Read more: Living dinosaurs: How birds took over the world
It had wings and feathers, but also the teeth, legs, claws and tail of a dinosaur 鈥� what kind of beast was archaeopteryx?
Archaeopteryx has always been considered to be the most primitive as well as the most ancient bird. Yet its strange mix of traits 鈥� the teeth, legs, claws and tail of a dinosaur but the wings and feathers of a bird 鈥� continues to raise doubts about its true affinities.
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Recent discoveries have only added to the enigma. It grew very slowly compared with modern birds. A 2009 study of growth patterns in its bones reveal that an adult would have taken over two-and-a-half years to mature, at least three times as long as a modern bird. This slow growth indicates a slow metabolism and suggests that archaeopteryx may have been sluggish compared with today鈥檚 active and warm-blooded birds.
鈥淎rchaeopteryx was much more like a dinosaur,鈥� says lead author Greg Erickson of Florida State University in Tallahassee (). His conclusion is that archaeopteryx should be regarded as a feathered dinosaur capable of flight.
That is not to say its flying skills were up to much. Archaeopteryx may not have been capable of flapping flight at all, according to a recent study of its feather strength. 鈥淭hey were not strong enough to withstand the forces generated during flight: they would have broken,鈥� says Robert Nudds of the University of Manchester, UK, who suggests that it might have been a glider instead (). These findings have been disputed by other palaeontologists but all sides agree that its flying ability was weak compared with modern birds.
However, archaeopteryx did have the large brain and excellent sight of a flying bird, according to a team who used computed tomography to scan the interior of its skull. A bird鈥檚 brain fits very tightly into the skull so a scan will provide a clear impression of its shape and structure. The shape and volume of the brain and inner ear were almost identical to those of modern birds. 鈥淚t was a flight-ready brain,鈥� says co-author Angela Milner of the Natural History Museum in London, home of the first archaeopteryx skeleton ever found ().
Most palaeontologists still regard archaeopteryx as the earliest known bird, but there is far less certainty about its taxonomic position than there used to be. Some now argue that it should be expelled from the Avialan group (modern and extinct birds) and placed closer to the dromaeosaurs and troodontids (see chart).FIG-mg27901201.jpg
The real question is, where do you draw the line between dinosaurs and birds? Ask different palaeontologists and you will get subtly different answers. That is because the distinction is basically arbitrary, says Xing Xu of the Institute of Vertebrate Paleontology and Paleoanthropology in Beijing, China, who discovered many of the Chinese fossils.
We now know that from the mid-Jurassic onwards, the world was full of feathered theropods, some more bird-like than others. One group gave rise to modern birds, but the magic moment when dinosaurs became birds is impossible to pin down.
Even so, there is hope of discovering bird fossils older than archaeopteryx. Xu thinks that birds split from the other dinosaurs between 175 and 161 million years ago, and is now hunting for fossils in rocks of this age.
It is even possible that we already have such fossils. Xu argues that two species of miniature feathered dinosaur discovered recently in 161-million-year-old rocks in Mongolia 鈥� Epidexipteryx and Epidendrosaurus, together known as the Scansoriopterygids 鈥� belong in the Avialans. If so, they usurp archaeopteryx as the earliest-known bird.
Whatever happens, archaeopteryx will remain a pivotal fossil. 鈥淎rchaeopteryx will always be the benchmark for discussing bird evolution,鈥� says Erickson.
