Research by an Otago University geology student has uncovered a strange pre-Ice Age world where primitive porpoises and baleen whales roamed the North Pacific alongside comparatively modern marine mammals.

Studying hundreds of fossil bones and teeth he excavated from the San Francisco Bay Area's Purisima Formation, Robert Boessenecker has put together a record of 21 marine mammal species including dwarf baleen whales, odd double-tusked walruses, porpoises with severe underbites and a dolphin closely related to the now-extinct Chinese river dolphin.

Among his findings documented in the international journal Geodiversitas is a new species of fossil whale, Balaenoptera bertae, a close relative of minke, fin and blue whales.

The findings represent eight years of research on bones fossilised 5 million to 2.5 million years ago.