Fossils are the preserved evidence of past life. They may include organic remains such as wood, shells, bones and teeth that have been buried, mineralised, and turned to stone. But the term fossil can also apply to anything dead and preserved. Fossils can be mummified, pickled, dried out or frozen. There are also trace fossils – records of animal behaviour such as burrowing, breathing, feeding, defecating, moving, resting, mating, nesting, growing, playing, exploring, fighting and hiding.
Most fossils are mineralised. To become a fossil a dead organism must be preserved, which requires eliminating the agents of decay and destruction – oxygen, bacteria, radiation and erosion. Fossil preservation varies greatly, and the optimum conditions for preservation rarely occur.
How fossils are preserved
When plants and animals die, they generally rot or are eaten. This happens quickly – in a matter of days or months. The hardest parts (wood, teeth and bone) are the last to decay. If they become buried in sediment such as gravel, sand, silt and especially mud, they are most likely to be fossilised. Fossilisation often involves shell, bone and wood being slowly replaced by minerals.
Sedimentary rocks and fossils are a history of past environmental conditions. Rock layers can be thought of as pages in a book and fossils as words. While the order of the pages is important, the words reveal the most.
Species are restricted to a certain geological time period; they evolve, live for a certain time and then become extinct. This makes fossils useful – finding a certain species, and knowing when it lived and died, enables geologists to date rocks.
Geologists use a chronology known as the geological timescale. It is divided into chunks of time called periods. The age scale of geological periods is measured in millions of years, and is denoted by the abbreviation Ma (Mega-annum in Latin). For example, 30 Ma means the rock formed 30 million years ago.
Geologists have relied on fossils to establish periods, and to tell them apart. A new period generally represents some abrupt change to life on earth. For example, the end of the Permian period is marked by a major extinction event. The following period, the Triassic, is characterised by very different fossils.
By international agreement, the major time periods have been subdivided into epochs. For example, the early Cenozoic period includes the Paleocene, Eocene and Oligocene epochs, and the Jurassic includes the early, middle and late Jurassic epochs. Epochs are further subdivided into stages, so the late Triassic epoch includes the Carnian, Norian and Rhaetian stages.
New Zealand has a very detailed local timescale with its own set of formal names. For example the late Miocene epoch includes the Tongaporutuan and Kapitean stages. These are named after places where rocks of this age are well exposed.
New Zealand’s fossils
New Zealand is rich in fossil-bearing sedimentary rocks. A huge diversity of past life is preserved, including trilobites, graptolites, brachiopods, belemnites (relatives of squid), ammonites, dinosaurs, marine reptiles, whales, turtles, crocodiles, fish, sharks, crabs, penguins and other aquatic birds. Many plants have lived on the ever-changing landmass of New Zealand, and these are seen as coal, fossil seeds, pollen, and up to full-size fossil tree parts. There are also large numbers of microfossils, which can only be seen clearly through a microscope.
While all geological periods from Cambrian (542–490 million years ago [Ma]) to Cenozoic (65–present day) are represented, New Zealand’s fossil record is not especially complete, and is poor for a few periods. For instance, very few Carboniferous (359–299 Ma) and Silurian (443–417 Ma) fossils have been found in New Zealand.
Joan Wiffen, the first fossil collector to extract dinosaur bones in New Zealand, explains the genesis of her interest:
‘As a child in the early 1930s, I roamed the bare brown hills of the Hawke’s Bay sheep country. There were shelly layers in the rough limestone outcrops, particularly where the weather and rubbing animal bodies wore ledges into the rockfaces along the tracks. I would run my fingers over the shells and wonder how they got there’. 1
The fossil record is dominated by the remains of marine life. The most readily visible fossils are late Cenozoic shells (less than 23.8 Ma). The most common fossils are of plankton – the skeletal remains of microscopic single-celled plants and animals.
There are also terrestrial or freshwater sedimentary rocks and fossils. The most widespread are coal deposits, which form from plant debris. These accumulated on land, mainly in lakes, rivers and swamps. Fossil pollen and spores are also abundant. Curiously, very few terrestrial animal fossils have been found, and the fossil record of modern birds (including moa and kiwi) and reptiles (tuatara, geckos, skinks) is surprisingly short – less than 2 million years.