Many birds have adapted to lose their ability to fly, but one group in particular sticks out. The ratites, or more accurately named the paleonaths, containing the largest living birds like ostriches, emus and cassowaries, sometimes weighing similar to or more than a human. They stand out because of their size, but also because of their primitive features like strong powerful legs and claws and often being covered in shaggy downy feathers instead of long flight feathers.
In fact, they are very similar looking to some herbivorous species of dinosaurs that lived during the Cretaceous period, like Ornithomimus and the Oviraptorids. Ornithomimus in particular having a very similar skeleton to a modern-day ostrich. But also, whereas many flightless birds evolved that way due to being isolated on an island void of predators, many of the ratites are flightless brutes that compete directly with many of the other large animals they share their ecosystem with. being known to kill animals as largest cheetahs.
In fact, the Palaeanaths are not just the largest birds, but some of the last living large land animals that aren't mammals. Their prehistoric features and large size may make them incredibly interesting animals, but the Rattites also have a very mysterious evolutionary history. They are closely related flightless birds, yet today, they are spread across most of the southern continents and islands. So how did they get there? Rattites get their name from the Latin word ratis, which means raft.
The vast majority of birds have a large piece of bone that protrudes from their sternum called a keel that is used to anchor their enormously powerful flight muscles to. Because Rattites have evolved away from being flightless for many millions of years, they have lost their keel, which is why they are known as rafts in Latin, because they don't have a keel. The Palaeanath group contains ostriches, emus, cassowaries, the rhea and the tinnimose that live in South America, and also the kiwi. And up until relatively recently, New Zealand was home to the giant Palaeanath moa that had massive 2-metre legs that made its head tower over a modern-day ostrich.
And the moa wasn't even the largest of these birds, because in Madagascar lived the monstrous elephant bird, weighing as much as the largest brown bears, making them the largest bird known to have lived. only going extinct around a thousand years ago. With the exception of the tinnimose, that can be found as far north as southern Mexico, all of these birds are only spread across the southern continents.
And there are even fossils of an ancient ratite known from Antarctica that would have lived there around 40 million years ago when it wasn't so cold. So being flightless, how did they reach these far off areas? In the past, it was thought that they may have all descended from one common ancestor, that lived on an ancient supercontinent comprised of the southern landmasses.
About 190 million years ago, in the early Jurassic period, the giant supercontinent Pangaea started to break apart. But when this happened, it didn't immediately break into separate continents, and instead the southern continents Africa, South America, Australia, and Antarctica remained connected together for some time, in a smaller supercontinent named Gondwana. Then when this small supercontinent broke apart around 100 million years ago in the Cretaceous, the modern continent started to take shape. So it was thought that all of the ratites may have descended from a flightless common ancestor on Gondwana, and then, when the continent split up, evolved differently, isolated on different continents.
Which would explain why a family of flightless birds have such a large spread around the southern parts of the globe. This seems to be a perfect explanation, however, it is now known that this can't be true. as the dates don't add up.
The Palaeanaths are one of the four distinct lineages of birds that survived the mass extinction that killed the non-avian dinosaurs. However, although the Palaeanaths can trace their origins back to before the dinosaurs went extinct, at the earliest they evolved in the very late Cretaceous period when Gondwana had long since broken up. Furthermore, DNA evidence shows the relationship between these birds to be very different to what you would expect if they all came from a large supercontinent that used to exist. For instance, the tiny kiwi is actually more closely related to the giant elephant bird in Madagascar than it is to any other ratite, including the cassowarian emu, despite living much closer, and even the moa that the kiwi used to share an ecosystem with.
So how did the paleonaths get to where they are today? It turns out that the ratites, or the paleonaths, may look very primitive, but they are actually not. The oldest paleonasts in the fossil record were a family of birds called the lithornithids.
They have some controversial fossils dating before the extinction of the dinosaurs, but they at the very least have existed from very soon after the dinosaurs went extinct, around 60 to 65 million years ago. The lithornophids were not flightless nor resembled modern ratites in any meaningful way. They had small legs and a keel that would have been developed enough for them to fly, so they would have fit in among the vast majority of birds living today. In fact, they actually looked more like pigeons or pheasants than an emu or an ostrich, which means that the Palaeanaths used to look a lot more like other birds, and then evolved most if not all of their primitive features.
So there may have been dinosaurs like Ornithomimus that looked similar to Rattites, but there is no relation, and the resemblance is just due to convergent evolution. So the Palaeanaths were still able to fly until much more recently than previously thought, and actually one species named the Tynemos, found in Central and South America, still can fly, although not very well. Which is why the name Palaeanaths is a more accurate word to describe these birds than Rattites, because at least one of them still has a keel. Because the Palaeanaths were able to fly until a lot later, this means they would have been able to fly to the different southern continents and then just evolved to lose their flight once there.
And there is some DNA evidence that supports this. But even better, in New Zealand the fossils of an ancient Kiwi have been discovered called Proapterix that was most likely able to fly, but co-existed with the Moa that was already flightless at this point, proving that they must have evolved flightlessness at different times. And this would make sense because unlike most other paleonoths, the moas had been flightless for so long they had evolved to lose their wing bones altogether.
However, this theory relies on an astounding amount of convergent evolution happening to the same group of animals. What are the chances that a normal bird-like ancestor would spread and settle on various continents and landmasses around the world, and at least twice in New Zealand? And then all but one of them, Tynemoths, would separately lose their keel, evolve flightlessness, and then many of them adapt independently into many of the largest birds that have ever existed.
Well, this can be possible, because sometimes animals can have a predisposition to evolve in a certain way because of their anatomy. For instance, silk production has evolved on as many as 10 times independently among arthropods, animals like insects and spiders, but vertebrates have never evolved the ability, and it may have been the same with the ratites. The common ancestor of the Rattites may have had certain features that made the evolutionary pathway of losing flight considerably more likely for these ancestors.
In order to fly, animals have to be incredibly specialized, and so it comes with a lot of trade-offs. It takes a lot of energy, which means animals that can fly, like birds and bats, have to have incredibly lightweight bodies to do it efficiently. And flying animals are burdened with a part of their body, their wings, that are absolutely crucial for their survival. making them more vulnerable to injury.
Flying birds have to be good enough at flying for all of these disadvantages to pay off, which means that animals that are clumsy at flying will likely lose the ability altogether pretty quickly, because they need to adapt to different methods of survival that will almost always come at a detriment to their flying ability. But why did the Rattites largely only settle in the southern hemisphere if they were able to fly anywhere? Well, this is a bit of a mystery, but it isn't just the case with Rattites. Nearly all of the large land animals living today are mammals, and while this has been the case for a long time in the northern continents, up until relatively recently this wasn't the case with places like South America and Australia that have a fossil record filled with strange animals by today's standards, like terrestrial crocodiles and other species of giant birds, including ones that filled predatory niches as well.
So in the past, places like South America and Australia seem to have been much more hospitable for non-mammalian animals to gain a foothold. So the Rattites are unusual and unique animals, but it turns out their unique body shapes and place in their ecosystem has actually evolved multiple times. Thank you for watching.
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