Dr James Crampton, paleontologist at GNS Science, explains how relative dating was used to determine that dinosaur and other fossils found in north-west Hawke’s Bay were from the Cretaceous period. Microscopic fossils, found with the dinosaurs, were correlated with others dated at other places to provide more precise dates.
DR JAMES CRAMPTON
Dating rocks traditionally has been done using the fossils they contain. The geological time scale in the first instance is really just an ordering of fossils. You put them in order, you know, from all around the world, this information is built up through hundreds and thousands of scientists. You know that fossils always appear in a certain order as you go up through the layers of strata and it’s really only relatively recently we’ve actually been able to put numbers on when those different species lived.
So dating Joan Wiffen’s fossils was firstly done just using the fossils themselves. First off, we knew, broadly speaking, the rocks in that area are what we call the Cretaceous period. Fossil shellfish and microscopic fossils were used to date the rocks. And then, not at Mangahouanga Stream itself but using information from other places, we can say, well, we found this fossil at Mangahouanga Stream and if we go somewhere else we can use other techniques and we can say, well, actually this fossil is about 80 million years old, and that’s a process called correlation – you get information from somewhere else but you use the fossils to equate the ages.
To date more precisely, what we’ve done is actually extracted little bits of the sandstone from right in the same bolus as the bones and we can look at the microscopic fossils that are found in that sandstone, and some of these microscopic fossils we know the ages of quite precisely, so this is very useful because even if a particular boulder that’s in the stream bed and it’s got a lovely bone in, even if that boulder has no other marine fossils in it, and even if we don’t know exactly where it came from in the formation, we can use these microscopic fossils to date it.
The key microscopic fossils we’ve used are things called dinoflagellates and also pollen, So dinoflagellates are single-celled algae – they’re abundant in the oceans today. In other places in New Zealand, we’ve got sequences of the different dinoflagellate species up through time, and we’ve dated those very precisely using a range of other methods – paleomagnetism and radiometric dating – so when we find them in the boulder with Joan Wiffen’s bones, we can say what age the bone is.
Charles Fleming Marine Fossil chart, courtesy of NZETC, Victoria University of Wellington Creative Commons Attribution ShareAlike 3.0
Australasian Pollen and Spore Atlas Creative Commons Attribution ShareAlike 3.0
Additional footage from GNS Science