Microfossil

A microfossil is a fossil that is generally between one micrometre and one millimetre in size, the visual study of which requires the use of light or electron microscopy. A fossil which can be studied with the naked eye or low-powered magnification, such as a hand lens, is referred to as a macrofossil. 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 microbial cysts from the Chrysophyta, Pyrrhophyta, Sarcodina, acritarchs and chitinozoans, together with pollen and spores from the vascular plants.

Overview

A microfossil is a descriptive term applied to fossilized plants and animals whose size is just at or below the level at which the fossil can be analyzed by the naked eye. A commonly applied cutoff point between "micro" and "macro" fossils is 1 mm. Microfossils may either be complete (or near-complete) organisms in themselves (such as the marine plankters foraminifera and coccolithophores) or component parts (such as small teeth or spores) of larger animals or plants. Microfossils are of critical importance as a reservoir of paleoclimate information, and are also commonly used by biostratigraphers to assist in the correlation of rock units. Microfossils are found in rocks and sediments as the microscopic remains of what were once life forms such as plants, animals, fungus, protists, bacteria and archaea. Terrestrial microfossils include pollen and spores. Marine microfossils found in marine sediments are the most common microfossils. Everywhere in the oceans, microscopic protist organisms multiply prolifically, and many grow tiny skeletons which readily fossilise. These include foraminifera, dinoflagellates and radiolarians. Palaeontologists (geologists who study fossils) are interested in these microfossils because they can use them to determine how environments and climates have changed in the past, and where oil and gas can be found today. Some microfossils are formed by colonial organisms such as Bryozoa (especially the Cheilostomata), which have relatively large colonies but are classified by fine skeletal details of the small individuals of the colony. As 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. In 2017, fossilized microorganisms, or microfossils, were discovered in hydrothermal vent precipitates in the Nuvvuagittuq Belt of Quebec, Canada that may be as old as 4.28 billion years old, the oldest record of life on Earth, suggesting "an almost instantaneous emergence of life" (in a geological time-scale), after ocean formation 4.41 billion years ago, and not long after the formation of the Earth 4.54 billion years ago. Nonetheless, life may have started even earlier, at nearly 4.5 billion years ago, as claimed by some researchers.

Index fossils

Index fossils, also known as guide fossils, indicator fossils or dating fossils, are the fossilized remains or traces of particular plants or animals that are characteristic of a particular span of geologic time or environment, and can be used to identify and date the containing rocks. To be practical, index fossils must have a limited vertical time range, wide geographic distribution, and rapid evolutionary trends. Rock formations separated by great distances but containing the same index fossil species are thereby known to have both formed during the limited time that the species lived. Index fossils were originally used to define and identify geologic units, then became a basis for defining geologic periods, and then for faunal stages and zones. Species of microfossils such as acritarchs, chitinozoans, conodonts, dinoflagellate cysts, ostracods, pollen, spores and foraminiferans are amongst the many species have been identified as index fossils that are widely used in biostratigraphy. Different fossils work well for sediments of different ages. To work well, the fossils used must be widespread geographically, so that they can be found in many different places. They must also be short lived as a species, so that the period of time during which they could be incorporated in the sediment is relatively narrow. The longer lived the species, the poorer the stratigraphic precision, so fossils that evolve rapidly. Often biostratigraphic correlations are based on a faunal assemblage, rather than an individual species — this allows greater precision as the time span in which all of the species in the assemblage existed together is narrower than the time spans of any of the members. Further, if only one species is present in a sample, it can mean either that (1) the strata were formed in the known fossil range of that organism; or (2) that the fossil range of the organism was incompletely known, and the strata extend the known fossil range. If the fossil is easy to preserve and easy to identify, more precise time estimating of the stratigraphic layers is possible.

Composition

Microfossils can be classified by their composition as: (a) siliceous, as in diatoms and radiolaria, (b) calcareous, as in coccoliths and foraminifera, (c) phosphatic, as in the study of some vertebrates, or (d) organic, as in the pollen and spores studied in palynology. This division focuses on differences in the mineralogical and chemical composition of microfossil remains rather than on taxonomic or ecological distinctions.

Siliceous microfossils: Siliceous microfossils include diatoms, radiolarians, silicoflagellates, ebridians, phytoliths, some scolecodonts (worm jaws), and sponge spicules. Calcareous microfossils: Calcareous (CaCO3) microfossils include coccoliths, foraminifera, calcareous dinoflagellate cysts, 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). 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.

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