Ducks may have been paddling about in primeval swamps when T. rex was king of the dinosaurs, scientists have announced in the journal Nature.
Some scientists believe many modern bird lineages existed as long as 100 million years ago
Fossil remains of a bird that lived 70 million years ago appear to belong to a relative of modern ducks and geese.
The partial skeleton, discovered on Vega Island, western Antarctica, is likely to stir up controversy.
Many scientists believe modern bird lineages did not evolve until the end of the dinosaurs' reign.
Although the first known primitive bird, Archaeopteryx, lived in the Jurassic Period 150 million years ago, researchers disagree over when modern birds made their first appearance.
One camp believes many modern bird lineages existed as long as 100 million years ago. According to this vision, familiar looking birds would have been running and flying about alongside dinosaurs.
In contrast, the other camp thinks that, although birds did exist during the Jurassic and Cretaceous Periods, they were largely wiped out by whatever killed the dinosaurs.
According to this theory, only a few lineages made it through the mass extinction and, subsequently, these lonely survivors blossomed into all the modern bird families we know today.
The fossil records so far support the latter version, known as the "big bang" theory of bird evolution.
But if the new find, known as Vegavis iaai, really is a relative of the duck, it would lend considerable weight to the idea that modern birds lived with dinosaurs and survived whatever catastrophe killed the "terrible lizards".
A team of scientists led by Dr Julia Clarke, from North Carolina State University, US, said Vegavis belonged to the waterfowl family and was "most closely related to Anatidae, which includes true ducks".
"Until now the fossil record has been ambiguous," added Dr Clarke. "But now we have a fossil which indicates that at least part of the diversification of living birds had begun before the extinction of non-avian dinosaurs."
If this species was a duck, and it did live in the Cretaceous Period, then other modern birds probably did, too.
"Chickens and their relatives belonged to the lineage that was closest to the duck lineage," Dr Clarke told the BBC News website.
"So if we had the duck lineage in the Cretaceous, the chicken lineage must have been present. Even though we don't have a chicken fossil yet, we know its lineage must have been there."
The researchers used a statistical analysis of certain bone features to identify the "duck"
However Vegavis has not managed to convince supporters of the big bang theory of bird evolution.
"This is basically an unidentifiable bundle of bones," Alan Feduccia, a bird expert from the University of North Carolina, US, said.
"This is a well-known specimen that has been kicking around since 1992, and it was originally described as belonging to an extinct group. And now all of a sudden it's a modern duck."
Sensitive to change
Julia Clarke and her team used a statistical analysis of certain bone features to identify Vegavis as a member of the duck family - but Professor Feduccia is unmoved by the interpretation.
"The analysis is based on very superficial features of bones; so I find it unreliable."
Professor Feduccia is sure that bird species could not have survived a major global extinction en masse.
"Birds are very sensitive to any environmental disturbance - in fact, they are a good indicator of environmental problems.
"But these people don't believe whatever caused the mass extinction had any affect on the birds, and that seems ludicrous."