Fancy Tail Feathers Of Fossilized Bird Show It Prioritized Fashion Over Function

If the Early Cretaceous had selfies, this would have been a scorcher. Image credit: Wang Min et al. 2021, Current Biology

Researchers have uncovered a fossil of a new species of Early Cretaceous bird who sported impressive tail feathers that were longer than its body. Such sexy booty plumage has never before been documented among dinosaurs and Mesozoic birds not belonging to Ornithuromorpha, the clade which gave rise to all living birds. Its presence demonstrates that sexual selection was a strong driving force for this species, leading to a body plan that prioritized fashion over function.

Evolution for the most part is about improving your fitness for a certain environment. That’s not to say being physically fit but adapting in a way that best complements your habitat, be that the flat, sand-colored body of an angelshark or the giant bug-eyes of nocturnal tarsiers. Sometimes, however, sexual selection can get in the way of an animal’s physical performance within its environment, in adorning the animal with features that, while attractive to potential mates, makes life just a little bit more exhausting.

Pheasants and peacocks are fine examples, burdened with enormous, ornate display feathers which while an effective courting tool (though not always) make fleeing in flight quite the energetic undertaking. This compromise between fitness and sexual success is highlighted in a new paper published in Current Biology, in which a fossil is described of a bird from the Early Cretaceous whose tail feathers were so fancy they were 150 percent the length of its body.

fossil tail feathers
An illustration showing what Yuanchuavis might have looked like in life. Image credit: Haozhen Zhang

The bird, named Yuanchuavis kompsosoura, sat within the Enantiornithes, the most successful group of Mesozoic birds considered to be the sister clade to Ornithuromorpha. Enantiornithes were mostly small, arboreal birds with lots of teeth that went extinct around 66 million years ago.

Y. kompsosoura is a cause for academic excitement as it represents the first time this combination of different tail feathers has been documented in a fossil bird from the Enantiornithes. Similar morphologies however pop up having evolved independently among the ornithuromorphs, as seen in Archaeorhynchus and many modern birds.

fossil tail feathers
A Malachite Sunbird, a modern bird with a pintail similar to what Yuanchuavis had. Image credit: Jason Weckstein

Wearing fancy kit is as much a symbol of privilege among animals as it is humans, as only when resources are abundant can you afford to prioritize your attractiveness over your ability to forage, flee, and fight.

“A seabird has no cover and often has to fly long distances without being able to land or has to land on water,” said study author and paleontologist at Chicago’s Field Museum Jingmai O’Connor to IFLScience. “This environment has shaped their plumage (the white and greys make it hard for fish, their prey, to see them; their plumages have no ornaments because they are adapted to be efficient flyers and thus do not evolve ornaments that are detrimental to flight).

“In contrast, forests provide ample cover, tropical forests have abundant resources, and you don't need to be particularly good at flying because the vegetation is so dense. This is why we see lots of birds with fancy ornaments living in this environment because having a tail that makes it harder to fly is less of an issue due to the structure of the environment in which they live.”

Clearly, then, Y. kompsosoura was quite the fancy lad, dining with ease as it flashed its fine feathers for all to see. You go, Y. kompsosoura.

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