New Zealand indicators of biostratigraphy
Many fossil groups are used for biostratigraphy in New Zealand. Large fossils (macrofossils), such as trilobites, graptolites or molluscs, have long been used by geologists to determine the ages of sedimentary rocks.
Examples of New Zealand fossils that are useful for identifying how old rocks are:
| Fossil Group
|
What they look like | Age Range |
| Belemnites |
 Bullet-like |
Late Mesozoic |
| Ammonites |
 Single spiral-shaped shell with several cavities |
Mesozoic |
| Gastropods |
 Coiled single shell with one cavity |
Late Palaeozoic to Cenozoic |
| Bivalves |
 Double-shelled, burrowing, attached or free lying |
Late Palaeozoic to Cenozoic |
| Bryozoans |
 Colonial; ‘lacy’ branched or domed |
Late Palaeozoic to Cenozoic |
| Brachiopods |
 Double-shelled ‘lamp shells’ |
Palaeozoic to Cenozoic |
| Conodonts |
 Tooth-like elements from chordates resembling eels |
|
| Corals | Solitary or colonial, cup-like | Palaeozoic to Cenozoic |
| Echinoderms |
Spiny, multi-plated, 5 rayed |
Early Palaeozoic to Cenozoic |
| Graptolites |
 Tree-like or branched, colonial |
Early Palaeozoic |
| Trilobites |
 Flattened, armoured, shrimp-like |
Palaeozoic |
Microfossils and biostratigraphy
Microfossils are especially important for dating geological sequences in most sediments from the Cenozoic Era (65–0 Ma). Pollen and spores are the main method used for dating terrestrial sequences.
There are several main types of microfossils, such as:
- those with calcareous shells composed of the calcium carbonate minerals calcite or aragonite, including foraminifera and nannofossils;
- those with shells of silica (silicon dioxide), which include radiolarians and diatoms;
- those composed of complex resistant organic compounds, which include pollen and spores; and
- those formed of calcium phosphate, which include conodonts.