Fossil plants from the Miocene of New Zealand.

For the next couple of weeks, I’m travelling in the southern South Island of New Zealand with a group of students from Lewis & Clark College in Oregon, USA.  The main objectives of their trip are biological, but there are inevitable cultural aspects to a trip like this too.  For me, a by-product of a trip like this is blog-fodder.

Lewis & Clark group at Bannockburn

Some of the most impressive evidence of evolution comes from fossils, especially fossils of animals and plants that no longer exist, but for which we can see clear similarities with living species.  But even if there were no fossils, there's still plenty of evidence of evolution.

Today (2 January 2012) we visited a fossil deposit near Bannockburn, Central Otago.  This is a flaky mudstone that was laid down in layers in an old lake bed, between 14 and 19 million years ago.  The fossils here are mostly leaves, but similar deposits have contained remnants of a crocodile and a small mammal.  Plant fossils from this formation and their implications have been described in a series of papers by Daphne Lee of Otago University and her colleagues.

There are bedded leaves that look almost like raked-up autumn leaves, and others that are impression fossils between layers of the mudstone.  The commonest plants here are Eucalyptus, Acacia, Casuarina, and the Protea family (Proteaceae).  These plants are mostly absent from New Zealand today, and they shout out an affinity with just one modern location: Australia. 

Eucalyptus ancient & modern.

Geologists tell us that New Zealand in the Miocene was low-lying with a subtropical climate.  The low relief implies low soil fertility as minerals are leached out of the stable top soils.  The lack of mountains suggests perhaps a lower rainfall than today too.  In other words, it was an environment where Australian plants would feel right at home.

Casuarina infructescence.

Among these Australian plants, there are fossils of familiar New Zealand groups.  We found leaves that look like rimu (Dacrydium cupressinum), tutu (Coriaria arborea), New Zealand flax (Phormium), Coprosma, and southern beech (Nothofagus).  (I say “look like” because identifying fossils is a specialised skill.)

This impression fossil of scale leaves spiraling on a stem looks to me like a conifer.

So what happened to the Australian plants that dominated the New Zealand flora in the Miocene?  From no more than 10 million years ago, mountain uplift began and the New Zealand alpine fault became active.  The main mountain uplift is even more recent, only 1–2 million years old.  With this uplift came erosion, releasing minerals from the disturbed rocks, higher rainfall as the mountains intercepted the prevailing winds, and these coincided with a global cooling of the climate.  Eucalyptus, Acacia, and Casuarina went extinct at this time.  We lost most of our Proteaceae too, with only Knightia and Toronia persisting in the modern flora.  Meanwhile other groups invaded and some of them became dominant, especially in the new mountains.

This view of New Zealand’s history is supported by the fossil record and by new techniques that compare the relationships of plants using DNA sequences.  It seems the old view of an ancient flora that persisted here since New Zealand split from Gondwana 80 million years ago just doesn’t stack up any more.

I hope to add more pictures, hyperlinks, and references as soon as I get a faster connection to the internet.  (Posted here 13 January 2012)