The eleven specimens of Archaeopteryx are some of the most iconic and captivating fossils in existence. The fingers end in claws, the tail is long and bony, and the head – arched back in the throes of death – contains toothed jaws. But the splayed arms are lined with the faint but unmistakeable outlines of feathers. This was an animal halfway between a small flesh-eating dinosaur and a modern bird. In fact, Archaeopteryx is widely heralded as the first bird, occupying a pivotal position in the origins of this group.
But Xing Xu from Linyi University thinks that this first bird was nothing of the sort. The Chinese palaeontologist, who has found one fascinating dinosaur after another, has identified a new species called Xiaotingia that threatens to oust Archaeopteryx from its position.
By comparing Xiaotingia's features with those of Archaeopteryx and other related birds and dinosaurs, Xu has drawn up a new family tree. In it, Archaeopteryx sits with Xiaotingia among the deinonychosaurs, a celebrity-filled group of small, predatory dinosaurs that includes Deinonychus and Velociraptor. The lineage that led to modern birds perches on a different branch of the tree.
This doesn't change the fact that birds evolved from dinosaurs – it merely relegates Archaeopteryx to the sidelines of that process. In its place, species like Epidexipteryx and Epidendrosaurus take up the mantle of earliest birds. It is a tentative revision but a bold one (Xu himself admits that the new family tree is statistically weak). "It's been a good run for Archaeopteryx," writes Larry Witmer in a related editorial. "This finding is likely to be met with considerable controversy (if not outright horror)."
Xiaotingia lived in China during the late Jurassic period, and was about the size of a pigeon. Until recently, its beautifully preserved fossil sat innocuously in the Shandong Tianyu Musuem of Nature, among a thousand-strong collection of feathered dinosaur skeletons. Xu noticed it when he visited the museum. "I immediately recognized that it was something new, but honestly did not expect that the discovery would change the dinosaur-bird family tree."
Xiaotingia's presence is crucial. When Xu re-ran his analysis with exactly the same species except for his new discovery, Archaeopteryx was restored to its original position as a proto-bird. This new specimen has made a big difference.
However, it's not the only fossil to have called Archaeopteryx's status into question. In recent years, fossil hunters have unearthed several species that are either primitive birds (or close relatives of them) that look very different to Archaeopteryx, including Epidexipteryx, Jeholornis, Sapenornis.
Meanwhile, others have noted that Archaeopteryx has many features that are only found among the deinonychosaurs. For example, they share a distinctive hip bone, and they both have a large hole above their noses (the "premaxillary fenestra") that other birds and dinosaurs lack. Any many of the features that supposedly characterise Archaeopteryx and other birds, such as feathers, a wishbone and long powerful forearms, are also found in deinonychosaurs.
This is more than just a matter of shuffling cards. Archaeopteryx's position has been so sacrosanct that its body had guided many of our ideas about the origins of birds. It grounds our understanding of this group. For example, it was previously thought that most primitive birds and their closest dinosaur relatives had lightly built skulls, which might have been useful for flight. "Instead, our study suggests that primitive birds had robust and rigid skulls," says Xu.
"Our study also suggests that most primitive birds were herbivorous animals," he says. "Previous studies which suggested that flight evolved in the [meat-eating] context (such as wings evolving for catching prey) need reconsideration." Witmer writes, "Clearly, without the safety net of good old Archaeopteryx at the base of the birds, we've got some fresh work to do."
But Gerald Mayr, who studies fossil birds at Germany's Sneckenburg Museum, is unimpressed with the new discovery. "I fear that it is a bit hyped and that the conclusions are not as novel as the authors claim," he says. Mayr is one of several palaeontologists who think that the deinonychosaurs are actually birds themselves. According to him, they're flightless members of a group that includes Archaeopteryx and modern birds, like smaller extinct versions of today's ostriches and emus.
The problem is that all of these reconstructions are weak. Mayr admitted as much about his own model back in 2006, and Xu says that his new family tree only has "tentative statistical support".
Creationists will doubtlessly pounce upon this story and quote-mine articles for supposedly damning phrases. But revisions and uncertainties like this are to be expected. As with all big evolutionary shifts, there wasn't a simple linear route from dinosaur to bird. Instead, animals at the time developed a whole range of different body shapes that were eventually tested and winnowed by natural selection. "There were lots of experimental trials", says Xu.
The fossil record is full of these failed experiments and evolutionary dead-ends, all preserved alongside the success stories that eventually gave rise to modern species. This makes it very difficult to construct a robust family tree. Witmer sums it up brilliantly in the conclusion of his editorial:
"Just as Xiaotingia moved Archaeopteryx out of the birds, the next find could move it back in - or to somewhere else within this fuzzy tangled knot that makes up the origins of birds and bird-like dinosaurs. That said, during this sesquicentennial anniversary of Archaeopteryx, which is being honoured with exhibits and commemorative coins, the bitter irony may be that it may not have been the bird we've always thought it was. But Archaeopteryx will remain an icon of evolution, perhaps even more so now, providing compelling evidence that, as we should expect, evolutionary origins are rather messy affairs."
Reference: Xu, You, Du & Han. 2011. An Archaeopteryx-like theropod from China and the origin of Avialae. Nature http://dx.doi.org/10.1038/nature…
This post originally appeared on Not Exactly Rocket Science.