WASHINGTON — Some ancient penguins may have been twice as big as today’s Emperor penguin but they lacked the dashing tuxedo.

Researchers unearthed remains of a nearly 5-foot-tall penguin that roamed what is now Peru about 36 million years ago, and they also discovered fossilized feathers that show back then, the flightless bird was a more motley mix of reddish-brown and gray.

Thursday’s report in the journal Science is more than a curiosity about color. Analyzing the fossil led to a new discovery about modern penguins, which in turn raises questions about how their feathers evolved to help them become such expert swimmers.

It’s one of the largest penguins that ever lived, estimated to have been twice as heavy as the average Emperor penguin of today. The second species of giant penguin discovered in Peru, it was given the name Inkayacu paracasensis, or Water King, part of a cluster of now-extinct penguin species that apparently ranged over much of the Southern Hemisphere.

A stroke of luck helped paleontologists find the feathers. A student on the dig team, from the Museo de Historia Natural in Lima, discovered the fossil’s foot and noticed it had scales, evidence of soft tissue that’s rarely preserved. Maybe there was more soft tissue, and if so, they’d have to excavate extra carefully.

“We got incredibly excited,” said paleontologist Julia Clarke of the University of Texas at Austin, who led the team. “Moving really slowly, flake by flake by flake through this giant block,” they eventually uncovered a flipper with layers of small feathers and under it, fossilized body feathers, too.

On the surface, they’re shaped like the feathers of modern penguins. Popsicle-shaped wing feathers were densely stacked on top of each other to create a stiffened flipper, Clarke said.

When they looked more deeply, the feathers were far different. The outer shape apparently evolved before some microscopic changes that may play a role in penguin’s underwater prowess.

Pigment is long gone in fossils. But left behind in feathers can be microscopic packets called melanosomes that in life contained color-producing pigments — and the shape of those melanosomes corresponds to different colors. So the researchers compared a library of melanosomes from living birds with these fossilized ones.

The big surprise is that it turns out modern penguins have large melanosomes packed into grape-like clusters, unlike those of any other known bird, while the extinct giant penguin’s smaller melanosomes resembled those of other birds, Clarke said.

The scientists can’t explain the difference. But they say it probably has to do with more than the black tuxedo coloration of today’s penguins.

Melanin, the pigment inside melanosomes, helps feathers resist breakage. So one possibility is that the melanosomes got bigger during later penguin evolution as the birds became better underwater swimmers and needed a more hydrodynamic covering. Clarke is anxious to get back to Peru and see if more fossil finds will help tell.

The work was funded by the National Science Foundation and National Geographic Society.