Introducing Yuanchuavis, an extinct bird that lived 120 million years ago in what is now China. Its elaborate tail feathers appear to contain a mix of functional and decorative characteristics, scientists say, revealing new insights into the evolution of ancient birds.
Research published today in Current Biology describes Yuanchuavis, an early Cretaceous bird with a pair of tail feathers longer than its body. Study co-author Jingmai O’Connor from Chicago’s Field Museum says these feathers weren’t very aerodynamic and that they likely served an ornamental purpose to attract mates.
But this bird was also equipped with a bundle of shorter tail feathers that likely enabled flight. For Yuanchuavis, it was a case of sexual selection being in tension with natural selection—a combination of pressures that resulted in an impressively distinctive creature. Paleontologist Wang Min from the Chinese Academy of Sciences is the paper’s first author.
There’s some very cool science here, but take a moment to marvel at that wonderful fossil. It was pulled from the Jehol Lagerstatten—a series of deposits in northeastern China “known for exquisitely preserved fossils that sometimes include traces of soft tissues like feathers,” explained O’Connor in an email. The excellent preservation is the result of rapid burial in ancient lakes, she added. The sediment from which this specimen was pulled dates back 120 million years to the early Cretaceous, and the extinct animal’s name comes from Yuanchu, a bird from Chinese mythology.
Yuanchuavis was relatively small, about the size of a blue jay. Its skeleton features a combination of primitive and newly derived features, which “speaks to the complexity of early avian evolution and the evolution of the modern bird,” O’Connor said.
The two long tail feathers, at 150% the total length of its body, are Yuanchuavis’s most distinctive physical trait. Importantly, however, the bird also had a pintail—the combination of the two long features and its short tail fan. Pintails are seen in modern birds such as sunbirds and quetzals, but this is the first known example of a pintail in Enantiornithes, a wildly successful group of Mesozoic birds.
“The pintail is a tail shape that is shaped by both natural and sexual selection, and it serves both purposes: increased chances of both survival and reproduction,” said O’Connor. “The tail fan provides lift to help during flight while the two elongated tail feathers are ornaments that can be used to attract mates.”
As a phenomenon in evolution, sexual selection is super interesting because it often works against a species when it comes to optimal function. Birds are particularly prone to it, developing elaborate feathers, decorations, vocalizations, and dances. Thing is, the demand for an eye-grabbing, sexy look often leads to the rise of superfluous, resource-heavy, and even detrimental features. But that’s the whole point of these male displays, as they communicate a clear message to curious females: “Despite these wacky features, I’m tough, fit, and someone who can help you pass down your genes.” That’s why evolutionary biologists refer to these sexual displays as “honest signals.”
The authors of the new paper argue that Yuanchuavis’s two elongated tails are exactly this: an honest signal.
“The tail feathers are very long, and long tails are pretty much always ornamental,” O’Connor explained. “The longer or bigger the feather the more energy that is needed to grow it, so it involves an initial energetic investment. Then the feathers create drag making it harder to fly so more energy is needed to move around.” It cost the bird a lot to have the two long feathers, she said, and they were not something that increased its chances of survival. “That means it must be an ornament,” O’Connor added. “How much of an impediment this ornament was is tough to say. Its cost may have been somewhat offset by the tail fan.”
Suffice to say, if male members of Yuanchuavis were able to fly, they flew very badly.
Michael Pittman, a vertebrate paleontologist at the University of Hong Kong who wasn’t involved in the research, said it’s entirely possible that sexual selection resulted in the elongated feathers, but that “knowledge gaps” still exist in this area. That said, Pittman said the new study “paves the way for a clearer understanding of early tail feather evolution that leverages what we already know about modern birds, where we can see the tail feathers themselves but we can also directly observe how the tail feathers are used.’’
The notion that sexual selection shaped the tail feathers of Cretaceous birds is nothing new, Dennis Voeten, a paleontologist at Uppsala University and an expert on ancient birds, explained in an email.
“It has even been suggested that sexual selection in small running dinosaurs may have boosted the elaboration of arm plumage, which would ultimately culminate in a flight-ready wing,” said Voeten, who wasn’t involved in the research. In other words, sexual selection may have given rise to wings in the first place!
The new study fits in well with the emerging idea that the feathers of dinosaurs, including birds, “are highly adaptive,” and it helps to explain the “bewildering” diversity of birds throughout evolutionary history, said Voeten.
Other reasonable assumptions about Yuanchuavis are that, as a bird that didn’t fly very well, it likely lived in a dense, resource-rich forest, and that males, burdened by their long tails, were probably not involved in the rearing of young.
What’s more, because Enantiornithes didn’t survive the Chicxulub mass extinction that wiped out all non-avian dinosaurs, the discovery of Yuanchuavis might help to explain why only a few birds survived.