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Archaeopteryx was not very bird-like

October 09, 2009

New research published this week clips the wings of Archaeopteryx. First found in Germany in the 1860's and dating to 150 million years ago, Archaeopteryx has long been considered the iconic first bird. But microscopic imaging of bone structure published in PLoS One shows that this famously feathered fossil grew much slower than living birds and more like non-avian dinosaurs. The bones of more recently discovered bird fossils like Confuciusornis, a primitive but toothless bird from the Yixian Formation in China that appeared after Archaeopteryx, demonstrate rapid growth more similar to that of modern birds. This means rapid bone growth-long thought a prerequisite for flight-was not necessary for taking to the air.

"Dinosaurs had a very different metabolism from today's birds. It would take years for individuals to mature, and we found evidence for this same pattern in Archaeopteryx and its closest relatives," says Gregory Erickson of the Department of Biological Science at Florida State University, who is also a Research Associate at the American Museum of Natural History and first author of this paper. "Living birds mature very quickly and grow really, really fast. This is why we see flocks of pigeons that all look the same and rarely see baby birds. Animals like Archaeopteryx would be very foreign to a bird watcher."

The first Archaeopteryx skeleton was found in Germany about the same time Darwin's Origin of Species was published. This was a fortuitously-timed discovery: because the fossil combined bird-like (feathers and a wishbone) and reptilian (teeth, three fingers on hands, and a long bony tail) traits, it helped convince many about the veracity of evolutionary theory. In fact, the first suggestion that birds are related to dinosaurs was made by early proponent of evolution Thomas Henry Huxley in the 1860's. "Archaeopteryx is the poster child for evolution," says Erickson. Ten skeletons and an isolated feather have been found.

"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. 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," says Mark Norell, Chair of the Division of Paleontology at the Museum and a co-author of the paper.

Norell, Erickson, and colleagues looked at growth rate in Archaeopteryx and in birds and dinosaurs up and down the family tree by removing tiny, 250-micron chips from the long bones (specifically thigh bones and one of the shinbones). This process required an enormous amount of cooperation. The Munich Archaeopteryx was sampled with the help of co-author Oliver Rauhut of the Bavarian State Collection for Palaeontology and Geology in Munich, who says: "The Munich specimen is one of the more recently discovered specimens. Nevertheless, 'cutting up' this fossil to take samples of the bones was almost considered blasphemous until recently. But because the samples were very small and were removed with great skill by our preparator, the knowledge gained was more than worth the sacrifice."

Chinese fossils were equally important to sample so that Archaeopteryx could be placed in context. Because of co-author Zhonghe Zhou of the Institute of Vertebrate Paleontology and Paleoanthropology at the Chinese Academy of Sciences, samples were taken from Jeholornis prima, a long-tailed specimen considered an early bird, and the short-tailed Sapeornis chaochengensi that had three fingers and teeth. The group also sampled the closest relatives among dinosaurs to the birds including Velociraptor mongoliensis and miniaturized species such as Mahakala omnogova that is similar in size to Archaeopteryx.

All chips were analyzed under polarizing microscopy. The bone types for the early birds Archaeopteryx, Jeholornis, and Sapeornis are comparable to those of same-sized dinosaurs-animals that grew considerably slower than living birds. The blood vessels are very small, the bone is very dense, and the "osteocyte lacunae," or the location of bone cells, are flattened and parallel.

The opposite pattern holds for more advanced bird fossils such as the roughly 94 million-year-old Ichthyornis dispar from North America. The bone is well-vascularized, which means that it was well-supplied with blood vessels, and the bone structure is woven, with randomly oriented bone fibers, indicating fast growth with no annual growth lines. This is the bone histology of modern birds.

Additional information came from physical examination of more than half of the Archaeopteryx specimens. Although the young Munich specimen did not have growth lines (a characteristic marker of annual growth in dinosaurs), both the larger London and Solnhofen specimens have peeling or delaminating bone suggestive of the presence of growth lines. This means that this animal grew for more than a year and that growth ceased for part of each year as is typical of small non-avian dinosaurs. Jeholornis and Sapeornis fossils also have growth lines that suggest they grew for more than a year. All of these traits point to a more reptilian growth pattern than seen in living birds.

"When first I looked at the Archaeopteryx, it looked like lizard bone," says Erickson. "This told me right off the bat that Archaeopteryx was an animal that grew slowly. This was a surprise and not what was expected for a same-sized living bird." In fact, analysis of numerous fossils showed the team that dinosaurs grew more slowly, pound for pound, than living birds, and that bird-like metabolic characteristics were not present in the ancestor of Archaeopteryx and modern birds. The team outlines a growth curve that indicates that Archaeopteryx reached adult size in about 970 days, that none of the known Archaeopteryx specimens are adults (confirming previous speculation), and that adult Archaeopteryx were probably the size of a raven, much larger than previously thought.

"Archaeopteryx had comparable metabolism to closely related Velociraptor," says Norell. "Although the genealogy of birds is well understood, the genesis of modern bird biology has been a huge mystery. 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."

Adds Erickson: "We show that avian flight was achieved with the physiology of a dinosaur."

American Museum of Natural History