Spectacular Fossil Shows a 120-Million-Year-Old Bird With a Highly Impractical Tail

Nothing says "Look at me!" quite like a set of tail feathers. A set of extravagant tail feathers is a great way to say "Look at me!" Modern birds are known to sacrifice agility to attract attention. However, fossil records show that there are rare examples of such behavior in their relatives.
Scientists now know that the remains of a 120 million-year-old feathered dinosaur have been found. It is roughly the same size as a bluejay and has a very long, extravagant behind.

The fossil, which is extremely detailed, was discovered in northeastern China. It was named Yuanchuavis (Yuanchuavis Kompsosoura), after a bird that resembles a phoenix in Chinese mythology.

This is the first time that a Mesozoic-era bird-like fossil has been found with such a complex array tail feathers. It includes a grey fantail as well as a pair long, black plumes.

These two long, dense dark feathers measure approximately 30 cm (12 inches) in length and are about 1.3 times as long as the bird's body. They create a pintail which, according to the authors, "strongly resembles" modern quetzals and sunbirds that use their ostentatious behinds in order to attract mates.

This discovery indicates that even the earliest birds used their feathers for similar purposes.

Being sexy has its downsides. A tail as flashy and large as that of Yuanchuavis could have made it difficult to fly and avoid predators.

Evidently, the risk of the booty call was worth it.

"Scientists refer to a trait such as a large fancy tail as an honest signal because it is harmful. If an animal can survive with that handicap, it's a sign it's truly fit," says Jingmai O'Connor, Chicago's Field Museum paleontologist.

"A female bird would see a male with ridiculously burdensome tail feathers and say, "Dang, he must have good genes if he can survive with such a ridiculous tail."

A fossil of Yuanchuavis and a matching illustration of the tail feathers. (Wang et al., Current Biology, 2021)

Yuanchuavis is part of a Mesozoic group of avians called Enantiornithes, or "opposite Birds", which are located on an evolutionary branch somewhere between the ancestor and modern birds.

Enantiornithes actually belong to the same sister clade as ornithuromorpha to which all living birds are members. These clades had coexisted side-by-side for 65 million years and only one survived the mass extinction that decimated most dinosaurs.

Understanding why some avian clades survived and others didn't can help us better understand modern birds' ability to survive in this world.

The Mesozoic era was when modern birds' ancestors did not have fan tails as long as Yuanchuavis. However, fossil records do indicate that they had fan tails.

For many years fan tails have been thought to have evolved in ornithuromorphs. However, this feature has been discovered in several fossils of enantiornithes.

Yuanchuavis are even more unusual. The unique tail, which consists of both fantail and paired plumes, is something no other bird in the clade can boast.

Although the authors are unsure of why Yuanchuavis developed such a complicated tail, considering how inconvenient these feathers were aerodynamically, they believe that this plumage was shaped through sexual selection.

There are many things you can hide behind in densely wooded environments, where the majority of enantiornithes live. If the tail is long enough, a cumbersome tail may not be so bad. That's where most birds of paradise are found today.

However, in the open and semi-aquatic environment where many Mesozoic ornithuromorphs lived, it might not have been as profitable to trade off survival for sex.

These landscapes may have been part of the reason why pintails appeared to have developed earlier in enantiornithines than in their sister clade.

Wang explains, "It is well-known that sexual selection plays an important role in speciation, recognition in modern birds,"

It is difficult to determine if a fossilized structure was shaped by sexual choice, given the imperfections of the fossil record. The well-preserved tail feathers of this fossil bird give us great information about the evolution of the avian tail through sexual selection.

Current Biology published the study.