Chickens the size of a horse roamed Australia as recently as 50,000 years ago.
Well, not exactly, but research into the evolutionary history of an extinct family of giant flightless birds, the Dromornithidae, has established that they were members of a larger group known as galloanseres – colloquially described as “fowl”, and therefore related to modern-day chickens, ducks and geese.
The Dromornithidae did not much resemble their distant cousins. Known also as giant mihirungs they were massive creatures, tipping the scales at as much as 650 kilograms, standing taller than the average human, and with huge bulbous beaks.
Research published in the journal Royal Society Open Science and led by Trevor Worthy of Flinders University in South Australia has established that the birds are members of the large and widespread fowl group, and not closely related to another group of giant flightless birds, the ratites, which includes the ostrich, emu and extinct New Zealand moa.
They are, however, quite closely tied to another family of massive flightless birds, the Gastornithidae, species from which once lived across a wide range of habitats in Europe and North America.
Australia’s mega-chickens first occur in the fossil record over 50 million years ago. This raises the question of how two related groups of birds could establish themselves at opposite ends of the globe, when neither could fly.
The team suggests a type of separated evolution, in which both groups descend from a common, much smaller flight-capable species.
The known diet of the giants provides a clue.
“Mihirungs were herbivores, just like typical ducks and geese,” says co-author Mike Lee. “Despite a five-hundred-fold increase in body size, they retained the diets of their much smaller ancestors.”
The suggestion that gigantism and flightlessness among the fowl evolved independently at least twice is not without parallel – appropriately enough, from the ratites.
Extensive research published in 2014 and led by ornithologist Allan Baker of the Royal Ontario Museum in Canada compared genetic and morphological data for the ostrich and all its relatives – which include a species called the tinamou (Eudromia elegans), a small bird perfectly capable of flying.
Baker and his colleagues concluded that a distant common ancestor had by means of flight settled in different parts of the world and had then evolved in many cases into flightless species such as the ostrich, emu, cassowary, rhea and moa.
“At the base of the family tree of the giant flightless ratites on each continent we know there was a small flying bird like the tinamou,” observes Worthy.
His team’s research also ruled out some other extinct species of birds from membership to the fowl group. A species known as vegavis, known from fossils found in Antarctica and thought to be part of the duck lineage was discovered to be much older in origin.
Surprisingly, the largest extinct flightless bird ever discovered in South America – the 300 kilogram Brontornis – was found to be strongly excluded from the chicken line, and related to a much more ancient group, the Phorusrhacids.
Andrew Masterson is a former editor of Cosmos.
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