When scientists discovered an unexpected link between New Zealand's extinct moa and a small South American bird in 2014, many biologists remained sceptical.
The moa were huge, couldn't fly and native to New Zealand, while the tinamou are small, live on the other side of the world and can fly. But DNA testing showed tinamou are the moa's closest living relative, not more obvious species like the kiwi or emu.
Now another link between the unlikely cousins have been found - they have very similar breathing, eating and vocalising structures, hidden deep in their throats and recently revealed thanks to new 3D scanning technology.
"Scanning lets us see details that we wouldn't be able to otherwise, including the shapes of internal structures, without causing damage to them," said study leader PhD student Phoebe McInerney of Flinders University in Adelaide.
She led a team looking at similarities between not just moa and tinamou, but Australia's own cassowary - another large flightless bird. They weren't surprised to find cassowary and emu had similar throats, but were surprised at how close the moa and tinamou were.
The study, published in journal BMC Evolutionary Biology, said "morphological support for the molecular evidence uniting these two taxa has previously remained elusive" - in other words, although DNA testing said they were close relatives, it was difficult to see how, based on their appearance.
"The morphology of the often-neglected larynx has shown to be far superior than the other anatomical traits biologists previously used to infer evolutionary relationships," said study co-author Trevor Worthy.
"The unexpected family tree for primitive birds based on genomic evidence is looking more and more convincing," added study co-author Michael Lee.
Despite its imposing reputation, the cassowary is more closely related to the kiwi than the moa, it turns out. Kiwis' closest ancestor is the extinct elephant bird from Madagascar DNA testing, also in 2014, has found.
The moa, cassowary, kiwi and emu all evolved from the same ancestor as the tinamou, the researchers said, independently losing the ability to fly over time.
