By Jon Todd, NHM London & Wolfgang Muller, Royal Holloway University of London
I’m Jon Todd, a scientist studying molluscs and based at the Natural History Museum, London. Together with Wolfgang Müller and Bill Wood, geochemists from Royal Holloway, University of London, we have formed a small exploration team. We set out with our driver each morning and explore around the available roads. The region around the town of Bontang where we are based is hilly and clothed in secondary rainforest, cut into by small farm plots, limestone and coal mines. Twenty years ago when the local map was made this area was sparsely inhabited rainforest with very few roads. So we use Google Earth and GPS units to map where the twisty roads run. As we drive around we look for the roadcuts and small quarries that pepper the landscape. When the rocks are freshly exposed they are grey or black and then we know that the fossil corals and molluscs will be well preserved. With time the rocks turn a more attractive orangey-red. Unfortunately the fossils are then very crumbly or sometimes have disappeared completely.
Yesterday, we walked up a track where we could see a few piles of mud that looked promising. At the end of the track we spotted a small drainage ditch maybe a metre deep which ran uphill. In the bottom of the ditch was dark lagoonal mud deposited, we think, about 10-15 million years ago. When the sea level rose a small coral reef of pencil-sized finger-corals grew directly on this mud. Today we returned to the outcrop with Emanuela, Sonja, two students, and Aries – a local palaeontologist from the Geological Survey of Indonesia in Bandung. They immediately started measuring the section, taking photographs and digging out big sacks of the corals and clams to be shipped back to Europe for study.
Tridacna, the Giant Clam, at one of our localities near Samarinda with camera case for scale
Wolfgang and I wandered off to look for nice specimens of the giant clam – Tridacna. This is the coral reef giant, up to a metre long, that legendarily clamps onto divers’ feet and drowns them – a myth of course. We had found a few scraps the previous day and were hopeful for more. Half an hour later I spotted a small piece of shell sticking out of the clay. We started digging, as we dug the clay away more and more shell became visible. It looked like we had a whole half shell (valve) of the giant clam that we had been seeking. But something wasn’t right – the shell was the wrong shape. A few more kilos of clay removed and it was clear we had a whole bivalved specimen. Now the challenge was to extract it from the ground. Two hours later, just as night fell and lightening sparked across the sky – we finally lifted the whole specimen from the ground. My job was over –now over to Wolfgang to tell you why this Tridacna might be very important...
Jon and Wolfgang hauling their heavy treasure back to the car
Well, Tridacnas for us (isotope) geochemists are essentially the perfect sample. In our quest to reconstruct the environmental conditions several million years ago, we use the hard parts of organisms to give us information about temperature, salinity, ocean productivity and so on. We call this ‘measuring proxies’ – namely, the concentrations of certain trace elements within the shell change systematically with temperature, salinity etc. And this is why Tridacna is so useful: we get more than a snapshot of life’s conditions some 10-15 million years ago, instead we obtain environmental information over several decades, possibly even 100s of years. This is possible because these shells grow like tree rings, regularly depositing shell material with time. By using a laser-based technique, we can reveal seasonally changing temperatures or salinities across these rings from our very distant past. Using this technique we hope to learn much more about oceanographic conditions in the past. This will help us understand how the current marine biodiversity hotspot developed.