Most modern penguins in South America are 2 feet (0.6 meter) tall or less.
(See a penguins photo gallery.)
Clarke said it's counterintuitive that some of the biggest penguin fossils were found in the lower latitudes, closer to the Equator. The commonly accepted belief has it that larger animals live in colder conditions closer to the Poles.
The study also describes the first complete skull of a giant penguin, which provided a tantalizing glimpse into ancient penguin lifestyle.
Differences in the flipper also suggest variations in the animals' walking and swimming styles, compared with modern penguins, Clarke said.
The neck and skull of the ancient species were connected with different arrangements of muscle, and their beaks were a foot (0.3 meter) long.
"It doesn't scale," Clarke said of the beak. "It's really pointed, and there's this texturing on the bone, a horny sheath. My speculation is that they're eating fish, using some kind of spearfishing."
Cooler Than They Used to Be
Scientists had previously believed that penguins migrated to northern South America during a cold era between four and eight million years ago.
But the new fossils, which Clarke analyzed with colleagues in the U.S. and Peru, are from a warm period more than 30 million years earlier.
(Read related story: "World's Oldest Penguin Fossils Suggest Birds Outlived Dinos" [April 11, 2006].)
The finding counters another theory published last year suggesting that today's penguins diversified all over the Earth during a cooling period.
Ewan Fordyce, head of the geology department at the University of Otago, New Zealand, said the new results shed light on penguin "structure, history, and lifestyles during an interval that is not well sampled."
He believes the study opens a door for more studies of marine vertebrates and their responses to climate change.
"For so long," he said, "people have viewed evolution and extinction as driven by classical biological interactions, such as competition.
"With the rise of plate tectonic views of the Earth, we are rapidly gaining an appreciation of physical driving forces, such as climate change and change in ocean circulation."
In the case of the penguins, ocean circulation patterns may have been more influential than global temperature in allowing the giant historical birds to inhabit equatorial regions.
Clarke and her colleagues have proposed that cold-water upwelling off the Peruvian coast may have benefited the penguins, either by providing colder waters or by providing nutrient-rich waters with high amounts of food.
The authors stressed that while the giant penguins of yesteryear may have thrived in warmer climates, modern penguins are cold adapted and quite vulnerable to warming.
(See an interactive map of the effects of global warming.)
"What we think is important to recognize is that for a very large early part of their history, they didn't have that constraint [to colder climates]," Clarke said.
"Whatever ecologies and limitations we see in living species, it's important not to attribute those to every part of that species' history."
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