(CNN)In the 1980s, paleontologists at the University of California Riverside visited Seymour Island, part of an island chain in the Antarctic Peninsula. They brought home a number of fossils -- including the foot bone and partial jaw bone of two prehistoric birds.
Antarctic fossil could have been the biggest flying bird ever, study finds
For decades, the fossils sat in a museum at the University of California Berkeley -- until a graduate student named Peter Kloess started poking around in 2015.
In a study published Monday in the journal "Scientific Reports," Kloess identified the birds as pelagornithids, a group of predators that roamed the Earth's southern oceans for at least 60 million years. They are known as "bony-toothed" birds because of their sharp teeth and long beaks, which helped them grab fish and squid from the ocean.
The birds were huge, with wingspans reaching up to 21 feet (6.4 meters). And the specific individuals that the fossils belong to may have been the biggest of them all, the study suggests.
Using the fossils' size and measurements, the researchers were able to estimate the rest of the individuals' size. The bird with the foot bone is "the largest specimen known for the entire extinct group of pelagornithids," while the bird with the jaw bone was likely "as big, if not bigger, than the largest known skeletons of the bony-toothed bird group."
"These Antarctic fossils ... likely represent not only the largest flying birds of the Eocene but also some of the largest volant birds that ever lived," said the study.
Kloess and other researchers determined that the foot bone dates back 50 million years, and the jaw bone is around 40 million years old -- evidence that the birds emerged in the Cenozoic Era, after an asteroid struck Earth and wiped out nearly all dinosaurs.
"Our fossil discovery, with its estimate of a 5-to-6-meter wingspan -- nearly 20 feet -- shows that birds evolved to a truly gigantic size relatively quickly after the extinction of the dinosaurs and ruled over the oceans for millions of years," Kloess said in a news release