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Slouching out of Gondwana

June 13, 2006 By Michael Hopkin This article courtesy of Nature News.

The ancestor of today's crocodiles was probably Australian.

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Modern crocodiles and alligators may be able to trace their roots back to Australia, say palaeontologists who have dug up the scaly beasts' most primitive known relative near a remote outback town.

The new crocodile-like species, unearthed in rural Queensland by a team led by Steven Salisbury of the University of Queensland in Brisbane, lived between 95 and 98 million years ago, during the Cretaceous period.

It was around that time that the group that includes today's crocs and alligators first emerged and began to diversify. Finding an early member of the group in Australia suggests that it arose in Gondwana, a supercontinent that consisted of much of the Southern Hemisphere's continents before they separated, say the researchers. They unveil the discovery in Proceedings of the Royal Society B1.

Salisbury and his team have christened the creature Isisfordia duncani, after the town of Isisford, near where the first fossil was discovered a decade ago by a local livestock farmer, Ian Duncan. Years of subsequent digging uncovered parts of several more skeletons.

Flexible friends

In Australia everyone loves crocodiles, so to discover they originated here - everyone's gonna be pretty chuffed.
Steven Salisbury
University of Queensland
The discovery fills in a crucial chapter in the evolutionary tale of crocodiles, Salisbury says. The first crocodile-like reptiles, called crocodyliforms, appeared more than 200 million years ago, but no fossils had previously been found documenting the appearance of modern crocodiles and alligators, which burst into the fossil record in a flurry of diversity around 80 million years ago.

The new species would therefore have lived just before the lineages that gave rise to today's species, meaning that it was probably closely related to their common ancestor, Salisbury adds.

I. duncani's skeleton may also hold clues as to how these animals managed to survive the mass extinction that killed off their fellow reptiles, the dinosaurs, 65 million years ago. At just 1.1 metres long, the creature was small compared with some of today's crocodiles, some of which can reach 7 metres. This diminutive size may also have helped it to avoid becoming too specialized, Salisbury suggests.

"The thing that separates crocodiles from other things is their ease of walking on land, but with limbs that are still flexible enough for swimming," Salisbury says. I. duncani seems to have shared this versatility, whereas other, larger reptiles may have been doomed by their lack of flexibility.

That is largely speculation, however, Salisbury warns; nobody really knows why some species came through the disaster thought to have been caused by a meteorite impact, and some didn't. "You have to say a lot of it was probably pot luck," he says.

A crocodile odyssey

As is the way with Australians, the Cretaceous crocodile's ancestors did not stay put, but headed out across the world.

The fossil record suggests that many of the modern species in the group, including those in Australia, originated in the Northern Hemisphere, particularly in North America, famed for its alligators. Australia's impressive crocodiles, which include the fearsome saltwater version (the world's biggest reptile), are descended from animals that went north and came back, Salisbury says.

But the new discovery suggests that the original granddaddy of today's crocs was a native inhabitant of the land Down Under. The implications for the nation's pride have not gone unnoticed. "In Australia everyone loves crocodiles," says Salisbury. "So to discover they originated here, everyone's gonna be pretty chuffed."

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References

  1. Salisbury S., Molnar R. E., Frey E., Frey E. & Willis P. M. A. Proc. R. Soc. B, doi:10.1098/rspb.2006.3613 (2006).

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