Fossil sheds light on evolution of birdsong
Scientists have reconstructed the "voicebox" of an extinct bird that lived at the time of the dinosaurs.
The bird may have honked, quacked or whistled, like a duck or goose.
Investigation of the oldest-known fossil of a bird's vocal organ - the syrinx - gives clues to how birdsong evolved.
The bird, Vegavis iaai, lived in what is now Antarctica about 66-68 million years ago. It belongs to the group that includes ducks, geese and swans.
Julia Clarke of the University of Texas at Austin said there had been virtually no work on the origin or early evolution of the unique way in which birds produce sound.
"While we've looked a lot at the evolution of the wing in birds," she said, "we have done very little with looking at the origin of what is perhaps one of the most striking characteristics of living birds - their songs."
Prof Clarke and her team scanned the fossil specimen using micro-CT, an X-ray scanning technique similar to hospital CT scans but on a smaller scale.
They made a 3D representation of the syrinx and then used another type of imaging that provides information on soft tissues to make comparisons with younger fossils and 12 living birds.
This allowed them to reconstruct the evolution of the tiny bony organ.
"We definitely think this voicebox is capable of honks or whistles," said Prof Clarke.
"But if we want to understand more precisely the frequency range or the variety of sounds, we'll have to build models and get more data from living ducks to constrain what might be the range of sounds produced by this structure."
Researchers think the syrinx may have arisen late in birds' evolution, well after the origin of flight.
No evidence for the syrinx had been found in non-avian dinosaurs.
The syrinx may be a relatively late arrival along the bird lineage, and in turn may have played a significant role in the diversification of birds, said Prof Patrick O'Connor of Ohio University.
"Their amazing diversity may in part be related to the evolution of the syrinx and any areas of the brain related to sound production and reception in the context of social interactions more generally," he explained.
Earlier this year, Prof Clarke's team deduced that some non-bird dinosaurs might have made a deep booming noise, giving a fuller picture of the sound landscape when dinosaurs roamed the planet.
"Larger-bodied animals like most non-bird dinosaurs are big and they're going to exhibit lower frequency sounds overall," she told BBC News.
"But then when we get the origin of flight and the origin of these smaller-bodied dinosaurs, including birds, then we think we're going to get open-mouthed sounds that are maybe a little bit jarring - quacks and honks and crows."
The oldest fossilised remains of a syrinx are described in a study published online in the journal, Nature.
The fossil was originally found by an expedition of scientists from the Argentine Antarctic Programme.
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