A tiny bird from the Jurassic period has revealed how dinosaurs lost their tails

The key discovery is the tail. In dinosaurs, it was usually long and consisted of dozens of vertebrae. In modern birds, the bony tail is short, and at its tip is the pygostyle – a fused bone to which the tail feathers are attached. In Zhengheornis, the tail was already significantly shortened, but the pygostyle had not yet developed.

This shows that birds did not ‘lose’ their tails in a single, abrupt evolutionary leap. First, the number and length of the tail vertebrae in early birds decreased, and only later did the short, fused tail of the modern type emerge.

Details

The fossil was found in the Nanyuan Formation in Fujian Province, China. The find is estimated to be approximately 148–150 million years old, that is, the Late Jurassic — a time when early birds and their close relatives were already actively experimenting with different body forms.

The skeleton is fairly well-preserved. Traces of feathers are visible around the bones, and the animal itself was very small: researchers estimate it weighed approximately 74–163 grams. This is smaller than many known early bird-like forms.

By way of comparison, Archaeopteryx, one of the best-known early bird-like animals, had around 23–24 tail vertebrae. Zhengheornis has only 15. However, these vertebrae had not yet fused into a pygostyle, as in modern birds.

Why the tail is so important

The tail is not simply a ‘superfluous part’ of the body. In dinosaurs, a long tail helped maintain balance. But for animals transitioning to active flight, a long, heavy tail became a problem.

A short tail makes the body lighter, reduces air resistance and helps shift the centre of gravity forwards. This is important for stability in the air. In modern birds, the tail feathers also act as a steering system: they help with manoeuvring, braking and changing direction.

Therefore, the question ‘how did the dinosaur’s tail disappear?’ is part of the wider question of how birds’ bodies became adapted for flight.

What the new bird revealed

Before this discovery, the picture seemed incomplete. The fossil record included early birds with long tails and later forms with short tails and a pygostyle. Because of this, it might have seemed that the transition occurred rapidly.

Zhengheornis fills an important gap. Its tail had already become short, but had not yet acquired its final, modern structure. This intermediate state suggests a step-by-step scenario: first, the tail shortened; then, its terminal vertebrae fused to form a pygostyle.

Put simply, evolution first ‘removed the excess length’ and only then assembled a new tail mechanism to control the feathers.

Why this is important

Birds are descended from theropod dinosaurs, and their bodies did not change all at once. Wings, feathers, a lightweight skeleton, a shortened tail, and a shifted centre of gravity — all these features developed gradually.

Zhengheornis illustrates one such step. It was already a small, feathered flying lizard, but its tail was not yet organised like that of modern birds. The discovery therefore helps us to see not the final result, but the very process of bodily restructuring.

The authors of the study also note that Zhengheornis is now the fourth early bird to be discovered in the Zhenghe fauna. This suggests that by the end of the Jurassic Period, early birds were more diverse than previously thought. Some species may have been better adapted to life on the ground, others to life in trees, whilst Zhengheornis does not show such obvious specialisation.

Background

The most famous early ‘bird-like’ dinosaur is Archaeopteryx. It lived during roughly the same ‘Great Jurassic Window’ and combined bird-like and dinosaur-like features: feathers and wings, but also teeth, claws and a long bony tail.

Modern birds are structured differently. Their tail vertebrae are greatly reduced, and at the end is the pygostyle. This bone helps to hold the fan of feathers in place, which is important for flight control.

For a long time, scientists lacked well-preserved fossils showing the intermediate stages between a long tail and a short tail with a pygostyle. Zhengheornis provides precisely such a rare ‘in-between’ example.

Source

Study: Min Wang et al., “Jurassic avialan reveals stepwise evolution of bony tail in birds”, Science Advances, 2026.