The creature, which may have reached lengths of up to 20 feet (6 meters), had a mouthful of teeth and likely feasted on giant penguins, sharks, and big bony fish, whose remains were also discovered with the jawbone.

The early whale swam polar waters during the Eocene period, some 49 million years ago. Its age suggests fully aquatic whales evolved from their mammalian ancestors more rapidly than previously thought, said researcher Thomas Mörs, paleozoologist at the Swedish Museum of Natural History.

Based on 53-million-year-old fossils of whale-like, semi-aquatic mammals, scientists had thought mammals gave rise to whales in a process that took 15 million years. The new find suggests it took just 4 million years.