A New Zealand geologist has dated a prehistoric whale fossil to a period much more recent than previously thought, raising new questions about the evolution of modern mammals.

The fossil of a dwarf baleen whale from northern California was estimated at 700,000 years old, compared with the previously youngest known fossils of the same species dated at about 3 million years, according to University of Otago researcher Robert Boessenecker.

The fossil showed that the 4- to 5-meter long Herpetocetus, thought to be the last survivor of the primitive baleen whale family called cetotheres, had survived from the pre-Ice Age Pliocene epoch, a time before many modern marine mammals appeared.

Baleen whales of the type were most common much earlier, about 10 million to 15 million years ago.

"That this whale survived the great climatic and ecological upheavals of the Ice Age and almost into the modern era is very surprising as nearly all fossil marine mammals found after the end of the Pliocene appear identical to modern species," Boessenecker said in a statement.

"Other baleen whales underwent extreme body size increases in response to the new environment, but this dwarf whale must have still had a niche to inhabit which has only recently disappeared."

The find indicated that the emergence of the modern marine mammals during the Ice Age may have happened more gradually than currently thought, he said.

Baleen whales lack teeth and instead use baleen to strain small prey like krill and fish from seawater.