Acritarch

Not to be confused with Antiarchi.

A supposed Ediacaran embryo contained within an acritarch from the Doushantuo formation

Acritarchs are small organic fossils, present from approximately 1,400 to 3,200 million years ago to the present. Their diversity reflects major ecological events such as the appearance of predation and the Cambrian explosion.

Definition

In general, any small, non-acid soluble (i.e. non-carbonate, 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 (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 of acritarchs into form genera is entirely artificial, it is not without merit, as the form taxa show traits similar to those of genuine taxa - for example an 'explosion' in the Cambrian and a mass extinction at the end of the Permian.

Classification

Acritarchs are most likely eukaryotes. While archaea and bacteria usually produce simple fossils of a very small size, eukaryotic unicellular fossils are 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.), they are predominantly eukaryotes, although simple eukaryote acritarchs also exist.

Occurrence

Acritarchs are found in sedimentary rocks from the present back into the Archean. They are typically isolated from siliciclastic sedimentary rocks using hydrofluoric acid 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 the organisms responsible for acritarchs started to increase in abundance, diversity, size, complexity of shape and especially size and number of spines. Their populations crashed during periods of extensive worldwide glaciations that covered the majority of the planet, but they proliferated in the Cambrian explosion and reached their highest diversity in the Paleozoic. 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 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)