Date a dinosaur

Dinosaurs used to live in New Zealand. We know this because their fossils have been found in a few places. The fossils of a number of different dinosaurs were found at the Mangahouanga Stream, in north-west Hawke’s Bay, by Joan Wiffen and her colleagues. They were found together with fossils of land plants, including pollen from trees and tree ferns, as well as with fossil marine animals.

The remains of dead land animals and plants were probably washed down a river and into a shallow bay, where they mixed with the remains of dead marine animals. This material was compressed together with sediments into a sandstone rock. The fossils and rock tell us about what lived in prehistoric Hawke’s Bay and what the environment was like.

To add to the picture of prehistoric Hawke’s Bay, paleontologists have tried to find out when the Mangahouanga fossils lived. These include Dr James Crampton and colleagues at GNS Science and others at Victoria University of Wellington.

Relative dating

James and the other paleontologists used relative dating first, which looks at where rocks fit in a sequence. The sandstone containing the fossils is above a layer of late Jurassic greywacke and under a layer of late Cretaceous mudstone. The sandstone and its fossils are therefore from a time somewhere in between, so all that could be said was that the fossils were from the Cretaceous period, which lasted for 80 million years.

Fossil correlation

The next dating technique was fossil correlation. Large fossil clams found at Mangahouanga were compared to fossils in rocks at other places that had already been dated. The evidence pointed towards the fossils being from the Piripauan and Haumurian stages of the late Cretaceous. This narrowed the dates of the Mangahouanga fossils down to a 20 million year range.

Dinoflagellate fossils

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The presence of these two fossil dinoflagellates in a rock can help date it to the late Cretaceous. The fossils are only about 200µm long.

Micrograph by Poul Schioler.

Rights: GNS Science

To narrow the dating down further, more correlation was done using microfossils found alongside the larger fossils. The microfossils that James and others study are single-celled marine organisms called dinoflagellates. Part of the dinoflagellate life cycle has a resistant case that survives well as a fossil.

Dinoflagellate fossils around the world have been studied in deep-sea cores that contain rock records going back many millions of years. The times when different species appeared and then became extinct have been dated using paleomagnetism and radiometric methods. Dinoflagellate fossils found at Mangahouanga were compared to this worldwide reference material.

Several pieces of rock from Mangahouanga, each containing a dinosaur or marine reptile fossil, were examined by scientists Martin Young and Michael Hannah. The rock was broken down using hydrofluoric acid, leaving the resistant dinoflagellate fossils. The species were identified using a microscope and then compared to species known and dated in deep-sea cores.

Late Cretaceous dinoflagellates

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The times when dinoflagellates appeared and became extinct have been dated in deep-sea cores around the world. Their presence is used to date rocks containing dinosaur fossils at Mangahouanga.

Rights: The University of Waikato Te Whare Wānanga o Waikato

One rock contained a mosasaur fossil alongside the dinoflagellate species Satyrodinium haumuriense and Vozzhennikovia spinulosa. These species lived together between 77 and 81 million years ago in the lower Haumurian stage, so we can say that the mosasaur lived at some time in this 4 million year period too.

The two rocks with dinosaur fossils only had a single dinoflagellate species each. This meant that the result was not so precise. One dinosaur fossil was found with the dinoflagellate Odontochitina porifera. This lived right from the Piripauan through to the middle of the upper Haumurian – a period of 16 million years. The other dinosaur fossil was associated with the dinoflagellate Odontochitina spinosa, which lived from the mid-lower Haumurian onwards.