The Silurian period — Recovery 1.0 ⋆ TheScientificRevelation
Namaste friends. Welcome to another chapter of the “History of Earth” series, The Silurian period — Recovery 1.0. We are in the third period of the Palaeozoic era and I know still things are a lot unfamiliar. We haven’t seen any known and common animals or plants. This can make you realize that our normal originated very late in the geological timescale. The Silurian period started 443.7 million years ago and ended 416 million years ago. Now let’s set the stage for the Silurian period. It started when the late Ordovician ice was melting off south America and Africa. However, Gondwana didn’t become fully iceless with the onset of the Silurian period. We have evidence that supports the presence of ice till the middle Silurian period.
When Silurian started biodiversity and life were handling a mass extinction, as the earth had been hit by one of the five major extinction. We shall remember these two points as we move forward to understand the Silurian period.
If there is magic on this planet, it is contained in water.
The Silurian period changed when the water had surged back into dried lands.
Scottish Rodrick Murchison named the Silurian period in the honor of native Silures who had resisted Roman conquest. The first rock layers dating back to the Silurian period found in Wales and neighboring England. It has four epochs Llandovery, Wenlock, Ludlow, Pridoli. It is quite easier to understand the period if we follow the flow of epochs. When Silurian started ice caps were melting off the southern supercontinent Gondwana. When the ice melted heavy erosion happened and eroded material flew along the ancient boundary of the supercontinent.
The onset of high productivity and sea-level rise
Firstly, along the ancient coastline, we have found sedimentary deposits. These sediments got deposited layer by layer and formed shale deposits. Secondly, erosion had saturated the seawater with many chemical compounds that had induced better productivity. Due to this surrounding water had high biodiversity. This incidence was like an Algal bloom. So a bonus point would be that after the algal bloom when those organisms died our their dead bodies drowned. Before any decomposers can act on them, they got buried under sediments and turned into organic-rich shales. These deposits are a big source of Silurian fossil fuels.
This nutrient-rich water had pushed biodiversity and evolution after a major extinction. New kinds of animals took hold of the land. We know that ancient plants had set the first steps on land in the Ordovician period itself. However, in Silurian ancient coastlines were dominated by liverwort like non-vascular plants. When glaciers melted water was freed in the oceans and sea levels had risen up. Water had again filled up continents creating epicontinental seas. The life that had disappeared from these regions came back as the water level surges high. I won’t say the Silurian period was very exciting as life was recovering from a major extinction. I think in Devonian and carboniferous were more exciting time periods for life. You will see how in future posts.
Positions of continents/paleogeography
The geography of Silurian was much like the Ordovician period. Panthalassic ocean again dominated the north pole and Gondwana had covered the south pole. The rest of the six continents straddled the equator and nearby regions. In the northern hemisphere, the climate was very static all around the year but in the southern hemisphere, climate depended very much on the interplay between land and water. Paleomagnetic data is a proven way by which we can find the orientation of landmasses in the past.
To get a brief idea, after a volcanic eruption, lava and other pyroclastic material rich in iron plus substitutes align themselves according to the magnetic field of the earth. You know by the simple law of magnetism how iron flakes act in the presence of a magnet. The same law applies to lava and when it solidifies those alignments remain the same forever. If that land doesn’t melt or recycled through subduction alignment of magnetic material never changes. Whenever that land travels it carries with itself that signature alignment and this helps us reconstruct how continents moved through time.
The story of Laurentia, Baltica, and Avalonia
Laurentia which included North America, Greenland, north Ireland, Scotland, and the Chukotskiy Peninsula of eastern Russia was situated between 30 degrees N and 30 degrees S of palaeoequator. During middle Silurian 65% of the Laurentia was submerged underwater. The narrow leptus ocean had separated Laurentia from Baltica. Laurentia had seen two important mountain-building events, Taconia ( landmass that became northeastern united states) and Pearya (it became the northern part of Ellesmere Island). In the same manner, Baltica had Uralian and Variscan-Hercynian mountain-building episodes that made eastern and southern boundaries of it. The Scandinavia part of Baltica was just below the palaeoequator.
The Caledonian highlands of Norway formed during this time. The Avalonia microcontinent was on the collision course with Baltica. In the Devonian period avalonia, baltica and Laurentia will coliide to form supercontinent Euramerica. This collision will form mountain ranges of Ireland, Scotland, Wales, Norway, and Sweden similar to the Taconia orogeny of Ordovician. You see in Permian all the continents were attached as a Pangea supercontinent and it all started long ago. Continents had started to come closer far before since Silurian times.
The southern continents of Gondwana, Siberia, and Kazakhstania
We see two more ocean oceans mentioned in the Silurian period. They probably were extensions of know bigger ocean bodies. Pleionic Ocean had separated Baltica from Siberia and Kazakhstania was situated in the east of it. North China (including Korea) and south china were separate continents situated in the equatorial region. Gondwana was in the southern hemisphere. It had Australia, Antarctica, India, Arabia, Africa, and South America, Silurian Gondwana also included smaller pieces of Florida, southern Europe, and the Cimmerian terranes. Cimmerian terranes with Turkey, Iran, Afghanistan, Tibet, and the Malay Peninsula were on the outer fringes of Gondwana. During the middle Silurian period India, Tibet, the Malay Peninsula, and Australia were in subtropical or tropical latitudes. In those times Rheic Ocean had separated the southern European sector of Gondwana from Baltica (it had northern and central Europe).
I have said it before that the northern hemisphere had a more stable climate because of a large ocean. On the other hand, the climate patterns in the southern hemisphere depended on the interplay between land and ocean. In the northern hemisphere, very stable polar easterlies flew towards the east. Midlatitudes were dominated by prevailing westerlies and northeast trade winds in equatorial regions, very much like today. If you want to read more about the weather and oceanic circulation of present times refer to my blog post on ocean currents.
The southern hemisphere, rainfall, and ITCZ
Now let’s move to the southern hemisphere. You can imagine the southern hemisphere as a mix of water bodies and lands spread across in an uneven manner. With so much going on the climate here had extreme variation lie winter Colling and summer heating much like many subtropical countries. The northern Panthalassic ocean had low-pressure zones and around 60 degrees N we had a low-pressure zone. This is an important point that here. Low-pressure zones also form near the equator. Here we see an intertropical convergence zone (ITCZ) where northern and southern trade winds force warm tropical air to rise and it produces regular cloud cover and rainfall. Because of this tropics get heavy rainfall.
In summers Heavy heating on southern continents might have shifted ITCZ northwards. Likewise in northern winter, ITCZ might have shifted southwards. We know this due to sedimentary deposits. Near the ancient boundaries of Laurentia and Baltica, we have clastic deposits. Clastic rocks are a type of sedimentary rocks that have fragments of older rocks. Both continents were in the southern hemisphere and have low-pressure zones. Heavy rainfall might have induced erosion due to which sedimentary deposits accumulated near coastlines. Australia and Antarctica had the highest rainfall according to their somewhat equatorial position.
Upwelling, high productivity, and black shale
Most of the Gondwana had mid-latitudinal westerly winds. Due to this, we had high upwelling around the margins of it. Upwelling is the process by which cold water from below replaces the warm surface water. Winds flow alongside the continental margins and they replace the surface water. Coldwater takes its place and brings rich nutrients from the ocean floor. Upwelling is always followed by increased productivity, algal bloom, and rich biodiversity. When these animals died out they settle on the ocean floor. The ocean depths are normally less oxygen-rich and in Silurian when the earth was still experimenting ocean depths were anoxic. Due to this Animals died out in high numbers and they settled on the ocean floor before they can get decomposed a new layer of sediments fell on them.
This process had made some of the most extensive black shales. These layers of the ancient Gondwana, are the biggest source of Silurian petroleum and natural gas. In spite of all these variations, Silurian had a stable climatic pattern.
The most profound feature of the Silurian environment was high sea levels and flooded continents much like the Ordovician period. The Silurian species had more globally distributed rather than being endemic to certain regions. Endemism was high in Ordovician times and those species were very successful. After the ice had melted and water had again submerged the continents, diversity could have gone back to previous times like Ordovician. Both endemic and global species were quite successful during Ordovician times. The end Ordovician extinction was equally damaging for both of them. At the start of the Silurian period when the intercontinental sea again came into the pictures life had taken a different route. Globally distributed species were more successful in the Silurian period.
Major marine communities
Two major kinds of life forms are visible in the Silurian period. Nearshore animals that lived in the shallow water and needed optimum sunlight as well as temperature. Another kind was offshore, deep water-dwelling kinds that lived near ocean depths. In early Silurian, we see a specific kind of Pentamerus community. It was dominated by large shelled brachiopod Pentamerus oblongus. The other species of large shelled brachiopod had dominated the younger or older equivalent of the Pentamerus community. These all have occupied a medium depth near continental margins. They required sunlight because we have found green algae in the fossils. And green algae need sunlight for photosynthesis. We know that these species lived near medium depth because they were susceptible to wave damage of surface water. These communities thrived between 30 to 60 meters of depth.
In wales which were part of Laurentia, have found fossil proves of brachiopod-dominated communities. They lived in all ranges from shallow waters to deep water settings. From shallower to deep we have Lingula, Eocoelia, Pentamerus, Stricklandia, and Clorinda communities. In the oceanic basin, we have fossils of graptolite shales with the same story as Gondwana shales. In other regions of Siberia and Laurentia, we have found Pentamerus, Stricklandia, and coral-stromatoporoid dominated communities. Coral-stromatoporoid communities dominated the shallow water settings with wave motion much like present coral reefs. They were also sensitive to sedimentation.
If you want to see a full picture inventory of all major Silurian life forms refer to the Silurian period by Natural history museum.
Sea scorpions- an apex predator
Eurypterids more commonly known as sea scorpions were the stars of the Silurian seas. They are the biggest arthropods ever roam our planet. Before fishes became dominant in the Devonian period, sea scorpion ruled. They were skilled swimmers and had claw-like appendages to walk on the ocean floor. A long tapering tail with some spike-like feature might be used for injecting venom.
Reef mounds and corals
Reefs were very successful since the Ordovician period except the difference being the species that inhabited them. The Silurian reefs were dominated by brachiopods, gastropods (class of mollusks that includes snails and slugs), crinoids( echinoderm class that includes sea lilies and feather stars), and trilobites. Reefs were most successful and diverse during the Wenlock epoch. We see differentiation of species/biozones like modern times. Tropics are most rich in biodiversity and as we go polewards this productivity decreases. The pentamerous communities, coral-stromatoporoid communities were present in tropical regions. In southern temperate zones, we had low-diversity Clarkeia (brachiopod) in Africa and South America. In the northern temperate region, we had Tuvaella (brachiopod)fauna restricted to magnolia and Siberia. Tuvaella fauna was present in china also. Modern coral reefs first appeared during the Silurian period.
Fishes and land plants
Fishes appeared in the scene in the Ordovician period itself. All kinds of fishes were present from 40 degrees north to 40 degrees south in all kinds of marine settings. We have fossils from full bodies to individual scales. Jawless fish(Agnatha) had orders Thelodonti, Heterostraci, Osteostraci, and Anaspida. As a bonus point, we also have Primitive jawed fishes in subclasses Acanthodii, Elasmobranchii, and Actinopterygii.
Vascular plants were becoming more diverse forwarding towards their extraordinary future. Liverwort like nonvascular plants had populated the swampy coastlines of all continents. Inland was still very much barren as they need water for reproduction. The distribution ranged from 45 degrees n to 30 degrees south. Cooksonia is among the most successful species with distribution in every continent. These plants had 5–6 cm, Hight, with green stem and no leaves. It is the first example of vascular plants. Baragwanathia was endemic to Australia in the Ludlow epoch. Along with these first fossils of arthropods have also discovered from the Silurian period. They were mainly annelids-like creatures and distant relatives of spiders, centipedes.
The land was getting crowded with plants and animals. Very soon even the most inhospitable regions had life living there. We will introduce New characters in upcoming posts.
- A very easy and comprehendible post on the Silurian period is by LiveScience, with climate, continents, and marine life.
- Another well-written blog post on Silurian times by Berkely education. It includes life, stratigraphy, tectonics, and paleoclimatic activities.
- A detailed and highly informative post on the Silurian period by Britannica. It has every perspective explained in detail with pictures.
- Few youtube documentaries on the Silurian period History of life part 2- Ordovician through Permian, lecture 2: Ordovician and Silurian, and The Silurian & Devonian periods.
I am quite abruptly ending this post as you can feel that a lot I have not included. While going through the reading material I have thought of finishing up Silurian in one post. However, when I was researching more I realized that I need to include many things. We haven’t discussed much geology, animal species, rock type, and a tiny extinction. I will involve these things in the next blog posts. Until then stay tuned and Do Revelation.
Originally published at https://www.thescientificrevelation.com on May 10, 2021.