Illustration by Ian Berry.

'Giant flying murder heads' and other creatures that ruled the ancient sky

Updated 1319 GMT (2119 HKT) September 27, 2021

Science has transformed our understanding of dinosaurs in the past two decades. Get caught up with what's new in this five-part CNN series.

(CNN)One close glimpse of a modern bird, especially around the eyes, and it may feel like you're looking into the ancient past, when massive creatures walked the Earth and soared across its skies. It's because birds are dinosaurs -- they just happened to survive the mass extinction event that wiped out their large relatives 66 million years ago.

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.
This is an artist's reconstruction of what Archaeopteryx looked like when it lived 155 million years ago, as seen in "Dinosaurs: New Visions of a Lost World."
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.