Palaeontologically speaking, New Zealand can’t boast glamour sites like the Burgess Shale or Dinosaur National Monument. However, we are particularly well endowed with fossiliferous marine sediments, which provide a record of the life and the environmental conditions that have prevailed in our part of the world over the past 500 million years.
It is not just the fossils themselves that are significant, but the fact that we have a complete listing of all known fossil localities from which collections have been made. Few other countries have such an inventory, and in this regard New Zealand is the envy of the palaeontological world.
The catalogue, called the New Zealand Fossil Record File, is maintained by the Geological Society of New Zealand and the Institute of Geological and Nuclear Sciences. Initiated in 1944 by one of New Zealand’s most notable geologists, Harold Wellman (see New Zealand Geographic, Issue 41), it includes about 80,000 entries, and is still growing. The file is in constant use as a database for oil and mineral exploration, research and mapping purposes.
Locality CH/f623 on the file is of greater significance than its humble designation suggests, for it was the very first fossil locality in New Zealand to be documented. Surprisingly, it is not one of the many mainland locations at which fossils are abundant, but is in an obscure corner of the country and was made by Johann Karl Ernst Dieffenbach in 1840.
Dieffenbach was a German explorer and geographer known for his high humanitarian ideals and his politically liberal views. He had fled Germany as a political refugee in 1836, ultimately reaching London. There he became known in natural history circles, and was recommended to the New Zealand Company by such scientific luminaries as Charles Darwin and Sir Charles Lyell.
Dieffenbach was appointed to the position of naturalist to the New Zealand Company in April 1839, at the age of 28, and accompanied Colonel William Wakefield on the Tory, arriving in Queen Charlotte Sound on August 16, 1839. This important voyage of exploration and surveying was the precursor to the establishment of the New Zealand Company colonies. It is likely that Dieffenbach was the first professional scientist to live and work in New Zealand.
Between May and July 1840, Dieffenbach visited the Chatham Islands in the Cuba, accompanied by Richard Davies Hanson, whose main objective was to purchase the islands for the New Zealand Company.
Dieffenbach is remembered in the landscape. Mount Dieffenbach, or Hemokawa, is a prominent hill rising 134 m above sea level—one of 15 small volcanic centres that dominate the otherwise low, flat landscape of northern Chatham Island.
It was during his time in northern Chatham Island that Dieffenbach recorded fossils. He gives a detailed description of observations made along Tumaio Beach, to the east of Tioriori, noting “a dark green friable sand,” above which lay “a horizontal bed, from half a foot to a foot [15-30 cm] thick, of broken decomposed shells of the oyster and nautilus tribe.”
On the modem geological map of Chatham Island, the “green friable sand” is called the Tutuiri Greensand, and the “bed of broken decomposed shells” the Tumaio Limestone. The fossils in these sediments indicate an early Eocene age, 53-55 million years old.
Dieffenbach left New Zealand in October 1841 and returned to England with his collections, including the fossils he had collected at Tioriori. Two species of oyster were subsequently recorded from the location.
For much of the last century and a half, Dieffenbach’s fossil locality has been hidden from view, buried under drifting dune sand. Then, in 1997, the sands dispersed and the site was able to be studied afresh.
We now know that the Tumaio Limestone contains a number of species of bryozoans, corals, brachiopods, bivalves, gastropods and echinoderms. About 75 species of foraminifera have also been identified. No nautiloid fossils have been recovered—despite Dieffenbach’s claim.
The deposit is the remnant of a blanket of limestone which covered the entire Chatham Islands area 55 to 50 million years ago. Perhaps the most spectacular expression of this limestone is the cuffed western shore of Te Whanga Lagoon, the great central lagoon on Chatham Island. The cliffs here are entirely made up of the skeletal remains of marine animals and plants that thrived in water depths of 150 m to 250 m. During the Eocene the Chathams were truly oceanic, and totally submerged but for a few volcanic islets.
Though extremely abundant, the fossil materials that make up the limestone are generally fragmented, and are derived from mainly small, inconspicuous plankton, various kinds of seaweed and sea urchins. Shelly fossils such as molluscs, brachiopods and barnacles are rather uncommon. The most prized fossils are the teeth of a giant prehistoric shark, Carcharadon, which may have reached 16 m in length.
The limestone is of particular interest to palaeontologists partly because it is so fossil-rich, but also because limestone of Eocene age is not known from mainland New Zealand. However, it is known from exploratory drillholes and dredge samples on the Chatham Rise, a broad submarine ridge that extends for over 1000 km due east of Banks Peninsula and reaches to within about 400 m of the surface along its crest. In a sense, the Chatham Islands are a window through which we can observe and sample the geology of the vast Chatham Rise.
Today, the Rise is immensely important to New Zealand’s fishing industry, and is rich in mineral resources. Around 100 million years ago, however, there was continuous land along the Chatham Rise, with mountains, forests, rivers and, no doubt, dinosaurs.
All was to change, though, and between 100 and 55 million years ago the New Zealand continent gently subsided with respect to the Earth’s surface, and much of it was submerged beneath the waves. Dieffenbach’s site was down there, too, receiving the gentle rain of biogenic calcium carbonate that would one day attract the roving explorer’s eye.
As you watch the millennial sunrise over Pitt Island, spare a thought for the ups and downs of the land, and of the fossil bed that was lost and then found.