SILURIAN: CHARACTERISTICS, SUBDIVISIONS, GEOLOGY, FLORA, FAUNA - GEOGRAFÍA - 2025

The Silurian was the third period of the Paleozoic era, located between the Ordovician and the Devonian. This period was characterized by intense geological activity, being evidenced in the formation of mountains, as well as the formation of a new supercontinent, Euramérica.

It was common that on the surface of existing continents there were shallow bodies of water, a product of the high sea level. The Silurian was a very interesting period for specialists, since at the level of biodiversity there were a lot of changes.

Trilobite fossil corresponding to Silurian. Source: DanielCD, from Wikimedia Commons

Plants managed to conquer the terrestrial environment and new species of arthropods, corals and fish appeared. Although it has been very well studied, there are still specialists who consider that there is still a lot of information to be discovered about the Silurian period.

General characteristics

Duration

The Silurian period lasted 25 million years, extending from about 444 million years ago to about 419 million years ago.

Mountain formation period

From the geological point of view, the Silurian period was characterized by the formation of mountain systems widely known today, such as the Appalachian Mountains of North America.

Diversification of life

During this period, the group of plants diversified widely, with the first vascular plants appearing. Likewise, animals also experienced significant evolution, especially corals and arthropods.

Divisions

The Silurian period was divided into four epochs: Llandovery, Wenlock, Ludlow, and Prídoli. Likewise, each era, with the exception of Prídoli, was divided into ages, a total of eight.

Extinction events

During the Silurian period there were a total of three extinction events considered to be of lesser degree. These became known as: Irekiven event, Mulde event and Lau event.

These events mainly affected organisms in marine habitats. 50% of the trilobite species disappeared.

geology

During this period, the supercontinent Gondwana continues to be located at the planet's south pole. The rest of the supercontinents - Laurentia, Baltic and Siberia - were in a position further north, with Siberia being the furthest away.

Likewise, the sea level rose as a result of the melting of the ice from the glaciation at the end of the previous period. This caused the so-called "epicontinental seas" to form on the surface of supercontinents. These were nothing more than small, shallow bodies of water.

Similarly, the effect of continental drift continues, and thanks to this process, the supercontinents Laurentia, Báltica and Avalonia collided to form a new, much larger supercontinent, which became known as Euramérica. This supercontinent was also known as the Old Red Sandstone continent.

Similarly, the Silurian period was characterized by the emergence of large areas of land. The oceans present on the planet in that period were:

• Panthalassa: it was the largest ocean on the planet, it occupied the entire northern hemisphere.
• Paleo Tethys: It was located between the newly formed supercontinent, Euramérica, and the great supercontinent Gondwana.
• Rheico: located between Gondwana and the supercontinents that were in the north, such as Baltica, Laurentia and Avalonia.
• Lapetus: (Iapetus) was located between Laurentia, Baltica and Avalonia. This ocean disappeared when the supercontinents united to form Euramérica.
• Ural: small ocean that occupied the space between Baltic and Siberia.

During this period, two orogenic processes took place: the Caledonian orogeny and the Acadian orogeny.

Caledonian orogeny

This was a geological process that consisted of the formation of mountains in the areas that today correspond to Ireland, England, Scotland, part of Norway and Wales.

It was originated by the collision of the supercontinents Baltic and northern Avalonia. Product of the Caledonian orogeny, the supercontinent Laurasia was formed.

Later, at the end of the period, the sea level decreased, revealing areas of land that suffered the onslaught of the erosive process.

Acadian orogeny

This was an orogenic process that began in this period and culminated in the Devonian. The consequence of it was the formation of one of the most recognized mountain ranges in North America, the Appalachians, which extend east from Canada to Alabama in the United States.

Weather

During this period, the planet's climate stabilized. Gone are the sudden variations in the weather.

In the Seluric the climate was mainly warm. The glaciers that had formed during the previous period, the Ordovician, were located towards the south pole of the planet.

In general, the climate in the Seluric period was warm, although there is fossil evidence that there were also a large number of storms in this period.

Later the ambient temperature seemed to decrease, cooling the environment a little, but without reaching the extremes of an ice age. At the end of the Silurian and already entered the Devonian, which was the following period, the climate ended up being humid and warm, with a significant number of precipitations.

Lifetime

Despite the fact that at the end of the previous period (Ordovician) there was a massive extinction event, during the Silurian life continued to develop successfully in marine ecosystems.

The species that managed to survive at the end of the Ordovician diversified and even some genera evolved. A greater number of species developed compared to the Ordovician period.

-Flora

In marine ecosystems there was a large amount of algae, mainly green algae, which contributed to the balance of the environment, since they were part of the trophic chains that developed there.

In this period, a milestone occurred in the development of plants: rudimentary vascular plants began to appear. Vascular plants are those that have conductive vessels: the xylem, through which water circulates; and the phloem, through which the nutrients products of photosynthesis and absorption pass through the roots.

At the beginning of the Silurian, the terrestrial landscape was far from the marine one. In the marine environment, life was bustling and life forms (plants and animals) became more and more diversified.

In contrast, in terrestrial habitats, the appearance was desolate and barren. Only long stretches of rocky and desert terrain were visible, perhaps with a bit of humus.

The first plants that developed in terrestrial habitats necessarily had to remain close to bodies of water, since this way they had availability of this element and nutrients.

This is so because they did not have conductive vessels or specialized structures of another nature such as roots or leaves. According to specialists in the area, these types of plants must have been similar to the bryophytes that are known today.

Cooksonia, an extinct Silurian plant. Source: Original by Smith609. Derivative by Peter coxhead, via Wikimedia Commons

Most of the plants that appeared in this period became extinct. None had differentiated specialized structures such as roots, leaves and stem, much less flowers. Among the first plants that colonized the terrestrial environment can be mentioned:

Cooksonia

According to the fossil records it was a ubiquitous plant, that is, it was found in a large number of places. It did not have a proper root, but was held firmly in the ground thanks to a structure known as a rhizome.

It did not have leaves, but its stem cells contained chlorophyll. Therefore they were able to carry out the photosynthesis process by expelling oxygen into the atmosphere. The stem was bifurcated, Y shaped. It reproduced through spores.

Psilophyton

These plants were of the herbaceous type, with dichotomously branched small stems. They lacked leaves and roots. They were anchored to the ground by a kind of rhizome.

According to those in the field, the stem cells had to contain chlorophyll for the plant to carry out the photosynthesis process. Its type of reproduction was through spores that were produced at the ends of the branches.

Baragwanathia

These plants were one step ahead in the evolutionary process. It is the first terrestrial vascular plant that is known. They had xylem and phloem through which water and nutrients circulated.

They had small leaves, so they carried out the photosynthesis process. Likewise, they had adventitious (aerial) roots through which they could absorb nutrients and water. Like the previous ones, they reproduced by spores.

-Fauna

At the end of the Ordovician there was a process of mass extinction that affected a large percentage of animals. Despite this, some of those that managed to survive this process managed to prosper during the Silurian, even new species appearing.

Arthropods

This was a group that experienced significant evolution in the Silurian period. Approximately 425 fossils representing individuals belonging to this phylum have been recovered from this period.

Trilobites, which declined in the previous period, continued to exist in marine habitats, but eventually became extinct.

Likewise, in the Silurian period the myriapods and chelicerates appeared for the first time, which began to populate terrestrial habitats.

Myriapods are animals whose body is divided into three parts: head, thorax and abdomen. In addition, the body is segmented into rings, each with one or two pairs of legs.

On their head they usually have antennae and a pair of eyes. Centipedes and millipedes can be mentioned among the most characteristic animals of this subphylum.

On the other hand, chelicerates are animals that have a segmented body. They also have two regions: cephalothorax and abdomen. They have four pairs of legs.

They owe their name to a structure known as chelicerae, an appendage that is very close to the mouth. This appendix can have several functions: taking prey and feeding or injecting poison into its victims.

Within this group, the existence of eurypterids, known as sea scorpions, was particularly significant. They were powerful predators of the marine habitat.

Mollusks

The group of mollusks was represented in this period by species of bivalves, gastropods. These lived mainly on the seabed.

Echinoderms

In this period there were crinoids, which are recognized as the oldest echinoderms on the planet. Even today there are specimens in the seas.

There were also another type of abundant echinoderms during this period, the euchinoid. These had a peduncle that fixed them to the substrate. They became extinct towards the end of the Silurian period.

Fishes

This was a group that experienced some diversification. In the previous period ostracoderms had appeared, which were jawless fish, considered the oldest vertebrates of which there are fossil records.

Ostracoderm. Source: Rod6807, from Wikimedia Commons

During the Silurian, other types of fish began to appear, among which the first fish with a jaw, known as placoderms, stand out. One of their most distinctive characteristics is that they presented a kind of armor on the front of the body.

Likewise, acanthods also appeared in this period. These are also known as spiny sharks, and are considered midway organisms between ostracoderms and cartilaginous fish.

This is so because they presented characteristics of both groups. For example, they had bone plates similar to those of ostracoderms at the head level and also had a cartilaginous skeleton.

Some specialists argue that cartilaginous fish made their appearance at the end of this period. However others refute it, stating that they appeared in the later period, the Devonian.

If it is true that they appeared in the Silurian, they did so when the period was about to end and they were not as large as those known today (sharks and rays).

Coral reefs

It is known that in the previous period, the Ordovician, the first coral reefs appeared. However, it was in the Silurian that truly large coral reefs formed.

This was because the existing coral species diversified and underwent adaptive radiation. The reefs were made up of very varied corals, there were many different forms.

Likewise, it was also common to observe in the reefs, sponges (cnidarians) and specimens of the crinoids, belonging to the echinoderms group.

Divisions

The Silurian period is divided into four ages, which in turn are divided into eight ages.

Llandovery

It is the first Silurian epoch. It lasted approximately 10 million years. It spread from about 443 million years ago, to about 433 million years ago. It was divided into three ages:

• Rhuddanian: with a duration of 3 million years.
• Aeronian: lasted about 2 million years.
• Telychiense: It spanned 5 million years.

Wenlock

It was the second epoch of the Silurian period. It lasted approximately 6 million years. It was divided into two ages:

• Sheinwoodian: extended from about 433 million years ago to about 430 million years ago.
• Homerian: spanned from about 430 million years ago to about 427 million years ago.

Ludlow

The third Silurian epoch spanned 4 million years. It was made up of two ages:

• Gorstiense: from about 427 million years ago to about 425 million years ago.
• Ludfordian: from about 425 million years ago to about 423 million years ago.

Pridoli

It was the last Silurian epoch. It was characterized by being the one that lasted the least (7 million years) and because it was not divided into ages.

References

1. Emiliani, Cesare, 1993. Planet Earth: Cosmology, Geology and the Evolution of Life and Enviro
2. Gradstein, FM; Ogg, JG; Smith, AG (2004). A Geologic Time Scale 2004. Cambridge university press
3. Harper, D. and Servais, T. (2013). Early Paleozoic Biogeography and Paleogeography. The Geological Society. London.
4. Pappas, S. (2013). Paleozoic era: Facts & Information. Retrieved from: Livescience.com
5. Sour Tovar, Francisco and Quiroz Barroso, Sara Alicia. (1998). The fauna of the Paleozoic. Science 52, October-December, 40-45.
6. Ziegler, AM, KS Hansen, ME Johnson, MA Kelly, MA Scotese, and CR van der Voo. 1977. Silurian continental distribution, paleogeography climatology and biogeography. Tectonophysics 40: 13-51.