyosaurs, they probably switched to sea turtles after the giant species went extinct and then switched to whales later when they evolved," says Vrijenhoek. "But nobody has reported boreholes in sea turtle remains. I hadn't even considered the possibility of diving birds. This is a truly novel find."

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Marine birds have existed continuously since the Cretaceous, and so would provide an alternative food source for Osedax.

However, the discovery does not resolve all of the evolutionary questions surrounding Osedax. To further support the theory that the worms evolved during the Cretaceous, scientists will need to find evidence of large marine reptile bones containing boreholes.

"I've checked a few plesiosaur bones from deep-water sediments and have not seen anything yet, but my searches have, so far, only been casual. I'd really like to get a systematic search going," says Kiel.