New Delhi, July 31: Only small and agile dinosaurs that evolved to climb trees, glide, and fly were the ancestors of birds, according to a study that corroborates an idea proposed by an Indian-origin paleontologist nearly 40 years ago.
While fossils have earlier hinted that birds evolved from large ground-dwelling dinosaurs called theropods, an international team of scientists has now shown that only a special group of theropods underwent sustained miniaturisation for over 50 million years to become birds.
“The ancestors of birds were the gymnasts of the dinosaur kingdom — they were small and agile, and they climbed trees and eventually learned to fly,” Michael Lee, a paleontologist at the South Australian Museum, Adelaide, told The Telegraph over the telephone.
The findings of the study by Lee and his co-workers at the University of Southampton in the UK and the University of Bologna, Italy, will appear in the US journal Science on Friday.
Their study corroborates a model of dinosaur-to-bird evolution suggested by an Indian-origin paleontologist Sankar Chatterjee at the Texas Tech University in his 1977 book The Rise of Birds where he had argued that small size was critical for flight.
“There is no doubt miniaturisation was one of the drivers,” Chatterjee said over the telephone today. “You have to be small to be tree-dwelling — big animals find it harder to climb trees than small animals.—
Lee and his team members applied statistical techniques to analyse how the anatomy of groups of dinosaurs evolved over time, using 1,549 skeletal characters to infer how changes in size and rates of innovations in anatomy.
Their study suggests that anatomical changes in the lineage of theropods that were ancestors to birds occurred up to four times faster than in other dinosaurs. The theropods underwent what the scientists say was a “prolonged miniaturisation”, and acquired several anatomical features — short snouts, large brains and eyes, and smaller teeth with reduced serations — associated with birds.
As the theropods grew smaller, they also acquired feathers. The 50-million year process of miniaturisation, Lee said, appears to have taken place between about 200 million years ago and 150 million years ago.
Fossils of an ancient creature called the archaeopteryx, widely viewed as the earliest bird, have been dated to about 150 million years — around the same time that the study suggests the transition from dinosaurs to birds had been completed.
The recent discovery of thousands of feathered dinosaurs from China suggests that the ancestors of birds also had arboreal habits.
“Wing-assisted climbing of tree trunks and living on the branches of trees would have given rise to balance and coordination, brain enlargement, acute vision, to feathered theropods, preparing them for flight,” Chatterjee said. “From wingless tree-climbing creatures, they learnt to parachute, glide, and then became fully-winged flapping birds.”