What happened

A fossil feather found inside dinosaur poop may help explain why some birds survived the mass extinction after the asteroid impact. Reports from the Field Museum, the University of Washington, and ScienceDaily describe the study, published in Current Biology.

The story began in 2016. David DeMar Jr., a University of Washington researcher, was collecting fish fossils in eastern Montana. He noticed a dark, reddish-brown nodule. It was about half the size of a golf ball. A tiny feather stuck out from its surface.

Researchers tested the mineral makeup of the nodule. They also used micro-CT scans. These scans combine thousands of X-rays into a digital view. They can show hidden parts without breaking the fossil. The scan revealed several feathers, tiny fish scales, and bird leg bones.

The nodule was a coprolite, or fossilized feces. A large meat-eating dinosaur probably produced it. The animal may have been a T. rex or Nanotyrannus. Researchers cannot identify the producer with certainty.

Reconstructing the last meal

The leg bones belonged to a hesperornithiform, an extinct group of water birds. They were ecologically similar to loons. Most could not fly. Instead, they used their feet to dive and catch fish. Because the feathers and bones appeared together, the team inferred that the feathers came from the same bird.

These are the first known hesperornithiform feathers. They also preserve a rare record of a predator and its prey. A large dinosaur ate a water bird. The bird had recently been eating fish. One small fossil therefore records several links in an ancient food web.

Why the feather structure matters

Two feathers had features found in living birds and their immediate ancestors. Their shafts were square in cross-section. Each shaft also had a sponge-like center. The researchers say no other Mesozoic feather has this exact combination.

Other feathers looked more primitive. Some were small and fuzzy body feathers. Body feathers help birds keep warm. The specimen also included feathers that looked modern and waterproof. This suggests the bird had more than one feather type.

What the evidence may mean

Birds are the only dinosaur lineage that survived the mass extinction. The surviving branch, called Neornithes, includes every bird alive today. Scientists have proposed that living near water helped this branch survive. Hesperornithiforms also lived in or near water, but they disappeared. Water habitat alone cannot explain the difference.

The team proposes another possibility. The extinct birds may have had less effective insulation. After the asteroid impact, dust and debris reduced sunlight. Earth entered a period of severe cooling called an impact winter. Better body feathers may have helped some birds endure that cold.

The researchers also mention molting, or replacing feathers, as another possible factor. These ideas fit the fossil evidence. They do not yet prove what caused the survival difference.

What remains unknown

This is one fossil, not a complete history of ancient birds. Researchers cannot yet measure exactly how warm the extinct feathers kept a body. They also cannot tell whether the bird’s molting pattern differed from that of early Neornithes. The exact dinosaur that produced the coprolite remains uncertain as well.

The feather’s owner was probably a hesperornithiform. Still, the fossil does not identify a precise species. More specimens are needed before scientists can test the survival hypothesis across several bird groups.

What researchers will look for next

The discovery opens a new use for coprolites. Museums and field sites hold many fossilized droppings that have never been scanned for feathers. Micro-CT imaging can inspect them without destroying their contents.

More scans could reveal feathers, bones, scales, diets, and predator-prey interactions. They may show whether better insulation was common in the bird branch that survived. For now, a lucky split in an ordinary-looking rock has turned dinosaur poop into a new window on one of paleontology’s biggest mysteries.