Even paleontology has ugly duckling stories.

In 2000, a fossil collector found a chunk of rock about as big as a deck of cards in a limestone quarry in Belgium. Sticking out of it were femur and shin bones the size of twigs that looked like the remains of a miniature chicken dinner.

Eighteen years later, Daniel Field, a paleontologist at the University of Cambridge in England, and Juan Benito, a doctoral student, took a CT scan of the chunk and found a prize within: an almost perfectly preserved ancient bird skull.

“It just totally blew our minds,” Dr. Field said.

The fossil, described by Dr. Field and colleagues today in Nature, is between 66.7 and 66.8 million years old, making it “the oldest known fossil that’s clearly part of the modern bird family tree,” Dr. Field said. It’s also the first fossil that resembles a modern bird without any ambiguity found in the northern hemisphere, he said, although at least one expert takes issue with this characterization.

Bhart-Anjan S. Bhullar, a paleontologist at Yale who was not involved in the study, said that the fossil is “one of those great discoveries that come up a few times in a lucky lifetime.”