Modern day birds may simply be dinosaurs that never grew up druing development, researchers say.
Evolutionary biologist Arkhat Abzhanov of Harvard University noted an apparent resemblance between the skulls of juvenile dinosaurs and adult birds and decided to do a more comprehensive study. With graduate student Bhart-Anjan Bhullar, he used CT scanners to examine dozens of skulls, including modern birds, theropods -- the dinosaurs most closely related to birds -- and earlier dinosaur species. By identifying various landmarks on the skulls, they were able to track how the skull shapes had changed over the years.
"We examined skulls form the entire lineage that gave rise to modern birds," Abzhanov said. "We looked back approximately 250 million years, to the Archosaurs, the group which gave rise to crocodiles and alligators as well as modern birds. Our goal was to look at these skulls to see how they changed, and try to understand exactly what happened during the evolution of the bird skull."
What they found was surprising. Early dinosaurs underwent vast morphological changes as they aged. Among other things, their snouts grew longer and their heads grew flatter. The skulls of juvenile and adult birds, in contrast, are remarkably similar. They concluded that the evolutionary changes that produced birds were a phenomenon known as paedomorphosis. "We can see that the adults of a species look increasingly like the juveniles of their ancestors," Abzhanov said. In the case of birds, he added, the phenomenon is caused by a process called progenesis, in which the descendants reach sexual maturity earlier. Birds can take as little as 12 weeks to reach maturity, while dinosaurs required months or years. Concluded Abzhanov: "When we look at birds, we are actually looking at juvenile dinosaurs."