Dr. Tarduno and his team published their findings on Monday in the journal Scientific Reports.

Scientists aren’t sure why Earth was stifling hot for several million years during the Cretaceous period, but according to Dr. Tarduno, the prevailing hypothesis is that the atmosphere was filled with heat-trapping carbon dioxide, most likely the result of extraordinary volcanic activity. The resulting greenhouse effect would have transformed the polar ecosystem into a place where Tingmiatornis arctica and its prey could thrive.

The warming period, known as the Turonian age, is estimated to have lasted from 93.9 million to 89.8 million years ago. At its coldest, it is estimated that the Arctic got around 57 degrees Fahrenheit.

Image An artist’s rendering of Tingmiatornis arctica in the ancient Arctic during a time when the climate was much warmer. Credit... Mike Osadciw

In his time exploring the snowcapped brown hills and thick glaciers of Nunavut, in the Canadian Arctic, Dr. Tarduno has come across two wing bones belonging to this species of bird. He uncovered the first humerus in 1999. It was relatively small and he didn’t pay it much mind until he found a second, larger bone a few years later. But even the second humerus didn’t catch his attention at first. Instead, he and his team were preoccupied with a large turtle shell that was on the other side of the same rock.

“We took it back to camp and went, ‘Oh, wait a minute, there’s another spectacular fossil on the other side,’ ” Dr. Tarduno said.