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Every day I'm shuffling: Xiaotingia & Aurornis There are many dinosaur groups whose interrelationships are uncertain, but none are as confused as the early birds and their relatives. For a long time, things seemed pretty simple: Archaeopteryx was a primitive bird (or, more properly, avialan). Dromaeosaurids and troodontids formed Deinonychosauria, which, along with the birds, formed Eumaniraptora ("true maniraptorans"); together with some more primitive forms, eumaniraptorans formed Paraves ("close to birds"). However, this would soon change. In 2011, a new paravian from Late Jurassic China was named: Xiaotingia ("of Zheng Xiaoting"). The study that introduced this new theropod found a new arrangement of the paravian family tree that was quite different from the previous consensus: not only did Xiaotingia and the supposed troodontid Anchiornis end up closely related to Archaeopteryx, but these forms were found to be primitive deinonychosaurs rather than birds! Predictably, popular media balleyhooed these new findings a lot. However, the authors of this study noted that support for this position was not terribly strong and was subject to change. And change it did. For the next year or so, these three generally bounced around between being primitive birds, primitive deinonychosaurs and primitive paravians. However, in early 2013, a new phylogeny arose. This new analysis found that Deinonychosauria was not a natural group; instead, troodontids were primitive paravians, while the narrow-snouted unenlagiines and four-winged microraptorians — previously regarded as primitive dromaeosaurids — were found to be birds even more primitive than Archaeopteryx, Anchiornis & Xiaotingia. Unfortunately, this analysis was plagued with multiple problems (most notably, the bird group was renamed "Averaptora" ["bird thieves"], which is redundant and best ignored), so its conclusions have not received much support. Fortunately, however, a new paravian from Late Jurassic China was announced a few short months later. Called Aurornis ("dawn bird"), the paper introducing this new form featured yet another new analysis — this one being bigger and better than any before. This one found that the bizarre scansoriopterygids, the supposed troodontid Eosinopteryx, dromaeosaurids (as in most analyses, unenlagiines and microraptorians were placed in the group), troodontids, Aurornis, Anchiornis, Archaeopteryx, Xiaotingia and a new group including Jeholornis (see below) and the Madagascan Rahonavis were progressively closer to modern birds. A later modification of the same analysis found Eosinopteryx to be closely related to Aurornis and scansoriopterygids to be closer to modern birds than Xiaotingia. In conclusion, it is unlikely that paravian relationships will be resolved anytime soon. Birds from ancient China: Confuciusornis & Jeholornis Confuciusornis lived in Early Cretaceous and was from the same famous Chinese Liaoning site in which the popular feathered dinosaur fossils come from. This animal had some evolved traits, for example had already lost its teeth (convergently from modern birds) and shortened its tail, but still retained an old legacy: three-clawed wings. As is easy to think from a Liaoning animal, the Confucius-bird has also preserved prints of feathers, which show two very elongated tail-feathers rather like peafowl. Zhongornis, originally thought to be a transitional form between long-tailed and short-tailed birds, may be a juvenile of a bird closely related to Confuciusornis. A long-tailed bird, Jeholornis from the same age and habitat, is also known as "Shenzhouraptor". Another bird from Early Cretaceous China, Sapeornis, has an interesting Science Marches On story. It was once thought to form a group (traditionally called Sapeornithidae, but properly Omnivoropterygidae) with three other birds: Omnivoropteryx, Didactylornis & Shenshiornis. However, further work suggests that these four birds were really just one all along; Sapeornis, as the oldest of these names, is the proper one. Yet another prehistoric Chinese bird, Gansus, was described in 1984, well before the other birds mentioned in this section. This form shows similarities to various groups of living birds (and it does indeed appear to be related to them), but it is most likely a diving bird also capable of flight. It is known from much of a skeleton, but the skull is currently unknown. As with several other feathered creatures from prehistoric China, the color of its feathers has been determined; it had dark-colored plumage. The Mirror Universe birds: Enantiornithines The most successful Late Cretaceous birds were the enantiornithes, whose name means "opposite birds". Why? Some skeletal features of the chest and feet are the exact opposite of those seen in modern fliers. They were a sort of middle-ways between the aforementioned Early Cretaceous birds and modern feathered guys, and were very diverse (predatory forms, aquatic forms, mud-probing forms, flightless forms, you name it). The Zerg Rush birds in the Walking with Dinosaurs episode about pterosaurs were enantiorns, as well as, arguably, those mentioned in the last episode which made the "omnipresent chorus" from the trees. One enantiorn (Avisaurus, "bird-lizard") was originally classfied as a birdlike dinosaur. The Mirror Universe birds went extinct along with non-avian dinos only after the comet/asteroid. The never-never birds: Pterosaurs mistaken for early birds In paleontology, it is very common for isolated bones to be misinterpreted as something very different from what they actually are. One of the best examples of this is the alleged bird Samrukia, from the Late Cretaceous of Kazakhstan. In 2011, Darren Naish of Tetrapod Zoology fame and his colleagues described a large jawbone, which they thought came from a large bird that they named Samrukia ("phoenix"). Later the same year, another paleontologist showed that this jaw did not come from a bird at all, but instead from a pterosaur. History repeated itself about a year later with the reexamination of three other Cretaceous birds. In 1986, two birds were named from the Early Cretaceous of Romania. The first of these, Palaeocursornis ("ancient running bird"), was known only from a leg that showed it to be a flightless runner; the second, Eurolimnornis ("European water bird"), was a conventional flyer. In 2002, another bird was named from Late Cretaceous rocks of the United States. Called Piksi ("big bird"), it appeared to be a fowl-like ground bird. In the latest part of 2012, these birds were shown to be pterosaurs instead, and the leg of Palaeocursornis was shown to actually be an arm. Ironically, all of these forms, originally used to support the theory that Mesozoic birds were surprisingly diverse, now show that pterosaurs truly were the dominant flyers of the time. Experiments in flightlessness: Patagopteryx & Balaur Many birds today, from the ratites to the penguins to the extinct dodo, have lost the power of flight for various reasons, and it was much the same in the Mesozoic. We will investigate two flightless Mesozoic birds in this section, starting with Patagopteryx ("Patagonian wing")., the oldest-known flightless bird. As the name implies, this Early Cretaceous hen-sized bird was found in South America. When it was named in 1992, it was assumed to be related to the modern palaeognaths (ostriches & kin), but several primitive features suggest it was instead part of a uniquely Mesozoic radiation. It may be closely related to the similarly flightless Gargantuavis from the Late Cretaceous of France, the largest Mesozoic bird. The other flightless bird discussed in this section is so completely specialized for a terrestrial life that it was not originally recognized as a bird. Named Balaur in 2010 after a mythical dragon, this cat-sized creature (which lived on the Late Cretaceous European archipelago) had a long tail and two retractable claws on each foot. As the claws superficially resembled those of the deinonychosaurs, it was assumed that it was a dromaeosaurid. However, detailed study reveals a suite of uniquely avian characters, and it is best interpreted as a late-surviving primitive bird. However, one Mesozoic lineage of flightless birds became far stranger than either Patagopteryx or Balaur; this group is discussed in the next section. The first full-birdies: Prehistoric neornithines Neornithes (meaning new birds) or colloquially "Neorns", is the name indicating the last common ancestor of all modern birds and all its descendents. Neornithes were the only Cretaceous birds which managed to overcome the mass-extinction and to make their way in the Cenozoic, the Mammal Age. It's worth noting that their descendants, our modern birdies, have much, much more species today than mammals. Most Cenozoic "new birds" were very similar to their descendants: some were rather generic-looking, while others were more specialized, but still not too different to modern avians. Furthermore, their fossil record is not as good as that of mammals; thus, evolution of the single modern-bird lineages is mostly unknown even today, and their phylogenetic tree is full of question marks. But don't worry...there were also many exceptions to this rule: we're going to talk about these.
The Magnificent Mihirungs: Dromornis & Genyornis Dromornithids were among the largest birds that ever lived (they varied in size from about as big as a cassowary to the largest and Trope Namer of the group, Dromornis stirtoni, 3 meters tall and half a ton in weight); and yet, they've not gained much consideration in popular media, unlike their American contemporary counterparts, the phorusrhacids. It's probably because they likely weren't, fast, vicious killers. Instead, the 'thunderbirds', with their vast bulk, thick, robust bones, hoof-like toes and strong, crushing beaks were browsing and grazing herbivores, slowly plodding across a wetter, more wooded ancient Australian outback. Typical of Australian things, they've been given many nicknames: "thunderbirds" obviously refers to their huge bulk and robust bones; "demon ducks of doom" refers to their closest living relative being the Australian magpie goose, and other waterfowl, and an old debate as to whether they were carnivores. Another addition to the list is "Mihirung", from an Aboriginal story that might mention them as the "mihirung paringmal" or Giant Emu: it is a certainty that the first people to arrive in Australia encountered them, and drove them to extinction (come on, what else wipes out species on a continent with relatively consistent climate and before the end of the ice age, eliminating climate as a variable?). Deadly feast: Teratornis We leave (almost) definitively the flighless bird's world and start to watch more traditional fliers. Among prehistoric flying birds, among the most commonly depicted (and most striking) are the teratorns, which were were very vulture-like animals. The namesake Teratornis is one of the most abundant birds in fossil record, and has been found in huge numbers in the famous Californian tar-pits in which mammalian sabretooths, giant wolves, mastodons and ground sloths have also been found. Arguably, they went to feed on the carcasses of these mammals, and remained stuck in tar just the same. A feathered airplane: Argentavis The aforementioned Teratornis had an earlier relative, which lived in South America 8 million years before: Argentavis (its name means "argentinian bird"). Why should we mention it separately? Well... simply because, along with giant pterosaurs, it deserves the Giant Flyer title more than every other prehistoric creature. Its wingspan was 25 ft, as much as Pteranodon; its weight 80 kg, as much as the two-times-wider-winged Quetzalcoatlus. Imagine a giant condor with a ostrich-sized body, huge roc-like wings, a sharp uncinated beak, and a love for carrion (and maybe even an occasional hunting attitude). With no doubt, the largest flying bird ever discovered (with the possible exception of Pelagornis sandersi, see below). And yet, Argentavis has yet to appear in fiction. And it has actually had one single apparition in documentaries to date: "Paleoworld", as a side-note of Phorusrhacids! Since Walking With Beasts producers did recreate Argentavis world (the Sabretooth episode)… they wasted a perfectly good opportunity. Cuban giant owl: Ornimegalonyx During the Pleistocene, Cuba was ruled by a large ground-dwelling owl known as Ornimegalonyx. Take the modern burrowing owl, scale it up to a meter high, and you have a good idea of what this bird looked like. The largest owl alive today, the Eurasian eagle owl, has been known to hunt prey as large as roe deer fawns, meaning that if this owl wanted to, it could possibly prey on a human being. Indeed, its prey is believed to have consisted of ground sloths. Granted, the Cuban ground sloths were quite small compared to their inland cousins due to island dwarfism, but species like Megalocnus could still weight up to 200 pounds, so that would still be an impressive kill. Sources are quite inconsistent whether this owl could fly or not; it's possible that, like the modern secretary bird, Ornimegalonyx only flew when necessary, especially since, with those long legs, it could run extremely quickly.
Toothy bird 3: Osteodontornis and Pelagornis sandersi However, Argentavis wasn't the only Giant Flyer in the Cenozoic: we have to add the Pelagorns. These were rather albatross-like or pelican-like marine birds, but they were actually more closely related to fowl like chicken & ducks. They had two cool traits: their wingspan reached 20 ft (a bit less than Argentavis) and their beak was toothed, seemingly revealing the trope Toothy Bird being a Real Life thing in the past. Sadly, this is not true: these "teeth" weren't real teeth, but their bill had an ondulating, pseudo-toothed edge, just like one modern bird, the duck-like Merganser. The only Real Life toothy-birds were those living alongside non-avian dinosaurs, such as Archaeopteryx, Hesperornis, and Ichthyornis (which weren't even closely related to any modern bird groups). Pelagorns were the new feathered version of Pteranodon, almost as large as it, and went extinct only 1 million years ago. Osteodontornis is the typical member of the group, though Pelagornis sandersi, discovered in 2014, is notable for possibly having a larger wingspan than the aforementioned Argentavis. (Though it wouldn't have weighed as much, so when it comes to weight, Argentavis still holds the title of largest flying bird.) A tale of two seabirds: Anthropornis & Copepteryx When hesperornithines went eventually extinct at the end of the Mesozoic, early representatives of modern seabird groups took their place. Among the largest of them, the giant penguin Anthropornis, was nearly as tall as a fully-grown human and weighed 200 kg, more than a modern ostrich; but it probably resembled modern penguins. A similarly-sized bird, Copepteryx, also resembled a giant penguin, but unlike Anthropornis, it was completely unrelated to them; details of its bones show that it was instead a closer ally of pelicans. Giant penguins swam in the southern seas (and Copepteryx in the north) for millions of years, until they were both outcompeted 20 million years ago by new groups of large marine animals, their mammalian equivalents: seals, sea lions, dolphins and porpoises.