The feathered creature called archaeopteryx, easily the world's most famous fossil remains, had been considered the first bird since Charles Darwin's day. When researchers put its celebrity bones under the microscope recently, though, they discovered that this icon of evolution might not have been a bird at all.

An examination of its bone cells revealed for the first time that the 150-million-year-old creature had the slow growth rate of a dinosaur, not a bird, an international research team reported this month. Comparing...