Courtesy Mark RyanWhen the first Archaeopteryx skeleton came to light in 1861 just two years after the publication of Charles Darwin’s On the Origin of Species, it was hailed as not only the earliest known bird but also as proof of evolution. The rare specimen (known as the London specimen) appeared to be somewhat of a chimera of both dinosaurian and bird features. Thomas Huxley, a fierce proponent of Darwin’s work, even suggested that birds were the descendents of small meat-eating dinosaurs. It took more than a century for the idea to become widely accepted. Today most paleontologists divide dinosaurs into two groups: avian dinosaurs (birds) and non-avian dinosaurs, the branch that died out at the end of the Cretaceous.
"For a long time, Archaeopteryx was considered the archetypical bird primarily because it had feathers, although it retained typical dinosaur features like a long tail and teeth,” said Mark Norell, Chair of the Division of Paleontology at the American Museum of Natural History. “But the discovery of classical bird features like feathers and wishbones have recently been found in many non-avian dinosaurs blurring the line of what constitutes a bird."
Now, a new study by Norell and co-author Gregory Erickson, a vertebrate paleontologist at Florida State University, shows that 150 million year-old Archaeopteryx (Greek for “ancient wing”) may have been much more like a non-avian dinosaur than a bird (avian dinosaur), at least in its bone structure and growth rate.
"Dinosaurs had a very different metabolism from today's birds,” Erickson said. “It would take years for individuals to mature, and we found evidence for this same pattern in Archaeopteryx and its closest relatives.”
For their research, the team removed tiny, 250-micron slivers of bone from the remains of several Archaeopteryx fossils, and from the bones of primitive but younger bird fossils (Jeholornis prima and Sapeornis chaochengensi, and Confuciornis sanctus) found recently in China. They also examined the bone structure of closely related non-avian dinosaurs such as Velociraptor mongoliensis.
Those samples from the Archaeopteryxes showed high bone-density, annual growth lines (a reptilian trait), and very small blood vessels similar to the bones sampled from the non-avian dinosaurs. The location of bone cells also appeared flattened and parallel – more like that of a velociraptor – and much different from the hollow, vascularized (i.e. filled with blood vessels), rapid growing bones found in younger-aged bird fossils, and most birds today. (Flightless birds such as ostriches and penguins have solid bones.)
"Although the genealogy of birds is well understood, the genesis of modern bird biology has been a huge mystery,” Norell said. “We knew that they are a kind of dinosaur, but we now know that the transition into true birds—physiologically and metabolically—happened well after Archaeopteryx."
What this implies is that Archaeopteryx was more of a dinosaur than it was a bird – at least in its metabolism and how fast it matured – and that the traits considered necessary for flight – such as light, hollow bone structure – evolved from a dinosaur body-plan sometime after Archaeopteryx.
"We show that avian flight was achieved with the physiology of a dinosaur," Erickson said.
The study can be found in a recent edition of the online journal PLoS ONE.