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Surprising 5 Species of Acritarchs Shaped the Evolution of Life on Earth

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What are acritarchs

Among the oldest micro-organisms, acritarchs are one of them. Acritarchs are organic-walled microfossils that range in size from 20- 80 micrometers. They are widely distributed from the Archean age of 3000 million years to the recent. Acritarchs are the most important microfossils among geologists because of their long geologic history of the existence.

Geologist tries to find the correlation between the first life on the earth with this type of unicellular micro-organism to convey the link between human micro-organisms from the past to the present. Through the study of acritarchs, a scientist can explore the paleoenvironment of the earth. 


Acritarchs are the unicellular, organic-walled micro-organism of unknown origin. They usually fall in the group of dinoflagellates. Because they are usually found in the rock along with the dinoflagellates type of microorganisms. They have a spike or ornamentation on their wall. Based on the spike and the shape of the acritarchs scientist can classify the acritarchs. Acritarchs are usually 20- 80 microns in size. So they can not be seen with the naked eye. Acritarchs are the most rigid and undegradable unicellular micro-organisms.

It is a non-carbonate, non-silicious, and non-acid soluble organism. So, in the rock sample, it is a very nicely preserved organism. It is very important for the geologist to explore the paleo-environments of the earth. The acritarchs first appeared in the Proterozoic and reached the maximum in the Early to Middle Paleozoic.

In the Carboniferous and Permian, they are nearly absent and the sudden absence of the acritarchs is called the “Phytoplankton Block Out”. Some scientists they try to reveal the similarity between the acritarch and the phytoplankton. Based on the value of the Carbon 13 of acritarchs and the Phytoplankton they found a similar value. Hence they emphasized that it may be in the phytoplankton type of microorganism. But still, the exact origin of acritarchs is unraveled!

How does acritarch evolve?

The evolutionary origins of acritarchs are still debated by scientists, as their lack of hard parts makes it difficult to determine their exact ancestry. However, it is widely believed that acritarchs represent a diverse group of single-celled eukaryotic organisms that evolved in the oceans during the late Proterozoic and early Phanerozoic eons, roughly between 1 billion and 540 million years ago.

The morphology and characteristics of acritarchs suggest that they evolved complex life cycles and reproductive strategies, including sexual reproduction and the formation of resistant cysts that allowed them to survive in harsh environmental conditions. Some acritarchs were able to form symbiotic relationships with other organisms, such as cyanobacteria or green algae.

As the environment of the Earth changed over time, so too did the distribution and diversity of acritarchs. They were particularly abundant during periods of rapid environmental change, such as the Cambrian explosion, when many new forms of life evolved. Some acritarchs even played a role in shaping the environment, such as the widespread formation of organic-rich shales during the Ordovician period, which were likely caused by the proliferation of certain types of acritarchs.

Overall, the evolution of acritarchs is still an active area of research, and their study continues to shed light on the early evolution of eukaryotic life on Earth.

How to Classify the Acritarch?

Based on the ornamentation and the geometrical shape of the vesicle of acritarchs, they can be classified into five different types. In a study from South China, Shangsi, Meishan, and some other section, geologists and paleontologists found a wide variety of acritarchs preserves in the samples. The Micrhystridium, Veryhachium type of acritarchs are very small in size usually 20 micrometers. 

Fig: Classification.

The classifications are;

  1. The Veryhachium cylindricum group: This group represents all the ellipsoidal shapes of the Acritarchs.
  2. The Veryhachium trispinosum group: This group includes all triangular-shaped vesicles.
  3. The Veryhachium lairdii group: This Group includes all rectangular forms of the acritarchs.
  4. The Micrhystridium pentagonale group: This group covers all the pentagonal shapes fo the acritarchs.
  5. The Micrhystridium breve group: This group includes all the spherical forms of acritarchs.

What is Known So Far About the Unknow-Origin of Acritarchs?

Acritarchs are sometimes called the phytoplankton origin of organisms. So, Phytoplankton and acritarchs may have relationships. But before the final discovery of the exact origin of acritarchs still, there is a huge debate among geologists, paleontologists, and other scientists.

  1. They are mainly marine phytoplankton or algae or unknown affinities of origin. Grice et al. (2005) state that similar values of stable carbon isotopic composition of the acritarchs derived biomarker (Geochemical fossils) and phytane sourced from chlorophyll a in phytoplankton
  2.  Another potential source was reported in cold-seep carbonate crusts (Pancost et al., 2001). 
  3. A series of linear diphenyl alkanes have been reported in sulfur-rich coals containing biomarkers also derived from Chlorobiaceae (Gorchs et al., 2003). 
  4. Some suggested that it might be derived from an organism that inhabited in a sulfide-rich environment (Grice et al., 2005; Fig. 15).

Though it is still a huge scope for the geologist to discover the exact sources of the acritarch in the Permian rocks or other older rocks to explore the paleo-history of the earth and can be linked up with the great mass extinction of the earth. During the last five mass extinction events, (Ordovician-Silurian, Devonian-carboniferous, Permian Triassic, Triassic Jurassic, and the last Cretaceous Paleogene mass extinction)  this micro-organism had been survived and experienced and until today we can find acritarchs in the recent sample.

The distribution of acritarchs in the geologic past:

Acritarchs are a type of microfossil that were present in various ancient sediments. Acritarchs lived in different environments such as island and oceanic waters and served as food for all kinds of organisms.

Different types of acritarchs and their presence were notable in various ancient sediments. The most reliable and widespread acritarchs are known to exist in the Cambrian era. Acritarchs of other types were also present in even more ancient sediments such as the Ordovician and Silurian periods.

The abundance of acritarchs may be associated with the Cambrian period, as it was first recorded during this time. During this period, the presence of acritarchs was prominent in sediments around the world.

Are acritarchs eukaryotes?

What do acritarchs represent?

Is an Acritarch a bacteria?

What is the Acritarch morphology?

Acritarchs are not definitively classified as either eukaryotes or prokaryotes, as they are not complete cells but rather organic-walled microfossils. However, many researchers consider them to be eukaryotic in nature, based on their complex morphologies.

Acritarchs represent the organic-walled remains of a diverse group of unicellular organisms that lived in various aquatic environments in the past. They have been found in rocks that date back to as early as 3 billion years ago and are used by scientists as a tool for dating and correlating rocks of similar age.

Acritarchs are not bacteria. They are thought to be the remains of a diverse group of unicellular organisms that have an uncertain taxonomic affinity, which means they do not clearly belong to any known group of organisms.

The morphology of acritarchs varies widely and can be complex. They can be spherical, elliptical, or spindle-shaped, and can have a variety of surface ornamentations and internal structures. Some acritarchs have distinctive spines, horns, or other projections, while others are smooth and featureless. The wide range of morphologies seen in acritarchs is one of the reasons why they are difficult to classify taxonomically.

Key finding:

Classification Organic-walled microfossils
Taxonomy Uncertain; may represent a diverse group of unicellular eukaryotes
Age range 3 billion years ago to present
Morphology Spherical, elliptical, or spindle-shaped; can have surface ornamentations and internal structures; some have spines, horns, or other projections
Abundance Widespread in various ancient sediments
Use Tool for dating and correlating rocks of similar age
Environmental Lived in various aquatic environments such as island and oceanic waters
Food source Served as food for all kinds of organisms
Significance Provides evidence of ancient life; helps to understand the evolution and diversification of eukaryotes
Challenges Difficult to classify taxonomically due to the wide range of morphologies seen in acritarchs
Importance Acritarchs are valuable for studying the evolution and diversification of eukaryotes, as they provide evidence of ancient life

How to use acritarch as a tool for researchers?

Acritarchs are used by geologists and paleontologists as a tool for dating and correlating rocks of similar age. This is because acritarchs have a relatively short geological range, meaning they only existed for a limited time period. By studying the presence and abundance of certain types of acritarchs in sedimentary rocks, scientists can establish a chronological sequence of events and can compare the relative ages of different rock formations.

Acritarchs are also valuable for understanding the evolution and diversification of eukaryotes. By studying the morphology of acritarchs, scientists can infer the ecological and environmental conditions in which they lived and can compare them to other organisms that lived during the same time period. This information can help scientists understand how eukaryotes evolved and diversified over time.

To use acritarchs for dating and correlation, scientists typically collect sedimentary rocks from different locations and study them under a microscope. They identify and classify the different types of acritarchs present in the rock, and compare them to other known acritarch species to establish a relative age. The abundance and distribution of different acritarch types can also be used to interpret environmental changes that occurred during the time period represented by the rock.

What is the oldest microorganism?

Determining the oldest microorganism is difficult, as fossils of microscopic organisms are rare and can be difficult to interpret. However, there are some microfossils that are thought to represent some of the oldest known life on Earth.

One of the oldest microorganisms that have been discovered is a type of bacteria-like microfossil known as a “stromatolite”. Stromatolites are layered structures that are formed by the activity of microbial communities and are found in rocks that date back as far as 3.5 billion years ago.

Another candidate for the oldest microorganism is a type of microfossil called “micro bacteria”. These tiny spherical structures were discovered in rocks that date back to 3.4 billion years ago and are believed to represent some of the earliest known forms of life on Earth.

Other ancient microorganisms include cyanobacteria, which are believed to have evolved as early as 3.5 billion years ago, and certain types of sulfur-metabolizing bacteria that have been found in rocks dating back to 3.4 billion years ago.

Overall, acritarchs are an important tool for understanding the geological and biological history of our planet, and their study continues to provide valuable insights into the evolution and diversification of life on Earth.

So, Acritarchs can live in stressful environments and are able to survive the event in the extinction span of time. Hence, we need more work on acritarchs to explore the past based on micro-fossils.