Chinese researchers reveal missing link in evolution of birds' tails

Scientists from the Chinese Academy of Sciences have discovered a well-preserved fossil of an ancient bird in Fujian Province, which has helped to shed light on one of the most contentious questions in evolution – exactly how birds lost their long tails and developed the short tails seen in modern species. This was reported by Science and Technology Daily, a partner of TV BRICS.

The main problem that scientists had long been unable to resolve lay in the sequence of changes. Dinosaurs and the earliest birds had long tails consisting of a large number of vertebrae – sometimes more than 30 – and the tail often exceeded the length of the body. In modern birds, the tail is short and ends in a specialised bony structure – the pygostyle – which is formed from fused vertebrae and helps to control the feathers and balance during flight. However, it was unclear whether the tail shortened first, or whether the vertebrae fused first.

The new discovery reveals a clear sequence of events. In the bird found, the first change had already taken place: the tail had become noticeably shorter, and the number of vertebrae had decreased to around 15. However, the second stage had not yet occurred: the vertebrae had not fused to form a pygostyle. This is the key transitional stage that was previously missing from the fossil record. It is now clear that the evolution of the tail proceeded in stages: first, a gradual shortening and reduction in the number of vertebrae, and only then their fusion into a single structure.

The shortening of the tail made the body lighter, shifted the centre of gravity closer to the wings and improved manoeuvrability in the air. In other words, such changes could have directly enhanced the ability to fly and manoeuvre, providing an evolutionary advantage.

Another important observation concerns the animal’s size. The ancient bird was very small: about 20 centimetres in length and weighing between 74 and 163 grammes. This suggests that the reduction in size across some avian lineages occurred quite rapidly, likely as an adaptation to life in trees and active flight.

Furthermore, the discovery was made in a region where various forms of ancient birds have already been found – ranging from more primitive long-tailed species to more modern ones with fused tails. This suggests that by the end of the Jurassic period (approximately 161 to 143 million years ago), birds were already highly diverse and evolution was proceeding along several paths simultaneously, rather than in a linear fashion.