Modern day birds may simply be dinosaurs that never grew up, researchers say. A comparison of fossilized skulls of juvenile dinosaurs with those of birds shows remarkable similarities, adding further evidence to the growing consensus that birds are evolutionary descendants of dinosaurs. A team from Harvard University reported online in the journal Nature that for some as-yet-unknown reason, some dinosaur infants began to mature much more rapidly than normal. That rapid maturation altered the expression of genes, changing the physical characteristics of the animals and keeping them much smaller in size. Those changes allowed the development of a relatively bigger brain and the development of flight.
A variety of evidence suggests an evolutionary link between the two species. At least 22 bones are found only in birds and dinosaurs, and in no other animals. Researchers have found fossils of feathered dinosaurs, fossils of dinosaurs with other bird-like features, and fossils of primitive birds with dinosaur-like features.
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."