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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
Belemnites

Bullet-like

Late Mesozoic
Ammonites
ammonites biostratigraphy

Single spiral-shaped shell with several cavities

Mesozoic
Gastropods
gastropods

Coiled single shell with one cavity

Late Palaeozoic to Cenozoic
Bivalves
bivalves

Double-shelled, burrowing, attached or free lying

Late Palaeozoic to Cenozoic
Bryozoans
Bryozans

Colonial; ‘lacy’ branched or domed

Late Palaeozoic to Cenozoic
Brachiopods
brachiopod biostratigraphy

Double-shelled ‘lamp shells’

Palaeozoic to Cenozoic
Conodonts
Conodonts

Tooth-like elements from chordates resembling eels

 
Corals Solitary or colonial, cup-like Palaeozoic to Cenozoic
Echinoderms
Echinoderms

Spiny, multi-plated, 5 rayed

Early Palaeozoic to Cenozoic
Graptolites
Graptolites

Tree-like or branched, colonial

Early Palaeozoic
Trilobites
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.