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So, penguins are pretty weird, right? They're flightless, waddling birds with wings built for swimming instead of flying.

They're pretty unique in our modern world... but that hasn't always been true. Because fossils indicate that North America was once home to its own flightless swimming birds: a group called plotopterids. They looked, walked, and swam like penguins, but they weren't penguins.

And studying their evolution can help us understand how they... and penguins... came to be. Both groups of birds were flightless, with long bills and flipper-like wings, but they lived in different places at different times. The oldest penguins in the fossil record are over 55 million years old, found in present-day New Zealand.

And throughout their history, penguins have mainly been limited to the Southern Hemisphere. Plotopterids, on the other hand, actually evolved more recently: around 35 million years ago. And their fossils are found in the Northern Hemisphere, specifically in parts of the United States, Canada, and Japan.

But despite these differences, the birds had similar body shapes for similar lifestyles. And both groups evolved enormous ancient species that stood at least 1.5 meters tall. Yes, that means penguins used to grow even bigger than emperor penguins do today — we are missing out on some seriously large birds.

But one of the most unique features that both groups share is their habit of using their wings to swim. Most swimming birds tend to swim with their feet — like ducks, for instance. But penguins use their flipper-like wings to propel themselves through the water.

And according to a 2020 study, so did plotopterids. The study compared the anatomy of several fossil penguin and plotopterid species, and found that both groups have similar adaptations in their shoulder and arm bones to allow the kinds of joint flexibility and muscle power needed for wing-swimming. But these birds are not closely related: they evolved wing-swimming independently, a result of convergent evolution.

Previous researchers have suggested this similarity might have had to do with their habitats, but this study proposed that part of the answer lies in the birds' ancestry. Because both groups of birds have close present-day cousins who use their wings to help them dive into the ocean to catch prey. Penguins are related to shearwaters, while plotopterids are related to gannets.

So it may be that both groups had ancestors who were already adapted for marine diving, and their behaviors and wing structures provided the perfect foundation for evolving true flippery-winged swimmers. Now, plotopterids, giant and otherwise, went extinct around 20 million years ago. And this is around the same time we stop seeing giant penguins in the fossil record.

These disappearances seem to coincide with the increasing abundance of certain predatory marine mammals, specifically toothed whales, seals, and sea lions. These mammals hunt in the oceans, where they would compete for food with big swimming birds. And they also show up in many of the same habitats inhabited by plotopterids and penguins in both the northern and southern hemispheres.

So with these new swimming predators on the scene, there may have been no room in the sea for giant wing-swimming birds — of either variety. Thank you for watching this episode of SciShow, which was brought to you with the help of our patrons on Patreon. While we can't send you a giant penguin doppelganger, we do have plenty of neat perks available as thanks, like an exclusive podcast and monthly bloopers.

If you'd like to check it out and get involved, you can go to patreon.com/scishow. [♪ OUTRO].