Feather trapped in dinosaur droppings provides new clues about why some birds survived extinction

One day in 2016, paleontologist David DeMar got very lucky. While examining the ground at Hell Creek in the U.S. state of Montana, he found a fossil the size of a golf ball. It was a coprolite — fossilized feces — from a theropod (probably a juvenile Tyrannosaurus rex or a Nanotyrannus). Inside the coprolite was a feather from an aquatic bird (not related to modern birds). With a high degree of confidence, researchers believe the bird spent the final moments of its life in the digestive tract of a teenage T. rex.

The bird’s misfortune has now enabled a team of paleontologists to make a remarkable discovery. In a study published Thursday in Current Biology, they argue that anatomically modern feathers already existed 66 million years ago. The finding could help explain why the ancestors of modern birds survived, while other avian lineages went extinct.

According to the paper, the key may be that modern feathers insulate against the cold better than primitive ones. That advantage could have proved crucial during the so-called “impact winter” that followed the asteroid strike, when global temperatures are thought to have dropped dramatically.

Jingmai O’Connor, the study’s lead author, speaks enthusiastically about Cretaceous feathers during a video call from an office decorated with dinosaurs. One of them watches over the scene from the top of a bookshelf: the head of a T. rex wearing a museum guard’s cap.

Thanks to CT scanning, O’Connor explains, the researchers realized that the coprolite also contained primitive feathers. This allowed her team to infer that the bird had a combination of both types: modern feathers, probably located on its arms, and primitive feathers covering the rest of its body. Despite having modern feathers, the fact that they were only on the bird’s limbs meant the species was not equipped to withstand the cold that followed the asteroid impact. It had feathers capable of providing insulation against freezing temperatures, but in the wrong place.

The importance of coprolites

For O’Connor, who works at the Field Museum of Natural History in Chicago, another important finding from the study is “the amount of information now obtainable from coprolites.”

Jesús Marugán, a paleontologist and evolution specialist at the Autonomous University of Madrid who was not involved in the study, agrees. In his view, the study of these fossils opens the door to analyzing countless aspects of ancient ecosystems, from “aspects of diet to trophic interactions between different organisms.” Although coprolites have been known since the earliest days of paleontology, their usefulness was limited until recently because, as Marugán notes by phone, “you had to break them open” in order to study them.

Today, thanks to CT scans and other available technologies, all the elements contained within fossilized feces can be examined without opening them. “Anything that provides contrast will show up,” the Spanish paleontologist explains.

What’s more, the Late Cretaceous — a period spanning roughly 100 to 66 million years ago — is one from which fossils of these soft tissues, essentially any part of the body that is not bone, are particularly rare. O’Connor explains that this is because their preservation requires “the organism to be buried in oxygen-free aquatic environments that prevent scavengers from feeding on it and allow microbial fixation to take place.” Such conditions are highly specific and, in the Late Cretaceous, the period that ended with the mass extinction, incredibly difficult to find. That makes this feather an exceptional rarity.

O’Connor says this is probably “the only Late Cretaceous feather” that can be analyzed in three dimensions. The few other feathers preserved from that period are generally trapped in amber and can only be studied in two dimensions.

A food chain

From the analysis of the feces, the team was able to identify a second victim. Alongside the feathers and bones, they also found scales. Based on their size, the researchers judged the scales likely belonged to a gar fish. The scientists thought it unlikely the theropod (that is, the young T. Rex or Nanotyrannus) had hunted those fish: they are simply too small and hard to catch to be its prey. Most likely, the researchers concluded, that fish was part of another animal’s diet.

And all the evidence contained in the fossilized feces pointed to a single candidate: the owner of the feather. The coprolite captures a snapshot of the food chain at that moment in time. The gar was eaten by the waterbird, which in turn was eaten by a theropod. The circle of life, reconstructed from the clues preserved in a piece of feces deposited more than 66 million years ago.

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