Only one penguin species lives in Africa now -- the black-footed penguin, also known as the jackass penguin for its loud, donkey-like bray, researchers said.
With fossil evidence suggesting many more species lived on the continent in the past, exactly why and when penguin diversity in Africa started to plummet is unclear, researchers from the U.S. National History Museum and the National Evolutionary Synthesis Center said.
New fossil findings represent the oldest evidence of these seabirds in Africa, predating previously described fossils by 5 million-7 million years, an NESCent release said Tuesday.
The fossils suggest the extinct species spanned nearly the full size spectrum for penguins living today, ranging from a pint-sized penguin that stood just about 1 foot tall to a species closer to 3 feet, the researchers said.
Gaps in the fossil record make it difficult to determine whether the extinctions were sudden or gradual, they said.
"[Because we have fossils from only two time periods,] it's like seeing two frames of a movie," study co-author Daniel Ksepka said. "We have a frame at 5 million years ago, and a frame at 10 (million)-12 million years ago but there's missing footage in between."
Humans probably aren't the cause of the extinctions, the researchers say, because by the time early modern humans arrived in South Africa all but one of the continent's species had already died out.
Rising and falling sea levels were a more likely cause, causing extinctions by destroying safe nesting sites, they said.
Numbers of the only remaining species -- the black-footed penguins --have declined by 80 percent in the last 50 years and in 2010 the species was classified as endangered.
"There's only one species left today, and it's up to us to keep it safe," Daniel Thomas of the National Museum of Natural History said.