Today, birds are one of the most diverse groups of vertebrates, and they've had a long time to evolve. The first "bird" in the fossil record is largely considered to be Archaeopteryx, which existed 155 million years ago. Around 55 million years ago, the first hummingbirds and parrots began to appear.
The more researchers learn, the more they realize that flight has evolved multiple times
across animals, helping them reach the same goal of flight in a variety of ways.
Birds weren't the only creatures taking to the skies back then. Flying reptiles called pterosaurs, which reached the size of small planes, dominated the skies as early as 215 million years ago. The fossil record has also shown evidence of flying dinosaurs, like microraptors, as well as other creatures that could glide from tree to tree.
The story of the origin and evolution of flight is a long and complicated one, made even more tangled and complex as new research restructures the roots and branches of this family tree.
Within the last 20 years, advances in technology, such as conducting computerized tomography, or CT scans of fossils, as well as the discovery of a wealth of fossils in China, are helping fill the gaps in the story of how animals transitioned from crawling on the ground to flying in the air.
Flying reptiles may be hard to imagine, but pterosaurs were the masters of powered flight in their day. They first appear in the fossil record about 215 million years ago, and they thrived until the mass extinction event that wiped out the dinosaurs 66 million years ago.
The fossils of these reptiles show they were perfectly adapted for powered flight. "Not only did they live alongside dinosaurs, but pterosaurs have a lot of the same features that birds later develop that makes them capable of powered flight," said Eugenia Gold, assistant professor in Suffolk University's biology department in Boston, Massachusetts, and research associate in the American Museum of Natural History's division of paleontology.
But pterosaurs achieved flight in a completely different way than birds. Pterosaurs had a muscular membrane stretched between a ridiculously long ring finger and their ankles -- almost like a modern flying squirrel, Gold said. They had a fluffy body covering to help them retain heat.
Like birds, pterosaurs had a keel on their sternum, a ridge that serves as the attachment for flight muscles, said Alex Dececchi, assistant professor of biology at Mount Marty University in South Dakota. But they were incredibly strong, with compact muscular bodies similar to gymnasts' and kite-like wings for soaring over great distances and oceans, he said.
On the ground, they walked on all fours, using their feet and wings. And like mythical dragons, they sported tails. The earliest pterosaurs had long tails, which grew shorter over time. While pterosaurs started out small and living in forest environments, they eventually reached gigantic proportions and soared over oceans before going extinct.
The pterosaur Cryodrakon boreas
was one of the largest flying animals that ever lived. The name means "frozen dragon of the north wind
," and it flew over North America 77 million years ago. This gigantic flying reptile had a wingspan up to 32.8 feet (10 meters).
The pterosaur Quetzalcoatlus, which lived during the Late Cretaceous 72 million years ago, was the size of a giraffe with a nearly 10-foot-long (3 meter-long) head. Some pterosaurs were also known for having flashy, colorful giant crests stretching up like huge mohawks from the tops of their heads.
"But nothing shows us what pterosaurs looked like before they adapted to fly," Gold said.
"All the evidence shows us the pterosaurs are cousins of dinosaurs and that both share a common ancestor," said Mike Benton
, professor of vertebrate palaeontology at the University of Bristol in the United Kingdom and author of the upcoming book, "Dinosaurs: New Visions of a Lost World
" publishing on October 21.
While the oldest fossils of dinosaurs and pterosaurs comes from the late Triassic 215 million years ago, evidence suggests that their predecessor lived 250 million years ago, creating a 35-million-year gap.
This mysterious common ancestor likely walked upright, was warm-blooded and probably had feathers, but no wings, Benton said. And it was probably about the size of a pigeon with short limbs.
However, no fossils for this ancestor have been found. It's possible that its remains weren't preserved in the environments where it lived because forests don't really fossilize, said Jingmai O'Connor
, associate curator of fossil reptiles at the Field Museum's Negaunee Integrative Research Center. Largely the creatures who die in forests are completely recycled by scavengers.
'Giant flying murder heads'
Researchers aren't exactly sure why pterosaurs reached the massive size they did before going extinct, some with wingspans of 10 meters (33 feet) or more. Some have suggested it was because small birds took over forest environments, leading pterosaurs to the open skies where larger wings were a benefit.