Researchers from Otago University have linked an ancient marine dolphin fossil, found in Waitaki, with an endangered dolphin that lives in the river systems of Nepal, India, and Bangladesh.

The fossil was discovered in the Hakataramea River valley, and is one of only a few other dolphins known to have existed from about 22.5 to 24.5 million years ago.

At that time, New Zealand was just a cluster of small, low islands with extensive shallow, warm seas.

Professor Ewan Fordyce and his PhD student Yoshi Tanaka have formally named the new fossil species Otekaikea huata, which refers to the marine limestone formation it was deposited in and the Maori word for spear.

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The dolphin fossil belongs in the once-diverse and widespread dolphin group called Platanistoidea. The group is now extinct in the oceans, and is represented only by the endangered Ganges river dolphin and Indus river dolphin subspecies.

"Our study is one of several showing that the South Asian river dolphin lineage was more diverse in the past," Prof Fordyce said.

"It is not clear why this lineage became extinct in the oceans, but it could have been due to long-term climate change and competition from later-evolving ocean dolphins."

The Ganges River and Indus River dolphins are considered endangered, with large dam and irrigation projects threatening their habitat.