Ancient dolphins – the fossil record
New Zealand provides important clues to the evolution and ancestry of modern whales and dolphins (cetaceans), thanks to an abundant fossil record dating back to the Oligocene period of 24–34 million years ago.
Pakistan has the earliest fossils of cetaceans, dating from about 50 million years ago. These animals were probably at home both in shallow waters and on land.
Over the next 15 million years ancient cetaceans evolved into diverse forms, exploiting the rich resources in the sea. They had well-developed legs and were dependent on the land for breeding, but their long, whale-like heads were adapted to searching the ocean for food. These toothed animals had not yet evolved sonar hunting methods (like modern dolphins), or filter-feeding (like baleen whales).
The cetacean species recognisable to us appeared about 5 million years ago. Between 12 and 15 million years ago the first true dolphins emerged, and there were key changes to body shapes in all species before they took on their present-day appearance.
Explosion of species
New Zealand fossils have played a key role in unravelling the secrets of cetacean evolution during the crucial Oligocene period. This saw an explosion in the diversification of species, and the appearance of the first toothed whales. The trigger was a cooling of the climate and the opening of the Southern (Antarctic) Ocean, which led to the development of massive quantities of krill and other small crustaceans – the favoured prey of baleen whales.
New Zealand fossil sites
There are dolphin and whale fossils in limestone and other marine rocks at sites at Punakaiki, Karamea, north-west Nelson, Wairarapa, Hawke’s Bay, Taranaki, and the South Auckland coast. But nowhere compares to North Otago’s Waitaki River valley for such fossils, partly because of the geological stability of the region. The rocks are less compacted than in the rest of the country, so the ancient bones are better preserved.
Here the sea reached 70 kilometres inland from the present-day coastline. Whales and dolphins used these calm waters for breeding, and as they died, their carcasses became embedded in marine sediments that turned to rock. Millions of years later the rocks were uplifted above the sea and then began to erode, leaving the fossil bones exposed.
Fossil finds around New Zealand
From the rocks, more than 30 Oligocene species have been uncovered. Among the finds are:
- a 34-million-year old baleen whale with characteristics of ancient and modern forms
- a shark-toothed dolphin (so-called because of its large triangular teeth) which was part of a widespread but now extinct group. Scientists speculate it was an agile fish eater which may also have preyed on penguins.
- a dolphin sporting a lance-like, 20-centimetre tusk. This fossil, closely related to the endangered Ganges River dolphin of India, is evidence that the ancestors of river dolphins were once widespread in the oceans.
- a fragmentary fossil of a sperm whale from early in the Miocene Epoch (23–24 million years ago). The sperm whale belongs to one of the oldest living whale families.
