A bluebell at the KT boundary.

Wahlenbergia matthewsii

The Geological time scale is divided up into Eras, Periods, and Epochs, and their boundaries are marked by evidence of rapid, even catastrophic, change.  The most famous is probably the one formed by the mass extinction at the end of the Mesozoic, known as the KT boundary (K for Cretaceous, to avoid confusion with the Cambrian; T for the Tertiary, nowadays called the Cenozoic with a slightly different definition).  This mass extinction is famous because it included the extinction of the dinosaurs, about 65 million years ago (I should say the non-Avian dinosaurs though, because small dinosaurs with wings, beaks, and feathers are very common today, only we call them birds because they seem so different from the lumbering dinosaurs that were their ancestors).

Wherever the boundary between Cretaceous and Tertiary rocks (we should perhaps say Cretaceous and Paleocene) is exposed on the Earth’s surface, it’s a place of great interest to geologists who want to examine fossils to see what was living in that location on either side of this mass extinction.

Students examining the KT boundary, Marlborough, New Zealand

In New Zealand there’s an accessible location on the Kaikoura coast where the KT boundary can be seen, and geologists have drilled cores out of the rocks on both sides of the junction.  The exposure is in a narrow limestone gorge that’s home to some of the iconic plants of the Marlborough region.

Wahlenbergia matthewsii and Pachystegia insignis on a
limestone cliff near the KT boundary in Marborough,
New Zealand

Among them is Wahlenbergia matthewsii (towards the top of the picture), a large-flowered native bluebell, and Pachystegia insignis (near the middle of the picture), the Marlborough rock daisy.  Not so long ago we used to think that many of our distinctive native plants were as old as the KT boundary.  It was thought that they, or at least their ancestors, had been in New Zealand ever since it broke off from Gondwana, 80 million years ago.  The opposite view has developed in recent years, and we now have good evidence that many of our plants arrived here very recently.  This is especially so for our alpine plants, which makes sense because alpine environments are relatively young here.  These newly arrived plants have come from all over, but by far the commonest source is Australia.  Some have come from South America or Africa, or even the northern hemisphere, but often these have established in Australia first, then crossed the Tasman to New Zealand.

The native bluebells are a case in point.  The genus Wahlenbergia seems to have originated in Africa about 30 million years ago.  Their relationships and evolution have been studied recently by Christopher Cupido in South Africa and Jessie Prebble in New Zealand using DNA sequences to trace their history.  From Africa, the group arrived first in Australia, where from a single ancestor it evolved into quite a large group over almost 5 million years.  Then, between 1.6 and 0.7 million years ago, two Australian bluebells crossed the Tasman to New Zealand, establishing two species groups in this country.  Another Australian species even seems to have found its way home to Africa.  At about the same time, some iconic Australian plants like Eucalyptus, Acacia, and Casuarina went extinct here in New Zealand.

Figure from Prebble et al. (2011) that shows the evolution and dispersal of southern bluebells.

Reference.

Prebble, JM; Cupido, CN; Meudt, HM; Garnock-Jones, PJ. 2011.  First phylogenetic and biogeographical study of the southern bluebells (Wahlenbergia, Campanulaceae).  Molecular Phylogenetics and Evolution 59: 636-648