As a guide, one of the first things I tell international visitors to Tiritiri Matangi is that New Zealand has no native land mammals apart from bats.  I suspect many other guides do the same.  It certainly is quite an unusual fact and accounts for so much of why New Zealand bird life (and indeed all New Zealand life) is the way it is.  I wonder how many guides go on to explain the absence of land mammals by some variation of the statement that New Zealand broke away from the rest of the world about 60 MYA (million years ago) before mammals evolved?  I know I did when I first started guiding.  It was one of those statements that I sort of grew up with and didn’t think to question.  I wasn’t guiding for long though before I did begin to wonder how true that statement really was, and so I began to dig a little deeper. It turns out there are several things wrong with the crude statement: “New Zealand broke away from the rest of the world about 60 MYA before mammals evolved.” The truth—or at least the best current guess at the truth (because science is always evolving as new facts come to light)—is even more fascinating.

This article and the next two give a summary of the latest thinking on the story of mammals in New Zealand. This story goes under three headings:

1. When did New Zealand really break away from rest of the world?
2. When did mammals really evolve?
3. Were there ever mammals in New Zealand before humans arrived (other than bats)?

This article begins the story by answering the first question. This is more of a question in geology and not really related to mammals explicitly. Nevertheless, it is necessary background information on which to overlay the answer to (2). I also think it’s rather an interesting question in its own right.

Before I begin with the first question, here are the short answers to all of them:

1. About 80 MYA.
2. About 205 to 210 MYA.
3. Yes!

Intrigued? Read on…

When did New Zealand really break away from the rest of the world?

The first question here is, who wants to know?  Most of the time the answer to when New Zealand broke away from the rest of the world is given by geologists, while much of the time the question is asked by people interested in plants and animals, i.e. people with an interest in biology.  The end result is confusion, because the two groups of people are often talking about two different things.  If you’re thinking about plants and animals, it’s rather fundamental to know whether they are on dry land or under water.  Thus when biologists want to know when New Zealand broke away from the rest of the world, they want to know when there was some water between the two landmasses.  Sounds obvious, doesn’t it?

Well, not to a geologist!  You see, geologists tend to talk about rocks more often than water.  When they talk about landmasses being separate, they often mean the underlying rocks of the two landmasses are separate rather than just being separated by sea.  The difference here is that it is possible for underlying rocks of two dry landmasses to be joined even when there is no dry land joining them.  When underlying rocks of two separate dry landmasses are joined, geologists often say the two landmasses themselves are joined, whereas biologists would say they aren’t. The North and South Islands are good examples here, as is Great Britain and the European continent.  Both these pairs of landmasses share common underlying rocks, yet both have sea separating them.

Are geologists just trying to be annoying and confuse everybody?  Not at all.  They are simply distinguishing between the two fundamental types of earth’s crust.  The earth’s crust, i.e. the outer layer of the earth, is composed of two fundamentally different types of crust: oceanic crust and continental crust.  Oceanic crust forms the bottom of the deep oceans and is generally at a depth of around 4km under the surface (but can be up to 11km in trenches).  Continental crust forms the large landmasses of the earth including where these landmasses slope down to meet the oceanic crust at depth under the sea (see Figure 1).  When geologists say two landmasses are separate, they tend to mean there is oceanic crust between them.  One implication of this is that there is sea at a depth of around 4km or more between the two landmasses.  This is quite different from saying there is no sea between the pieces of land at all!  The reason geologists tend to talk about landmasses as pieces separated by oceanic crust, rather than pieces separated by sea, is that each piece of continental crust behaves as a unit, i.e. it changes and moves around as a segment of the earth’s crust independently of where the sea happens to be at any one time. 

What’s all this got to do with when New Zealand broke away from the rest of the world?  Well, that rough figure of 60 million years for when this occurred is actually more of a geologist’s answer.  It’s roughly the time when the continental crust of New Zealand became properly separated from any other piece of continental crust and there was oceanic crust completely surrounding New Zealand.  But there would have been an awful lot of sea between New Zealand and any other landmass then—an awful lot of sea that land animals wouldn’t have been able to cross for quite some time before this. 

So what is the answer for biologists? When was there water between New Zealand and the rest of the world?  The short answer is given above, i.e. about 80 MYA.  To explain this, i.e. to reconcile the two answers, we need to return to geology for the story of the formation of New Zealand.  The outline of this story has been known for a few decades now.  In 2022, though, GNS Science released a series of fifteen maps showing New Zealand’s formation from 98 MYA ago to the present day (1).  These maps give a much clearer picture than ever before of the story of New Zealand’s formation.  A summary of this story is as follows (see Figures 3–6 for a selection of the GNS Science maps illustrating this story—and see the footnotes for examples of the processes described operating in today’s world if you’re interested): (2)

• About 180 MYA there was a supercontinent called Gondwana (see Figure 2).  It comprised the land that is now India and the continents of the southern hemisphere (including Antarctica).
• By about 105 MYA Gondwana had broken up into various pieces, but New Zealand, Australia and Antarctica were still joined together.
• A rift began to form from this time separating New Zealand from the combined continent of Australia and Antarctica.  As the rift formed, the land around it stretched and sank. (3)
• Sea began to appear in the rift soon after it formed. (4)
• The rift worked like a zip, unzipping New Zealand from Antarctica first and then working its way up towards and then along the coast of Australia.
• Oceanic crust began to appear in the rift about 85 MYA. (5)
• Sea completely surrounded New Zealand by about 80 MYA.
• The continental crust that New Zealand sits on (6) became completely isolated from the continental crust of Australia and Antarctica about 60 MYA. (7)
• Oceanic crust continued to form between the two pieces of continental crust until about 55 MYA. (8)

So when did New Zealand break away from the remnants of Gondwana?  Well, to be precise, it’s a story spread over many millions of years.  If one really needs to put a single date on it though, a geologist might want to say about 60 MYA, being the time New Zealand was surrounded by oceanic crust.  Alternatively, a geologist might want to say about 85 MYA for when oceanic crust first appeared.  If, however, one is interested in New Zealand life (which includes most Tiritiri Matangi people),

then the best figure is probably about 80 MYA for when New Zealand was completely surrounded by sea.

So there you have it—the first piece of the puzzle in the story of mammals in New Zealand.  Part two of this series will give a brief summary of the evolution of mammals globally, with an emphasis on the story of mammals in Australia.  Given that New Zealand is a chip off the Australian (and Antarctic) block, the Australian story forms the opening chapter of the story of mammals in New Zealand.

Footnotes:

(1) These maps can be downloaded and a video of the map sequence can be found here: https://www.gns.cri.nz/data-and-resources/the-100-million-year-history-of-te-riu-a-maui-zealandia-in-maps/.

(2) Several relatively recent sources give dates for the events described, such as Mortimer and Cambell’s Zealandia and Gibbs’ Ghosts of Gondwana.  Most of the dates given above come from the former source.  Where possible, I have estimated other dates from the more recent pictures by GNS Science, e.g. the date of 80 MYA for New Zealand being completely surrounded by sea.

(3) A contemporary example of a rift forming in a continent is the Great Rift Valley of east Africa that runs through countries such as Kenya, Tanzania, Uganda and Ethiopia.  If this rift continues, east Africa will eventually separate from the rest of Africa like New Zealand has from Australia and Antarctica.

(4) The Red Sea is a more contemporary example of sea flooding into a continental rift.  In fact, the Red Sea is part of the rift forming the Great Rift Valley in Africa (see previous footnote).

(5) Oceanic crust has appeared in the Red Sea (see previous footnote).

(6) This piece of continental crust is called Zealandia. In addition to New Zealand, it includes other pieces of dry land, such as the distant Chatham Islands, the sub-Antarctic islands, and even Norfolk Island and New Caledonia.

(7) Madagascar is similar to New Zealand in that it split from remnants of Gondwana and is now completely surrounded by oceanic crust.

(8) Oceanic crust continues to be formed today in rifts formed a very long time ago. One of the most well-known examples is the Mid-Atlantic Ridge that runs all the way down the middle of the Atlantic Ocean. The oceanic crust being formed there widens the Atlantic Ocean by about 2–5cm per year.

References:

Gibbs, G. 2016. Ghost of Gondwana: The History of Life in New Zealand (revised edition). Potter & Burton, Nelson. (Chapter 4)

Lomolino, M.V., Riddle, B.R. & Whittaker, R.J. 2018. Biogeography: Biological Diversity across Space and Time (5th ed.). Sinauer Associates, Sunderland, MA. (pp. 250–254, 258)

Mortimer, N., Campbell, H. 2014. Zealandia: Our Continent Revealed. Penguin Books, Auckland. (pp. 202–204)

Pough, F.H, Bemis, W.E, McGuire, B. & Janis, C.M. 2023. Vertebrate Life (11th ed.). Sinauer Associates, New York. (p. 492)

Wicander, R. & Monroe, J.S. 2016. Historical Geology: Evolution of Earth & Life Through Time 8th ed.). Engage Learning, Boston. (p. 50)

Picture credits:

• Figure 1: My own work (can you tell?!)
• Figure 2: Zealandia website: https://www.visitzealandia.com/About/History/The-Continent-of-Zealandia (slightly modified).
• Figures 3–6: https://www.gns.cri.nz/assets/15-Maps-of-Zealandias-History.zip. The original source of the maps is:
  Strogen, D. P., Seebeck, H., Hines, B. R., Bland, K. J., & Crampton, J. S. 2022. Palaeogeographic evolution of Zealandia: mid-Cretaceous to present. New Zealand Journal of Geology and Geophysics, 66(3), pp. 528–557. (https://doi.org/10.1080/00288306.2022.2115520)