Acritarchs are small organic fossil

Fossil

Fossils are the preserved remains or traces of animals , plants, and other organisms from the remote past...

s, present from approximately to the present. Their diversity reflects major ecological events such as the appearance of predation and the Cambrian explosion

Cambrian explosion

The Cambrian explosion or Cambrian radiation was the relatively rapid appearance, around , of most major phyla, as demonstrated in the fossil record, accompanied by major diversification of other organisms, including animals, phytoplankton, and calcimicrobes...

.

Definition

In general, any small, non-acid

Acid

An acid is a substance which reacts with a base. Commonly, acids can be identified as tasting sour, reacting with metals such as calcium, and bases like sodium carbonate. Aqueous acids have a pH of less than 7, where an acid of lower pH is typically stronger, and turn blue litmus paper red...

 soluble (i.e. non-carbonate

Carbonate

In chemistry, a carbonate is a salt of carbonic acid, characterized by the presence of the carbonate ion, . The name may also mean an ester of carbonic acid, an organic compound containing the carbonate group C2....

, non-siliceous) organic structure that can not otherwise be accounted for is classified as an acritarch.

Acritarchs include the remains of a wide range of quite different kinds of organisms - ranging from the egg cases of small metazoans to resting cysts of many different kinds of chlorophyta

Chlorophyta

Chlorophyta is a division of green algae, informally called chlorophytes. The name is used in two very different senses so that care is needed to determine the use by a particular author...

 (green algae). It is likely that some acritarch species represent the resting stages (cysts) of algae that were ancestral to the dinoflagellates. The nature of the organisms associated with older acritarchs is generally not clear, though many are probably related to unicellular marine algae. In theory, when the biological source (taxon) of an acritarch does become known, that particular microfossil is removed from the acritarchs and classified with its proper group.

While the classification

Biological classification

Biological classification, or scientific classification in biology, is a method to group and categorize organisms by biological type, such as genus or species. Biological classification is part of scientific taxonomy....

 of acritarchs into form genera

Form taxon

Form classification is the classification of organisms based on their morphology, which does not necessarily reflect their biological relationships...

 is entirely artificial, it is not without merit, as the form taxa show traits similar to those of genuine taxa

Taxon

|thumb|270px|[[African elephants]] form a widely-accepted taxon, the [[genus]] LoxodontaA taxon is a group of organisms, which a taxonomist adjudges to be a unit. Usually a taxon is given a name and a rank, although neither is a requirement...

 - for example an 'explosion

Cambrian explosion

The Cambrian explosion or Cambrian radiation was the relatively rapid appearance, around , of most major phyla, as demonstrated in the fossil record, accompanied by major diversification of other organisms, including animals, phytoplankton, and calcimicrobes...

' in the Cambrian

Cambrian

The Cambrian is the first geological period of the Paleozoic Era, lasting from Mya ; it is succeeded by the Ordovician. Its subdivisions, and indeed its base, are somewhat in flux. The period was established by Adam Sedgwick, who named it after Cambria, the Latin name for Wales, where Britain's...

 and a mass extinction at the end

Permian-Triassic extinction event

The Permian–Triassic extinction event, informally known as the Great Dying, was an extinction event that occurred 252.28 Ma ago, forming the boundary between the Permian and Triassic geologic periods, as well as the Paleozoic and Mesozoic eras...

 of the Permian

Permian

The PermianThe term "Permian" was introduced into geology in 1841 by Sir Sir R. I. Murchison, president of the Geological Society of London, who identified typical strata in extensive Russian explorations undertaken with Edouard de Verneuil; Murchison asserted in 1841 that he named his "Permian...

.

Affinity

Acritarchs may represent the remains of any of the three domains of life, the archaea, the bacteria, and the eukaryotes. Archaea and bacteria usually produce fossils of a very small size, although the sheaths of some bacteria can reach the millimetre scale. Eukaryotes can sometimes be identified by complex traits such as ornamentation or projections, although simple eukaryote acritarchs also exist.

Occurrence

Acritarchs are found in sedimentary rocks from the present back into the Archean

Archean

The Archean , also spelled Archeozoic or Archæozoic) is a geologic eon before the Paleoproterozoic Era of the Proterozoic Eon, before 2.5 Ga ago. Instead of being based on stratigraphy, this date is defined chronometrically...

. They are typically isolated from siliciclastic sedimentary rocks using hydrofluoric acid

Hydrofluoric acid

Hydrofluoric acid is a solution of hydrogen fluoride in water. It is a valued source of fluorine and is the precursor to numerous pharmaceuticals such as fluoxetine and diverse materials such as PTFE ....

 but are occasionally extracted from carbonate-rich rocks. They are excellent candidates for index fossils used for dating rock formations in the Paleozoic Era and when other fossils are not available. Because most acritarchs are thought to be marine (pre-Triassic), they are also useful for palaeoenvironmental interpretation.

Diversity

Acritarchs have been recovered from sediments deposited as long as 3.2 billion years ago, but at about 1 billion years ago they started to increase in abundance, diversity, size, complexity of shape and especially size and number of spines. Their populations crashed during the Snowball Earth

Snowball Earth

The Snowball Earth hypothesis posits that the Earth's surface became entirely or nearly entirely frozen at least once, some time earlier than 650 Ma . Proponents of the hypothesis argue that it best explains sedimentary deposits generally regarded as of glacial origin at tropical...

 episodes, when all or very nearly all of the Earth's surface was covered by ice or snow, but they proliferated in the Cambrian explosion and reached their highest diversity in the Paleozoic

Paleozoic

The Paleozoic era is the earliest of three geologic eras of the Phanerozoic eon, spanning from roughly...

. The increased spininess 1 billion years ago possibly resulted from the need for defence against predators, especially predators large enough to swallow them or tear them apart. Other groups of small organisms from the Neoproterozoic

Neoproterozoic

The Neoproterozoic Era is the unit of geologic time from 1,000 to 542.0 ± 1.0 million years ago. The terminal Era of the formal Proterozoic Eon , it is further subdivided into the Tonian, Cryogenian, and Ediacaran Periods...

 era also show signs of anti-predator defences.

Further evidence that acritarchs were subject to herbivory around this time comes from a consideration of taxon longevity. The abundance of planktonic organisms that evolved between 1,700 and 1,400 million years ago was limited by nutrient availability – a situation which limits the origination of new species because the existing organisms are so specialised to their niches, and no other niches are available for occupation. Approximately 1,000 million years ago, species longevity fell sharply, suggesting that predation pressure, probably by protist herbivores, became an important factor. Predation would have kept populations in check, meaning that some nutrients were left unused, and new niches were available for new species to occupy.

Etymology

Acritarch was coined in 1963 from the Greek ákritos meaning confused (a kritēs, without critic) and arch meaning origin (confer archaic).

External links

• CIMP Subcommission on Acritarchs
• Commission Internationale de Microflore du Paléozoique (CIMP), international commission for Palaeozoic palynology.
• The Micropalaeontological Society
• The American Association of Stratigraphic Palynologists (AASP)