Silurian period- The Recovery 2.0 ⋆ TheScientificRevelation
Hallo, friends. Welcome to another post of the “History of earth” series. In the previous post The Silurian period- Recovery 1.0, we had talked about the position of continents, geology, environment, and general trends in life forms. This post will be an extension of previous topics and we will see other aspects of the Silurian period. The more we move towards the present the natural history will get messier and vast. The past was astonishing with The formation of the earth, The oxidation event, snowball earth, the Ediacaran biota, The Ordovician period, and the Cambrian explosion. There is so much to learn and our recent post, Silurian period- The Recovery 2.0 will get us further.
Silurian didn’t have a major extinction, so few species got unlucky in the cycle of ongoing mild climate changes. During the lower Silurian( starting epochs) earth was recovering from an extinction event. Glaciers were melting and supercontinent Gondwana was having high erosion. The up and downs in sea level are the major reasons behind extinctions and extinctions are always followed by radiation events. When glaciers melted around the late Ordovician and early Silurian sea levels increased and continental landmasses were flooded. Globe was full of epicontinental seas and water splashed coastal areas. Life recovered fast from preceding extinction. The most important thing to remember here is that Gondwana hasn’t moved from its position on the south pole and some portions still had a significant amount of ice in the Silurian period. Scientists have found glacier tillies and sediments in brazil dating back to the Wenlock epoch of the Silurian period.
When sea levels were high it was a good time for life to flourish. However, Gondwana was still on the south pole and the land had the capacity to build multiple layers of ice so there were miner glaciations. Not much severe but enough to lower sea level by few meters. You might be thinking that why sea-level fluctuations hadn’t brought another extinction. It’s because few meters were no big problem. When sea levels fall the life forms had migrated little downslope to survive and vice versa. Benthic fauna adopted to their particular Bathymetric levels and living habitat was not lost in any way. However, some species were affected by variation in temperature and went extinct. The Ordovician had high endemism where particular species were thriving in certain areas only. In Silurian, we see the global distribution of species. With little variation here and there they were very similar species.
I had read a book that goes by the name The story of earth- first 4.5 billion years. It was a fascinating book and you people may give it a try. In that book, the author had written something very true. We study biology and mineralogy as separate subjects. Actually, we tend to classify everything with a distinctive boundary and tend to think nature follows our way. Physics and biology are totally different. Natural history has nothing to do with anatomy and art and science have no connection. But we tend to forget as Leonardo Di Vinci had said “everything is connected to everything”.
The rock formation, mineralogy were affected by life and climate and vice versa. Everything is connected to everything else. The geology of Silurian was heavily affected by climatic conditions. The major rock types of Silurian are as follows and along with it, we will also tell what type of conditions had formed them.
Firstly, I want to talk about sedimentary rocks. The Ordovician glaciation had an immense effect on life and the planet. A lot of water was stores as ice caps. Sea levels drooped almost 200 feet in the Ordovician period. All epicontinental seas were bone dry. When these glaciers melted margins of Gondwana had high erosion. Ice after melting turned into series of rivers and they transported soils and rocks in the surrounding ocean. Paleovally of Arabia and northern Africa had extensive glacier features like tillies, glacier boulders, and signs of ice cap movements.
One type of sedimentary rock was limestones which formed out of carbonate detritus of coral animals, shells, and algae. Limestones can only be formed when these carbonate materials form in large quantities and rapidly buried. Otherwise, they will dissolve in cold water. Due to this wherever we have found limestone deposits it was part of tropics or subtropics in the Silurian period. The process of deposition is very similar to the present time but because Silurian was a lot warmer and watery limestone deposits are fundamentally different. In the Silurian period, Laurentia, Baltica, and Siberia were present in the tropics so they had high limestone deposits.
North America, have stretches of limestones from northern Greenland to west Texas and from Quebec’s Anticosti Island to the Great Basin of Utah and Nevada. In Baltica, we have prominent deposits in Gotland in Sweden, Estonia, and the Ukrainian region of Podolia. Most of Siberia had layers of limestone deposits of almost 650 to 1000 feet. England, south china and Australia also have limestone deposits dating back to the Wenlock epoch.
Evaporites are salt(halite) anhydrite and gypsum layers of crystal that accumulate inside the continents after the evaporation of water. During low stands of the Silurian period, epicontinental shallow seas were most affected. Water in some portion drained out or evaporated but left dissolved minerals to turn into evaporite. This process is more effective under arid climates of the tropics. Again Laurentia, Baltica, and Siberia had maximum deposits of evaporites because of their paleo position. Michigan, Ohio, and New York have the most well-developed evaporite from the upper Silurian (Ludlow and Pridoli) period. The Yangadin and Holuhan formations of Siberia, Latvia, and Lithuania have extensive gypsum beds. Western Australia had different types of evaporite basins dating back to the Pridoli epoch of Silurian.
Clastic beds also trace their origin to glacier erosion. After the ice caps melted of Gondwana surplus water made rivers reach oceans. These rivers had swept past the vast land and carried with them tons of clastic material. Clastic means terrigenous material that originated from land erosion. This clastic material doesn’t deposit as layers but instead, makes wedges that are normally 1 km thick. They are always deposited just next to continents. Clastic material includes sandstones, conglomerates, and shale.
Taconia mountains chain had produced heavy clastic material around Laurentia. The conglomerate deposits in New York, Tuscarora-Clinch sandstones in central Pennsylvania, and western Virginia are from Taconic highlands. Pearya highlands of Canada had shed its clastic deposits in nearby Hazen Trough. In the same manner, Caledonian highlands are responsible for clastic deposits of Baltica. The water had carried away an immense amount of clasts farther away from their sources and deposits in layers. Wind-driven clastic deposits are very rare including the Mereenie Sandstone in central Australia.
The shale also makes another type of structure, Platform margins. For example, the 500-meter shale of the Road River group in Yukon, Canada. Platform margins form due to the accumulation of eroded material. Other than land erosion, underwater volcanoes and submarine avalanches are also responsible for platform margins. Their name itself suggests that these layers are comparatively thinner and mostly ocean-based. Platform margins are shale-based. For example, in the Mackenzie mountains, only a few k.m. of distance separates deep water and shallow water shale.
Platform margins are normally run in smaller areas but we have few exceptions. For instance, the Longmaqi Formation of south china covers an extensive area including, Yunnan, Shaanxi, Hubei, Guizhou, Yunnan, and Hunan. These shales have formed in quite shallow waters of low oxygen. Their black color is due to the accumulation of organic matter. It didn’t decompose because of low oxygen.
The Ordovician period had extensive glaciers and glacial deposits. It is full of tillies and boulders carried by glaciers. Tillies are broken parts of sedimentary rocks. They travel with glaciers to far and wide places. When glaciers retreat in the interglacial period they left tillites. These sedimentary rock pieces are very distinctive and normally belong to the polar region. So, if glaciers had reached lower latitude near tropics it left the tillies thereafter melting away. During studies and research when we find these deposits we can trace them back to the origin.
Silurian sandstones and shale layers lie above the glacier deposits of Ordovician. Northern Africa was the center of glacial deposits and here sedimentary rocks lay above unweathered glacial boulder ex. Tabuk Formation of Saudi Arabia as well as the whole of North Africa. In South America glaciers well survived till the Wenlock epoch, the middle of the Silurian. Here glacier tillite layers 60 meters thick extend from Argentina to Peru and Bolivia. In Brazil, we have Diamictite (Conglomerate made) layers full of glacial tillites.
Compared with other periods, Silurian was very quiet in terms of volcanic activity. There were moderate volcanic eruptions notably, in the British Isles, Canadian maritime, and coastal New England. Skomer volcanoes in Wales ( part of Baltica) had produced 1000 meters of basalt flow in the Llandovery epoch. There are some other eruptions in Massachusetts, Maine, New brownswick (Laurentia). Perhaps the most extensive volcanic events happened in eastern Australia. In South Wales, Victoria, and Queensland activity initiated during the Wenlock epoch and finished during Pridoli time. These volcanic activities occurred in the terrestrial environment.
As I have written that sea-level changes were not extreme. A major portion of Gondwana was still on the south pole. Moreover, south America had ice until the middle of the Silurian period. In Llandovery time alone we had 4 global high stands ( higher sea levels). To study sea levels we look for biological communities living in that particular environment. Although moving a bit downslope or upslope can not alter species behavior drastically, we can still see differences. While study if we find evidence of sea-level changes we compare it interregional and intercontinental. Other periods of Silurian had also seen sea-level changes but data is scarce.
Silurian had a fair share of sea-level low stand and it had affected ocean circulation. The epicontinental seas had water shortages which led to evaporite depositions. Evaporites are chemicals like sodium, magnesium dissolved in water. After the evaporation of water, they crystallized into rock layers. Low stands were more local affecting a particular region more. Underwater volcanism or tectonic movements could be the reason. We have proof of at least 3 successive smaller ice ages in the brazil sector or Gondwana. halite and anhydrite occur in Carnarvon Basin. Extensive beds of both or either of these is found in the Canning and Bonaparte Gulf basins.
Silurian life forms
Silurian was a rather exciting period for many firsts. We have found fossil remains of the first land plant, first vascular plant, and first land animals. First land plants only grew near swampy areas like the coast and estuaries because they reproduced by spores. CookSonia had more of global distribution and they were only a few inches tall with no leaves and green bare branches. Baragwanathia was endemic to Australia and it was a large plant
The Silurian life forms unfamiliar but a step further towards our normality. I know you people might be eagerly waiting for Devonian fishes, Giant insects, the great dying, or dinosaurs, mammals. However, this slow journey might show the similarities between organisms. Atrypa is an extinct genus of brachiopods (Shelly animals). It was a global species that swam in Silurian waters from Silurian to the early Carboniferous period. Baragwanathia is an endemic plant species of Australia, the biggest land plant of that time. It had true leaves and a vascular system, reproduction was through spores. The first fossil dates back to Late Silurian times.
The starships of biodiversity
Baragwanathia comes under the genus Lycopsids which also includes Mosses. It was a giant according to past standards growing 11 inches in height. Cyathocrinites is an extinct genus of sea lilies. They lived from Silurian to Permian in ocean water. It came into the limelight in the Carboniferous period. There are many species and groups of brachiopods, lobsters, corals in the Silurian period.
However, the star and apex predators of Silurian were giant water scorpions. They are part of the extinct subclass Eurypterida of Arthropoda. You just imagine terrestrial scorpion and you can the idea of how these predators looked. The body anatomy is quite similar and water scorpion were bottom dwellers. Nearly 2.5 meters(8 feet). The fossil has recovered from brackish and freshwater regions. They probably developed in oceans and then moved to freshwater regions. They had used their claws to catch the prey and their appendages in the tale had poison similar to a present-day scorpion.
- The Silurian period by Britannica.com is indeed one of the most detailed posts on this topic. It has everything in a brief and concise manner.
- A post by LiveScience.com on the Silurian period can be a good read if want.
- You can watch this video on the Silurian period that had explained everything in audio format.
- Other resources are as similar as the previous post The Silurian period- Recovery 1.0.
I am getting more excited as we are moving on ahead. Now it’s getting spicy. The Devonian period is the time of fishes. Now you will hear about animals beyond savage. It s action time so, stay tuned and Do Revelation.
Originally published at https://www.thescientificrevelation.com on May 25, 2021.