An 85-million-year rift
About 85 million years ago a large fragment of eastern Gondwana began to break away, and the sea flooded the rift, forming the Tasman Sea. This land mass, today's New Zealand, drifted north. By 55 million years ago it was located between latitudes 60° and 50° south. It lay 2,000 kilometres from eastern Australia, a distance it has more or less maintained ever since.
New Zealand was surrounded by warm temperate to subtropical waters. As the land moved north, marine corals and tropical mangroves fringed the shores, and tropical and subtropical-type forests dominated. The land was gradually flooded by the sea and worn down by erosion. As the landscape and climate changed, plants and animals evolved or became extinct.
Similarities with Australia
For a long time New Zealand plants and animals were similar to those in Australia, as the climate and soils were alike. Typically, plants such as Casuarina (she-oaks), eucalypts (gum trees), Banksia (bottlebrushes) and Acacia (wattles) flourished in both lands.
By 5 million years ago Australia had moved northward into a drier subtropical zone. New Zealand lay further south in cooler, moister latitudes swept by westerly winds. Glaciers advanced and retreated as the climate repeatedly cooled and warmed.
New Zealand life forms which had adapted to warm temperate and subtropical conditions rapidly became extinct, and some distinctive groups evolved.
Primitive remnants or recent arrivals?
There has been much controversy about whether today’s native plants and animals are direct descendants of those on eastern Gondwana. Some New Zealand plants and animals evolved more than 85 million years ago – so they might already have been living there when the land split away. However, it is possible that they have not been in New Zealand continuously since the break-up, but spread later from other parts of Gondwana.
There is an excellent fossil record of flowering plants and conifers – making it easier to assess whether they have been in New Zealand continuously since it broke away from Gondwana, or arrived later. Recently, scientists have examined DNA differences between related species to estimate their age of origin.
Very few plants, if any, can be shown to have been continuously present. Many common plant families (for example, grasses, daisies and forget-me-nots) did not evolve until long after New Zealand had separated from Gondwana. They arrived by crossing the ocean.
Almost all living things contain DNA – the genetic blueprint that contains coded instructions as to how to make a particular species. The DNA of two different species can be compared to estimate how long ago they diverged from a common ancestor, on the basis that minor differences accumulate at a steady rate. This process is referred to as the molecular clock. The bigger the differences in DNA, the longer ago the two species diverged on separate evolutionary paths.
Even ancient groups may have arrived relatively recently. For years it was believed that southern beech (Nothofagus) had been in New Zealand for 85 million years. The presence of Nothofagus (or its fossil leaves) on many of the continental fragments of Gondwana (South America, Antarctica, Australia, New Caledonia and New Zealand) seemed to support this idea. The family Nothofagus has been in New Zealand for 85 million years, but fossil finds and DNA research suggest that today’s southern beech species are much younger. They evolved from an ancestor that arrived around 30–40 million years ago.
Animal groups may have evolved from life forms that were in New Zealand when it broke away from Gondwana. But so few fossils have been found that it is difficult to be sure. Groups that were probably part of the Gondwana fauna include tuatara, geckos, kākāpō, moa, wrens and some primitive groups of insects, spiders and earthworms.
Some of New Zealand’s plants and animals are primitive – but this does not mean they have been present for 85 million years. Some relatively primitive groups such as skinks and cicadas spread from New Caledonia and Australia. Skinks may have arrived around 30 million years ago, and cicadas within the last 24 million years.
It seems that the ancestors of many of New Zealand’s plants and animals made landfall long after the country split from Gondwana. This is astounding given the sheer distance between New Zealand and the nearest large land mass, Australia.