Jan 31, 2007
Terror birds were top predators of their time
It had been thought the fearsome beasts became extinct as little as 10,000 years ago - a time when humans shared their North American habitat.
But a US team has now revised this date to about two million years earlier.
The study, published in Geology, also sheds light on the flightless birds' migration to North America.
T. walleri is thought to be the largest species of the terror bird family. It would have stood 2m (7ft) tall and weighed 150kg (330lb).
The flightless species, which inhabited South and North America, had an enormous beak and lived up to its terror tag by being a top predator of its time.
The researchers looked at fossils of the birds that had been uncovered in Florida and Texas.
By analysing the distribution of a group of chemicals, known as rare earth elements, within the bones, the team was able determine the age of the North American remains.
"Previously, scientists believed that Titanis became extinct about 10,000 years ago," explained Professor Bruce MacFadden, a palaeontologist at the Florida Museum of Natural History and lead author of the Geology paper.
This would have coincided with the mass extinction of other mega-fauna that occurred in North America at the end of the Pleistocene; a period of biodiversity loss which has been blamed in part on humans and their novel spear technologies at the time.
But analysis of fossil bones found in the Sante Fe River, Florida, which are thought to represent the last known T. walleri, suggests the terror bird had disappeared long before this.
"The last occurrence of Titanis, as far as we know it, dates to about two million years ago. It didn't persist into the last ice age," Professor MacFadden told the BBC News website.
Dating of another fossil in Texas - the earliest known example of a terror bird in North America - also yielded a surprising result for the team.
Most researchers thought the big bird had migrated from South America to North America after the two continents had become connected by the Panamanian land bridge about 3.5 million years ago, explained Professor MacFadden.
"But based on the new chemical dates that we have established, that previous hypothesis is no longer correct," he added.
"What we now believe, based on the age of the Titanis from Texas, is that Titanis dispersed from South America into North America about five million years ago, significantly earlier than the land bridge formed."
He said the researchers did not yet know how the flightless beast could have crossed unconnected continents.
"Did it swim across? Or did it raft across on a float? There were a series of closely spaced volcanic islands, which now forms Panama, so maybe it swam from one to the other - but we really don't know," he said.