Microfossil

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.

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.

Everywhere in the oceans, microscopic protist organisms multiply prolifically, and many grow tiny skeletons which readily fossilise.

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.

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.

To be practical, index fossils must have a limited vertical time range, wide geographic distribution, and rapid evolutionary trends.

Pollen has an outer sheath, called a sporopollenin, which affords it some resistance to the rigours of the fossilisation process that destroy weaker objects.

A spore is a unit of sexual or asexual reproduction that may be adapted for dispersal and for survival, often for extended periods of time, in unfavourable conditions.

[15] Common from the Ordovician to Devonian periods (i.e. the mid-Paleozoic), the millimetre-scale organisms are abundant in almost all types of marine sediment across the globe.

The organisms have been better understood as improvements in microscopy facilitated the study of their fine structure, and it has been suggested that they represent either the eggs or juvenile stage of a marine animal.

While archaea, bacteria and cyanobacteria (prokaryotes) usually produce simple fossils of a very small size, eukaryotic unicellular fossils are usually larger and more complex, with external morphological projections and ornamentation such as spines and hairs that only eukaryotes can produce; as most acritarchs have external projections (e.g., hair, spines, thick cell membranes, etc.

[41] Siliceous oozes are largely composed of the silica based skeletons of microscopic marine organisms such as diatoms and radiolarians.

Other components of siliceous oozes near continental margins may include terrestrially derived silica particles and sponge spicules.

[42] Distance from land masses, water depth and ocean fertility are all factors that affect the opal silica content in seawater and the presence of siliceous oozes.

Calcareous sediments (limestone) are usually deposited in shallow water near land, since the carbonate is precipitated by marine organisms that need land-derived nutrients.

Below this depth, calcium carbonate begins to dissolve in the ocean, and only non-calcareous sediments are stable, such as siliceous ooze or pelagic red clay.

For many years, conodonts were known only from enigmatic tooth-like microfossils (200 micrometres to 5 millimetres in length) which occur commonly, but not always in isolation, and were not associated with any other fossil.

Composed of highly resistant organic substance, the scolecodonts are frequently found as fossils from the rocks as old as the late Cambrian.

Since the worms themselves were soft-bodied and hence extremely rarely preserved in the fossil record, their jaws constitute the main evidence of polychaetes in the geological past, and the only way to restore the evolution of this important group of animals.

[59] The cloudinids were an early metazoan family that lived in the late Ediacaran period about 550 million years ago,[60][61] and became extinct at the base of the Cambrian.

[62] They formed small millimetre size conical fossils consisting of calcareous cones nested within one another; the appearance of the organism itself remains unknown.

[65] Cloudinids had a wide geographic range, reflected in the present distribution of localities in which their fossils are found, and are an abundant component of some deposits.

The classification of the cloudinids has proved difficult: they were initially regarded as polychaete worms, and then as coral-like cnidarians on the basis of what look like buds on some specimens.

In 2020, a new study showed the presence of Nephrozoan type guts, the oldest on record, supporting the bilaterian interpretation.

The most widely supported answer is that their shells are a defense against predators, as some Cloudina specimens from China bear the marks of multiple attacks, which suggests they survived at least a few of them.

These two points suggest that predators attacked in a selective manner, and the evolutionary arms race which this indicates is commonly cited as a cause of the Cambrian explosion of animal diversity and complexity.

Dinoflagellates are mainly represented in the fossil record by these dinocysts, typically 15 to 100 micrometres in diameter, which accumulate in sediments as microfossils.

[66][67] Dinocysts have a long geological record with geochemical markers suggest a presence that goes back to the Early Cambrian.