Scientists finally have the skinny on two extinct species of giant "svelte" penguins that lived in New Zealand 25 million years ago, a new study says.

For decades, study co-author Ewan Fordyce, a paleontologist at New Zealand's University of Otago, had been happening across bones of the species while searching for fossil whales and dolphins.

Only recently, though, has a team reconstructed a full skeleton. This composite—created using a model of a modern-day king penguin—represents both species, which were quite similar.

The result is "quite a streamlined animal—it wouldn't look like any penguin that's alive today," said study leader Dan Ksepka, an avian paleontologist at North Carolina State University.

Instead of a modern penguin's rotund shape, each of the newly named species had a narrow chest; long, tapering flippers; and a narrow beak—a body specialized for hunting fish.

Standing about 4.3 feet (1.3 meters) tall, both species would have been taller than the tallest living penguin species, the emperor penguin, which can reach 4 feet (1.2 meters) tall.

Prehistoric New Zealand

In the penguins' time, New Zealand itself was mostly underwater—only a smattering of islets were above the surface. Shallow waves rich with food and protection from predators would've made the habitat ideal for the birds.

Picturing the scene 25 million years ago, Ksepka sees an assortment of various penguins crowded onto remote, rocky outcrops—much like the areas inhabited by modern penguins in New Zealand, though the researcher imagines active volcanoes possibly turning prehistoric skies cloudy with ash.

In a hat tip to that ancient homeland, the team has named the species Kairuku waitaki and Kairuku grebneffi—"kairuku" roughly meaning "diver who returns with food" in the language of New Zealand's indigenous Maori people.

Giant Penguins Not Alone

"We've been gaining new insights into giant penguins from other parts of the world," said University of Texas vertebrate paleontologist Julia Clarke, who wasn't involved in the new study.

Clarke herself led a 2010 study describing a new species of ancient giant penguin, the water king, the first fossil penguin discovered with feathers.

But "in New Zealand, where they have one of the peak areas for [giant penguin] diversity, they haven't been really fully described"—making the new reconstruction an important advance, said Clarke, whose work has been supported by the National Geographic Society's Expeditions Council. (The Society owns National Geographic News.)

Both of the new species lived alongside four other penguin species, each of which likely ate different fish prey—a level of local diversity rare today, study leader Ksepka noted.

"The fact they're five species all standing on the same beaches—it's pretty incredible," he said.