Ostriches, emus, rheas and other large, flightless birds are found on six landmasses separated by oceans, but how they reached such far-apart places without the ability to fly has remained an enduring mystery.
One idea was that the ancestors of this group of birds, known as paleognaths, just walked to those locations when most of the planet was harnessed together as the supercontinent Pangaea (320 million to 195 million years ago) and that, when this giant landmass split up, the birds were already in those locations.
To work out what happened, Klara Widrig, a vertebrate zoologist at the Smithsonian National Museum of Natural History in Washington, D.C., and her colleagues analyzed a specimen of the ancient paleognath Lithornis promiscuous. Although it lived around 59 to 56 million years ago, it is the oldest fossil palaeognath found in such pristine condition.
"We can't tell for sure if Lithornis was the direct ancestor of our living paleognaths — it is entirely possible that the true ancestor is yet to be discovered — but it represents our best guess as to what the ancestor would have looked like," Widrig told Live Science.
Previous investigation of preserved feathers of a slightly more distantly related lithornithid called Calciavis grandei indicated that it could have flown, but it wasn't clear how far. No one had done a quantitative analysis of the shape of lithornithid bones to try to answer that question.
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So, in the new study, published Wednesday (Sept. 17) in the journal Biology Letters, Widrig and her colleagues compared the shape of the sternum, or breastbone, of L. promiscuous to those of living birds and used a three-dimensional geometric dataset to work out how well the animal could have flown.
"The sternum is very important for flight because that's where the big pectoral flight muscles anchor," Widrig said.
The shape of the sternum indicated it could have handled a range of aerobic, flapping flight styles, which would have enabled lengthy flights.
"We found that the shape of the breastbone was really similar to that of living birds that are capable of flying very long distances across oceans, like great egrets and herons," Widrig said.
"This is very interesting because the great egret is a cosmopolitan species in that it travels from continent to continent," said Peter Hosner, curator of birds at the Natural History Museum of Denmark, who wasn't involved in the work.
"Such species are actually quite rare in birds," he told Live Science. "We get biased in the Northern Hemisphere, where many birds are migratory and cover long distances. But globally, most birds are residents found in one continent, island or small area, and don't really move that much."
The finding suggests that ancient paleognaths may have flown to the distant landmasses and established populations that later independently evolved into the large and generally flightless birds we know today.
"It seems to be a spectacular case of convergent evolution," Hosner said.
Today, there are about 60 species of living paleognaths. They include about 45 species of tinamou (which can fly in short bursts much like pheasants do), up to five species of kiwi, one species of emu, three species of cassowary, two species of ostrich, and either one or two species of rhea, Widrig said.
"In order for a bird to become flightless, two conditions have to be met," she said. "It has to be able to get all of its food on the ground, so it can't be relying on food that's up in trees, for example. And there can't be any predators that it would need flight to escape from."
In more recent times, that would have happened only in predatorless island environments, Widrig said, such as with the dodo (Raphus cucullatus). But after the Cretaceous-Paleogene extinction event some 66 million years ago wiped out the nonavian dinosaurs, it was very different.
"The world was cleared of predators generally, and mammalian predators had not evolved yet — so any ground-feeding bird had a free pass essentially to become flightless," Widrig said. "Flight is hard work, and it's a lot easier to be flightless if you don't have to get away from anything.
When bigger predators did emerge, she said, the flightless birds would have had time to adapt either by becoming big and intimidating, like the cassowary, or by becoming swift runners, like the ostrich.
But all these similar changes evolved independently. "It's not as if they got on a conference call with each other and said, 'Okay, you go to Africa and you're going to evolve into an ostrich. I'm gonna go to South America. I'm gonna evolve into a rhea,'" Widrig said.
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