Perhaps the most surprising thing about Nothofagus leaf fossils in New Zealand is – not finding them. Nothofagus is another name for the southern beech trees that form forests in New Zealand, as well as Australia, Patagonia, New Caledonia and New Guinea. As a group, the beeches are… Read more
One of the more evocative Miocene fossils you might pick up near Bannockburn, New Zealand, are she-oak ‘cones’ (see the featured image). The Latin name is Casuarina (but see ‘Technical Details’, below). This is a plant that no-longer grows naturally in New Zealand, but is a tree in… Read more
Most conifer leaves have just one vein, whether they be the needles of pines, or the much broader leaves of some tropical conifers. This limits their size and shape (they mostly stay small and can’t do fancy stuff like many flowering plant leaves). Just two… Read more
The biodiversity of Blue Lake, at St Bathans, New Zealand, is precisely zero. It is an artificial lake partly filling a hole blasted out in the search for gold in the 19th century. The hole is directly in front of one of St Bathan’s and New Zealand’s gems – the Vulcan Hotel. If you ran out the front door and forgot to stop at the cliff marking the edge of the old workings – you would land in one of the most biodiverse fossil plant sites anywhere on the planet.
The gold miners were sluicing quartz-rich gravel and sands. But in among those are thinner beds and lenses of mud. These are often full of plant fossils of Miocene age – say around 17-18 million years. Sometimes these are amazingly-preserved whole leaves – the featured image and one below show fossil leaves that have been floated out of the mudstone and then set in glycerine jelly between sheets of plastic. But often the fossils are just a hash of leaf fragments. This looks like a handful of compost at first, but it’s this hash, plus the extremely good preservation of that hash – that can make just a single handful of mud rich with fossils.
On the slope marking the edge of the Blue Lake digging and in front of the Vulcan Hotel there used to be a pine tree. It was my marker to locate a lens of mud remarkable by itself for its fossil plant content. In a few handfuls of mud from that lens were seven genera of conifers and at least 48 flowering plants. The conifers included some of our familiar New Zealand trees – like matai (Prumnopitys) and rimu (Dacrydium). But there were also surprises. There was Acmopyle – unique as a ‘hairy’-leaved conifer, and only growing in Fiji and New Caledonia now. Also Retrophyllum, a conifer with a distinctive paired arrangement of leaves along its shoot, but now found only in New Caledonia, Melanesia and South America.
And that little lens was just the start of it. Blue Lake has many such lenses, and over the hill is a ‘sister-lake’ – one of the local names being ‘Grey Lake’. It has a similar kind of geology. The sands and mud exposed in these lakes were deposited in an ancient river flowing along what geologist Barry Douglas (1986) has called the ‘St Bathans PaleoValley’ (part of the Manuherikia Group). It came from uplands in the west, to the coast somewhere to the east. In both Blue and Grey Lake muds I’ve now recorded a total of 13 conifers, 144 flowering-plant types and a further two cycad-like ones. To put this biodiversity in context, As a comparison, today there are around 215 species of tree in the entire New Zealand region (including the subtropical Kermadec Island; McGlone et al., 2010) and nine genera of conifers. So in an area of just a few hectares – there are more fossil conifer types than in all of New Zealand today. This is one of the most biodiverse, and perhaps the most, Miocene plant fossil localities anywhere.
What caused this high biodiversity? It was certainly warmer – the climate was warm temperate or even subtropical. Average temperatures would have been at least 6-7 degrees warmer than today. But perhaps more importantly, the cooler temperatures would have been much warmer. The harsh frosts and snow that St Bathans gets now, would have been entirely unknown. The kinds of rainforest plant fossils found at Blue Lake prove that rainfall too, would have been higher and more consistent, quite unlike the low and drought-ridden climate that the area has now. On top of that, Douglas considered that the ancient river was ‘braided’. This is a type of river that has many channels, and switches between them from time to time. This process keeps vegetation in various stages of ‘succession’, allowing many species a chance to find their niche.
The Miocene plant fossils of Blue and Grey Lakes are a treasure -trove and much remains yet to be understood. Many of the fragmentary plant fossils are still unidentified. They are clearly not plants living in New Zealand today – but where will similar plants turn-up? New Caledonia? Patagonia? Madagascar? And just what were the plant communities that lived in the St Bathans Paleovalley? How many new fossils wait to be found?
And that first lens of mud? The one beside the pine tree? Pine trees are an introduced, often invasive conifer in New Zealand. Their spread across parts of New Zealand is causing problems (they shade out smaller natives, acidify the soil and are a fire hazard) and so the Department of Conservation is doing their best to control the pines. That pine, one of the few trees in an otherwise naturally vegetation-free spot, was removed, and along with it, much of that biodiverse fossil lens. In fact, with the pine tree gone and the disturbance that created, I’ve had trouble re-locating it.
Something very ironic there!
Links will take you to a site to download pdfs of the papers.
Pocknall, D.T., 1982. Pollen and spores from Blue Lake, St Bathans (H41) and Harliwichs Lignite Pit, Roxburgh (G43), Central Otago, New Zealand. Palynology Section, NZGS, Lower Hutt.
McGlone, M. S., S. J. Richardson, et al. 2010. Comparative biogeography of New Zealand trees: species richness, height, leaf traits and range sizes. New Zealand Journal of Ecology 34: 1-15.
Pea pod fossils in New Zealand were first found by Aline Holden, a pioneer of New Zealand plant fossil research. She found the first ones at Bannockburn in 1981, while working on her PhD, and then found more in the Nevis Valley.
In 1987, my PhD professor, J.D. Campbell and his wife, Anne, dropped in unexpectedly in Alexandra (my home). They were on their way to the Nevis Valley and wanted to know if I would come too. The Nevis Valley has a well-known oil shale deposit, and this includes fossil leaves and scattered fish bones of Miocene age (c. 18-17 million years). Once in the area, ‘JDC’ focussed on shale near the track, while I set off to explore up a shrubby side valley. I was on the way back when I spotted a likely outcrop up on the valley side. It was solid gold – to a fossil plant person that it is. Technically it was mudstone, but stuffed full of not only fossil leaves, but pea pods as well. I worked out as large a chunk as I could, strapped it to my pack frame, and then staggered back to the car with it.
Back in the lab, chipping away the overlying mudstone, revealed fossil pods (legumes) up to 130 mm long, and with 14 peas (seeds).
Today New Zealand has just four groups of peas (the legume family). There’s kowhai (Sophora), Kakabeak (Clianthus), the scree pea (Montigena) and the many brooms (Carmichaelia). All of these are part of the legume family that have classic ‘pea flowers’. One of the technical terms for this group is (or was) the Papilionoideae (think of the French word for butterfly – ‘papillion’).
But traditionally, there are also two other large groups of legumes. One is the Caesalpinioideae – with quite showy flowers (think Bauhinia), then the Mimosoideae. These have very reduced flowers, looking a bit like pom-poms (think of the wattles/Acacia).
The lucky find in the Nevis that day was not just the large fossil pea pods, some with the seeds (peas) in them, but some scattered leaves that were surely produced by the same plant. Based on the combination of evidence, the legume expert Dr Les Pedley, suggested most likely identification of the fossils was Serianthes. This plant is now found in New Caledonia, Fiji, and French Polynesia.
Serianthes is one of the Mimosoideae group. The Mimosoideae was already known in New Zealand based on fossil pollen, although in those cases, Acacia was considered the more likely parent plant (Pocknall and Mildenahll 1984; Mildenhall and Pocknall 1989).
Together, the pods, leaves and pollen make a nice addition to what used to grow in New Zealand. But what do these fossil mean? Based on where Serianthes grows today, the average annual temperature may have been more than 20 C. That’s about twice what it is now. Also a suspicion that the vegetation it was growing in was relatively dry.
I seem to spend half my life staggering kilometres with a pile of rock, either on my back or worse, carrying it in my arms. But in this case, it was well worth it.
Links will take you to my academia.edu site where you can download a pdf.
Mildenhall, D.C., Pocknall, D.T., 1989. Miocene-Pleistocene spores and pollen from Central Otago, South Island, New Zealand. New Zealand Geological Survey Palaeontological Bulletin 59, 1-128.
Pocknall, D.T., Mildenhall, D.C., 1984. Late Oligocene -Early Miocene spores and pollen from Southland, New Zealand. New Zealand Geological Survey Paleontological Bulletin 51, 1-66.
Pole, M.S., 1992. Fossils of Leguminosae from the Miocene Manuherikia Group of New Zealand, in: Herendeen, P.S., Dilcher, D.L. (Eds.), Advances in Legume Systematics: Part 4. The Fossil Record. The Royal Botanic Gardens, Kew, pp. 251-258.