The Ordovician period- A Prequel in time ⋆ TheScientificRevelation
Holla friends welcome to another post of The scientific Revelation where we will jump right into the Ordovician period-A prequel in time. You must be thinking we have discussed Ordovician extinction in the previous post so why discuss the period in general. I haven’t included a lot in the extinction episode which can be useful to understand our journey. In this post of “The History of Earth,” series we will learn about Ordovician as a whole. Take it as when we have to increase the speed of our bike it moves a little backward before speeding ahead. This post is a little backward push before we speed away into forward.
If there is magic on this planet, it is contained in water.
Few major points
The Ordovician period started 488.3 million years ago and ended 443.7 million years ago flowed by the Silurian period. Up until now the amount of research paper I have read on this period there are two things by which you can classify it, high water level with tropical climate in the beginning and low water level with ice age in the end. Keep these two things in mind and you will see the connections. This period is credited with the second most important diversification event called The Great Ordovician diversification event and the second most widespread extinction, The end Ordovician extinction event. The diversification had introduced more advanced animals in the particular phylum.
The acceptance story of the Ordovician period is interesting. In 1835 the English geologist Adam Sedgwick had named Cambrian system and at the same time, Scottish geologist Roderick Murchison had named Silurian period. Both periods started to overlap and to calm the dispute in 1879 Geologist Charles Lapworth proposed the Ordovician period. Ordovician has 3 epochs early (488.3–471.8 million years ago), middle (471.8–460.9 million year ago), and late (460.9–483.7 million years ago). Basically in every epoch climate was a little unique with different dominant species. Along with the main extinction event Ordovician had many local smaller extinctions.
During Ordovician, there were three oceans and four major continents. How are we so sure of the position of continents? Well, we can never be truly sure of the past but with scientific methods, we can go to the closest possible. By paleomagnetism, geography, fossils of animals and plants, and orogeny(mountain building) we can very precisely estimate the position of continents. Paleomagnetic data is very important and precise. During the process of seafloor building and volcanic eruption, the lava that came out has magnetic elements like iron and nickel. The magnetic elements in lava align themselves according to the current magnetic poles’ position.
You should know that every thousand years magnetic south and north poles interchange their positions means magnetic north becomes south and vice versa. The intensity of magnetic alignment also depends on the latitude that means up in the higher latitude we see more magnetic alignment in comparison to tropics. This information helps a lot to find the paleogeography of previous times. Along with it mountain building, sedimentation, fossils remains help to figure out where that particular continent was in past. A fun fact many of you know our cold and white Antarctic but it was not always like that. Once continent Antarctic was green with a beautiful tropical environment. How do we know that? With the help of the above-mentioned methods, especially by fossils of tropical animals and plants, we found its position in the past.
Position of landmasses
The craton (stable interior parts of the continent that have faced the test of time and remained intact) of Laurentia that includes North America, Greenland, and Scotland was the equator. The craton of Siberia- Kazakhstan was east of Laurentia. So Laurentia was in the western hemisphere and Siberia- Kazakhstan was in the east in opposes to their current position in higher alpine latitude. The Iapetus sea was in between them and the craton of Baltica was in the south. Baltica consisted of present-day Scandinavia and north-central Europe. In the west of Baltica, the microcontinent of Avalonia was situated. It had some portions of England and maritime Canada.
The paleo Tethys ocean had separated Avalonia, Baltica, and Siberia- Kazakhstan from Gondwana in the south. Gondwana was the southernmost continent straddled between Paleo Tethys and the Panthalassic ocean. Gondwana had present-day Australia, India, South America, Africa, Mexico, Antarctica, Some parts of Arabia, and China. The Panthalassic ocean was the biggest and had covered the whole northern hemisphere.
As I have told you earlier there was a lot of water in Ordovician. No Icecaps whatsoever were present and all the continents were flooded with extensive intercontinental seas. When supercontinent Rodina broke apart during Cambrian heavy sedimentation happened and many shallow seas formed near continental margins. The seafloor spreading was high during this period and when oceanic mountain ridges formed water was displaced which also had increased the sea levels. This excess water had flooded the coasts and entered inside the continents. Ordovician was a turbulent time. Sea levels kept on decreasing and increasing throughout the period. The end-extinction was also linked with it.
When the temperature dropped down and glaciers formed water level decreased. Most animals were residing near continental margins in shallow water. Falling sea levels have exposed them to the scorching sun. It had destroyed their habitat and wiped the species out. When this ice melted water came crashing away and might have brought a tiny level of extinction. Apart from these two big events, there was the local and shorter sea-level fluctuation in sea levels. We are not sure of the reasons but plate motion and mountain building might be responsible.
We can not say for surety about the ocean currents that prevailed in the Ordovician period but our current understanding can help us a bit. The Panthalassic ocean was in the northern hemisphere and there were no continents in between so it had a westward circumpolar current. Westward means for example an ocean current going from Africa to the Americas is westward (towards the west). Laurentia and Gondwana were interjected from the south in the ocean so there were two smaller clockwise Gyre formations. The westward circumpolar current flowed above 60 degrees north and the two gyres formed between 60 degrees N and the equator.
Paleo Tethys and Iapetus oceans were present in the southern hemisphere. Paleo Tethys had counter clock gyre south of 60 degree S and Lepetus had clockwise gyre formation between 60 degree S and equator. Smaller variations were everywhere due to volcanic islands. In millions of years, ocean circulation kept on changing according to tectonic motion. There may have monsoon rain on some portions of subtropical Gondwana. This ocean circulation was disrupted during glaciation. The water near the south pole around Gondwana froze a bit. This cold heavy water by sinking had increased upwelling and started something like thermohaline circulation. Read my blog post on ocean currents to know more about modern ocean circulation.
One of the most important geologic cycles and the main reason behind the earth being itself, tectonic motions were very active in the Ordovician. Along with volcanic activities and displacement of Gondwana in the south, high mountain buildings happened in Ordovician. When Rodinia broke apart and new continents formed, seafloor spreading was high. This had created Paleotethys, Iapetus, and Panthalassic oceans. The seafloor spreading is a regular process where new ocean floors form lava and to balance this out in the boundary of continents we have subduction zones. These subduction zones are places where the ocean floor slides under the continental plate and mixes with the mantle. Scientists think the Panthalassic ocean was bordered from all sides by subduction zones, much like the modern pacific ocean.
From this period itself continents had started to come together for the formation of Pangea. Baltica traveled to Laurentia from the south and while closing some portion of Lepetus sea collided with it in the Silurian period. Paleotethys sea also narrowed when Gondwana started traveling towards Laurentia. It had rotated and Africa was right on the top of the south pole.
Volcanism was very extensive during Ordovician. Two major peaks happened at the Cambrian-Ordovician boundary and between the middle and late Ordovician epochs. Heavy volcanic activity was the reason behind the tropical climate in the Ordovician period. The constant gas eruption had kept the atmosphere saturated with carbon dioxide and other greenhouse gases. This had maintained the greenhouse effect and kept the temperatures high. To get a good understanding of Plate tectonics refer to my blog posts, Plate tectonics and continental drift, Plate tectonics 2.0- past and present, and Plate tectonics 3.0- A unique shapeshifting.
The level of atmospheric carbon was higher during Ordovician, almost 14 to 16 times higher than today. Volcanism was high and most continents were flooded underwater. Due to this fewer rocks were exposed to weathering. Weathering is one of the processes by which atmospheric carbon dioxide break down the rock into soil. Furthermore, there was no prominent vegetation on land so no photosynthesis. In short, more carbon oxide was being added into the atmosphere than stored. High carbon dioxide had higher temperatures from the equator to the poles.
Whenever people talk about climate change you must have heard them saying extreme weather. Well, it is because if the temperature will be more hurricane and extreme storms will naturally come. You might know that tropical storms absorb power from the surface of the water. Higher temperatures can feed more severe storms and that is what happened in the Ordovician period. It was more or less a greenhouse planet.
Ordovician didn’t have life as we know it but it was more complex than Cambrian. The updated predators and new species had made the ecosystem more lively. In the Cambrian period, organisms lived on or near the seafloor. Mostly were bottom feeders and lived near the floor. However, in Ordovician, we had tiers in the ecosystem much like today’s forest. Apex predators like Orthoceras were good swimmers. Coral reefs were more vivid and diverse like modern time and species like Platform and Horn corals originated in Ordovician.
Ordovician had seen the rise of early Jawless fishes like Arandaspis. We are not sure of the morphology of it but with the fossil bone, we can deduce that it was a bottom feeder of shallow oceans. Mollusks were the star of the period spanning many species across the family. Predator and prey both were unique and advancing towards a better form.
First land plants
The land was barren for the most part but land plants have started to colonize it. Bryophytes and Cryptospores appeared around the middle Ordovician epochs. We have found remains of spores from this period from China and Argentina. There were as far as four distinct species. These Cryptospores had cuticle covering which means it was a protection for spore against harsher conditions. This cuticle is the main reason these spores have fossilized. We have two major groups of land planets from this period.
Firstly, Cryptospores were more widespread and have given rise to modern nonvascular plants like Liverwort and bryophytes. The other one Triletes had formed spores through meiosis, four cells. Because of its modern style of reproduction they might be ancestors of vascular plants. Triletes were geography confided to certain localities in comparison to more Cryptospores. Studies say these first plants were responsible for Colling down the planet bringing the ice age. This is a mind-blowing new direction to look towards glaciation and subsequent glaciation followed after it. Refer to this research paper ‘ The first plant cooled the Ordovician ‘ for more into the subject.
We don’t have any fossil of land animals from this period but trace fossils tell a story about millipede like animals. Perhaps Arthropods have spread to the coastal area transformed that environment.
The intercontinental oceans have Limestone and Dolomite rocks in Laurentia, Kazakhstan, Siberia, and south china. These rocks formed by the Calcareous skeleton of animals, carbonate mud, and Peloids (small grain-sized particles formed by algae and mud). The evaporite deposits formed between 30 degrees N and 30 degrees S, proper tropics in Kazakhstan and Laurentia. It proves that these two continents were right smacked in the equator. Coarse-grained siliciclastic rocks (formed by broken silicate rocks) either formed in higher latitudes with high carbon or lower latitude with orogeny (mountain building) activities. These types of rocks are sandstones and conglomerates. North American Appalachians are filled with them. These sandstones are also abundant in Gondwana specifically South America, Antarctica, and Africa. Mind you Gondwana was in higher latitudes in the Ordovician period. They are also a major part of Glacial deposits like dropstones and tillies found in Africa.
Life is only a flicker of melted ice.
Fine-grained siliciclastic rocks like shale and mudstones have accumulated in deepwater settings of Ordovician oceans and the intercontinental sea. Black shale symbolizes anoxic condition and grey or red shale means high dissolved oxygen. Volcanic ashes are also common form this period as volcanism was high. They are found interbedded with other types of rock like sandstone, limestone, and shale. Other volcanic rocks are also common near subduction zones currently present in China and Kazakhstan. These carbonate and limestone rocks are major sources of carbon fuel in the united states of America and Canada. The Ordovician phosphate is useful as fertilizers.
The stages of Ordovician
There are 7 stages in Ordovician, two in each early and middle epoch and three in the late epoch. They are Tremadocian (1), Floian (2) of early Ordovician; Dapingian (3), Darriwilian (4) of middle Ordovician, Sandbian (5), Katian (6), and Hirnantian (7) of late Ordovician epoch. Scientists have used brachiopods (lamp shells) and trilobites for correlation and differentiation of stages. These two animal groups are widespread and have endemism so they are a good candidate for study. All the stages of Ordovician have sea-level fluctuations and few species have gone extinct. The Ice age happened in The Hirnantian Stage which brought the major extinction of fauna. To know more about Ordovician extinction refer to my previous blog post “ The end Ordovician extinction- first of major five “.
- read this brilliant and detailed research paper on the Ordovician period b Harry Blackmore Whittington and Alwyn Williams.
- There is a book by Columbia University Press on the great Ordovician Biodiversification event. Read the 8th and 9th chapters on diversification and glaciation respectively.
- If you are more interested in the Great Ordovician biodiversification event refer to this research paper on Oxygenation as a driver for diversification.
- The major inspiration for this post has come from a book, The Paleozoic Era: Diversification of Plant and Animal Life. Do read it, you will get a lot of information.
You see things are getting very interesting as we are moving forward. More action is waiting on the horizon. This story will get even more spicy and intriguing when we will enter the Mesozoic and Cenozoic eras. Until then stay tuned and do revelation.
Originally published at https://www.thescientificrevelation.com on April 26, 2021.