Micropaleontology

Micropaleontology (also sometimes spelled as micropalaeontology) is that branch of paleontology which studies microfossils. Microfossils are fossils generally not larger than four millimeters, and commonly smaller than one millimeter, the study of which requires the use of light or electron microscopy. Fossils which can be studied with the naked eye or low-powered magnification, such as a hand lens, are referred to as macrofossils. Obviously, it can be hard to decide whether or not some organisms should be considered microfossils, and so there is no fixed-size boundary.

For example, some colonial organisms, such as bryozoa (especially the Cheilostomata) have relatively large colonies, but are classified on the basis of fine skeletal details of the tiny individuals of the colony. Most bryozoan specialists tend to consider themselves paleontologists, rather than micropaleontologists, but many micropaleontologists also study bryozoa.

In another example, many fossil genera of Foraminifera, which are protists, are known from shells (called "tests") that were as big as coins, such as the genus Nummulites.

Microfossils are a common feature of the geological record, from the Precambrian to the Holocene. They are most common in deposits of marine environments, but also occur in brackish water, fresh water and terrestrial sedimentary deposits. While every kingdom of life is represented in the microfossil record, the most abundant forms are protist skeletons or cysts from the Chrysophyta, Pyrrhophyta, Sarcodina, acritarchs and chitinozoans, together with pollen and spores from the vascular plants.

Micropaleontology can be roughly divided into four areas of study on the basis of microfossil composition: (a) calcareous, as in coccoliths and foraminifera, (b) phosphatic, as in the study of some vertebrates, (c) siliceous, as in diatoms and radiolaria, or (d) organic, as in the pollen and spores studied in palynology.

This division reflects differences in the mineralogical and chemical composition of microfossil remains (and therefore in the methods of fossil recovery) rather than any strict taxonomic or ecological distinctions. Most researchers in this field, known as micropaleontologists, are typically specialists in one or more taxonomic groups.

Calcareous microfossils
Calcareous [CaCO3] microfossils include Coccoliths, Foraminifera, Calcareous dinoflagellates, and Ostracods (seed shrimp).

Phosphatic microfossils
Phosphatic microfossils include Conodonts (tiny oral structures of an extinct chordate group), some scolecodonts ("worm" jaws), Shark spines and teeth, and other Fish remains (collectively called "ichthyoliths").

Siliceous microfossils
Siliceous microfossils include Diatoms, Radiolaria, Silicoflagellates, phytoliths, some scolecodonts ("worm" jaws), and spicules.

Organic microfossils
The study of organic microfossils is called palynology. Organic microfossils include pollen, spores, Chitinozoans (thought to be the egg cases of marine invertebrates), Scolecodonts ("worm" jaws), Acritarchs, Dinoflagellate cysts, and fungal remains.

Methods
Sediment or rock samples are collected from either cores or outcrops, and the microfossils they contain extracted by a variety of physical and chemical laboratory techniques, including sieving, density separation by centrifuge, and chemical digestion of the unwanted fraction. The resulting concentrated sample of microfossils is then mounted on a slide for analysis, usually by light microscope. Taxa are then identified and counted. The very large numbers of microfossils that a small sediment sample can often yield allows the collection of statistically robust datasets which can be subjected to multivariate analysis. A typical microfossil study will involve identification of a few hundred specimens from each of ten to a hundred samples.

Applications of Micropaleontology
Microfossils are especially noteworthy for their importance in biostratigraphy. Since microfossils are often extremely abundant, widespread, and quick to appear and disappear from the stratigraphic record, they constitute ideal index fossils from a biostratigraphic perspective. In addition, the planktonic and nektonic habits of some microfossils gives them the added bonus of appearing across a wide range of facies or paleoenvironments, and having near-global distribution making biostratigraphic correlation even more powerful and effective.

Microfossils also provide some of the most important records of global environmental change on long-timescales, particularly from deep-sea sediments. Across vast areas of the ocean floor the shells of planktonic micro-ogranisms sinking from surface waters provide the dominant source of sediment and they continuously accumulate (typically at rates of 20-50m/million years). Study of changes in assemblages of microfossils and of changes in their shell chemistry (e.g oxygen isotope composition) are fundamental to research on climate change in the geological past.

In addition to providing an excellent tool for sedimentary rock-body dating and for paleoenvironmental reconstruction -- heavily used in both petroleum geology and paleoceanography -- micropaleontology has also found a number of less orthodox applications, such as its growing role in forensic police investigation or in provenancing archaeological artefacts.