Perhaps no bird so effectively displays its connection to dinosaurs as the cassowary. These large, flightless, and well-armed birds stalk the rainforests of New Guinea and northern Australia, earning the nickname "prehistoric murder chickens" and avoiding a lot of the scientific study that comes with more accessible locations.
Flinders University PhD student Phoebe McInerney conducted a study of one of the cassowary's lesser-know parts, its throat, and has tipped the balance in a debate about these mighty birds' wider family tree.
McInerney conducted a 3D image of the cassowary's throat using CT scans. Getting a bird that large and obstreperous to stay still would have been challenging, but McInerney told IFLScience she was provided with the remains of an individual hit by a car – a common fate in Far North Queensland.
The cassowary's syrinx, hyoid, and larynx are described in great detail in BMC Evolutionary Biology, but it was when McInerney and co-authors sought to broaden their studies that things got really interesting.
Visually, cassowaries are the closest survivors to New Zealand's extinct moa, which until their extinction 600 years ago were the largest living birds. On the other hand, genetic analysis suggested the moa were most closely related to tinamou, birds that at first appear utterly different. These inhabitants of Central and South America are modest in size and can fly, even if they don't do it all that often.
“The molecular data still has some issues but it is relatively reliable," said McInerney. Nevertheless, some traditional biologists had trouble accepting the result, noting, in McInerney's words, that “there was nothing similar between” moa and tinamou.
McInerney and colleagues decided to compare their cassowary results with scans of moa neckparts, since the throat bones can be found in many museum collections. The study revealed substantial differences, but when McInerney extended the comparison to tinamou, she found a much closer match.
The ancestor of all these birds was presumably flighted, explaining the journeys between Australia, New Zealand, and South America. In the first two, it found a niche for large, flightless birds and settled in, with almost everything about the ancestral bird changing other than the shape of its inner throat.
The finding vindicates the genetic analysis in this case, and arguably bolsters it for other families where molecular resemblances conflict with similarities in animals' shapes. The work leaves open the question of whether or not it is a coincidence that when given the opportunity these flighted birds evolved multiple times into fairly similar shapes.
“There seems to have been something about this group that they grew large and flightless everywhere they settled,” said McInerney. Remote branches of the bird family tree, such as terror birds and Australia's Bullockornis (the duck of doom) grew to similar sizes but did so far less often.