New analysis suggests New Zealand's giant penguins and a much younger group of Northern Hemisphere birds, the plotopterids, were physically quite similar.

The research, published Monday in the Journal of Zoological Systematics and Evolutionary Research, could help scientists figure out how birds evolved wings better suited for swimming than flying.

Fossil remains suggest as many as nine different species once swam the tropical seas that washed over most of what's now New Zealand, some 62 million years ago. While some species were the size of modern penguins, others grew to heights of more than five feet.

Plotopterids don't appear in the Northern Hemisphere fossil record until 30 million years later. Their remains have been recovered from several sites in Japan and North America. Like penguins, plotopterids used flipper-like wings to navigate coastal seas. But while the relatives of New Zealand's ancient penguins can still be found today, plotopterids went extinct around 25 million years ago.

For the new study, scientists compared the fossilized remains of plotopterids recovered from Japan with the fossils of three giant penguin species. In addition to boasting similar wings, the analysis showed both groups of birds possessed long beaks with slit-like nostrils, as well as chest and shoulder bones conducive to swimming. Like the giant penguins, some plotopterid species were oversized, growing to heights of more than six feet.

Despite their physical similarities, plotopterids and penguins aren't particularly close relatives. Plotopterids are more closely related to other seaworthy birds like boobies, gannets and cormorants.

"What's remarkable about all this is that plotopterids and ancient penguins evolved these shared features independently," study co-author Vanesa De Pietri, curator at the Canterbury Museum in New Zealand, said in a news release. "This is an example of what we call convergent evolution, when distantly related organisms develop similar morphological traits under similar environmental conditions."

Though plotopterids and giant penguins were separated by several thousand miles and nearly 30 million years, had they lived side-by-side, they would have been hard to distinguish.

"Plotopterids looked like penguins, they swam like penguins, they probably ate like penguins -- but they weren't penguins," said Paul Scofield, study co-author and Canterbury curator.

The newly published comparison of the two ancient bird groups has helped scientists begin to develop an explanation for why some birds developed wings for swimming.

"Wing-propelled diving is quite rare among birds; most swimming birds use their feet," said study co-author Gerald Mayr, scientist at the Senckenberg Research Institute and Natural History Museum in Germany.

"We think both penguins and plotodopterids had flying ancestors that would plunge from the air into the water in search of food," Mayr said. "Over time these ancestor species got better at swimming and worse at flying."

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