Zealandia, the New Zealand continent, broke away from Gondwana about 85 million years ago. As it did so, the floor of the Tasman Sea formed. Zealandia’s drift and the spreading of the Tasman Sea floor continued for some 20 million years.
As Zealandia drifted, it sank. This went on for 60 million years, until about 25 million years ago, it was almost totally underwater, and therefore almost all of New Zealand’s Cenozoic fossils are marine. The most commonly found New Zealand fossils, which people can stumble across in road cuttings, river beds, and along the sea shore, are fossil shells from marine molluscs.
Around 24 million years ago collisions began along the boundary of the Pacific and Australian plates. This process was responsible for the emergence of the New Zealand landmass and the modern land surface, and uplift is still happening.
The age of marine mammals
The last 65 million years, the Cenozoic, is referred to as the age of mammals – in the New Zealand context it is more accurately called the age of marine mammals.
New Zealand has a rich and diverse fossil record of marine mammals (mainly whales with some seals). Widespread limestone formations (32–22 million years ago [Ma]), especially in Canterbury and North Otago, have provided most whale fossils. Along with them are fossil penguin-like birds. So far no terrestrial mammal fossils have been found.
Cenozoic fossil-bearing sedimentary rocks drape the older Cretaceous land (145.5–65 Ma) like blankets on a bed. The oldest Cenozoic (65–23.8 Ma) fossils are shallow water shells, with deeper water forms appearing successively in younger rocks. Younger Cenozoic sediments are characterised by remarkably widespread limestone. So around 25 million years ago New Zealand was a huge underwater platform with perhaps a few small islands poking up above the waves.
Great, great white
The great white shark (Carcharodon carcharias) is the most feared predator in the sea, growing up to 7 metres long. Fossil teeth of a giant shark (Carcharodon megalodon), which lived from 1.6 to 25 million years ago, have been found in New Zealand. The largest fossil teeth are about the size of a person’s hand. Going by the size of its teeth, this shark would have been 13 metres in length.
Cenozoic fossils include many marine invertebrates. There are also cetaceans (whales), turtles, penguins, sharks and bony fish. New Zealand’s oldest whale fossils date from 40 million years ago. New Zealand has some of the oldest known early penguin-like bird fossils (55 Ma). The first fossil seals appear in the late Cenozoic period (23.8–0 Ma).
Swamps, coconuts and crocodiles
Coal deposits continued to form on land through the early Cenozoic period with associated wood, leaf, spore and pollen fossils. From about 24 million years ago as the land re-emerged, swamps established in low-lying areas (Southland, Waikato, and the King Country). When swamp deposits were buried they formed lignite (a poor-grade coal).
In other areas mountains were forming. Terrestrial plant fossils include wood, legumes, leaves, flowers, seeds, coconuts, pollen and spores. Miocene fossil-bearing sediments (23.8–5.3 Ma) are widespread. Fossils of freshwater crocodiles, birds, fish, crayfish and molluscs have also been unearthed from Miocene lake sediments in Central Otago. The climate was warmer at times during the early Miocene, with Eucalyptus and Casuarina appearing. Northland was almost tropical and a type of coconut palm (Cocos zeylandica) grew there.