Carboniferous rainforest collapse and Pangaea ⋆ TheScientificRevelation
Welcome to another post of “the history of earth” series. Carboniferous rainforest collapse and Pangaea have one thing in common. Firstly, these both were history-defining movements in earth’s past. Moreover, they both happened due to plate tectonics. Folks if you haven’t already read my posts on plate tectonics go give them a try. Plate tectonics- past and present, plate tectonics- a unique shape-shifting and plate tectonics- continental drift will give you a better subject on our star culprit.
This title is enough to make the blog post interesting. We have finally come this far where we can see the aggregation of the supercontinent Pangaea. At the end of the Carboniferous period, all the landmasses came together to form the massive single supercontinent Pangaea. It was surrounded by superocean Panthalassa. Now let’s jump into the post.
The transition from wet to dry
The Carboniferous period was wet, warm, and tropical. To put it simply, continents were like giant Tropical islands with lush forests. The fragmentation of continents allowed the moistures from the ocean to travel across continents which in turn enabled the growth of identical tropical forests. Landmasses weren’t far from each other. They were coming together to form the future Pangaea from the Devonian period. Earth’s climate was constantly changing and animals were adapting to it. The lush water-rich environment of carboniferous, specifically early carboniferous, had promoted the growth of amphibians and insects. They thrived in those swampy wet ecosystems.
Life in the Carboniferous period
The oxygen levels were highest in carboniferous, reaching almost 35% of the atmospheric constitution. This has enabled insects to go bigger. You must be familiar with dragonflies. We had meganeuras, very similar to a dragonfly with a wingspan of 1 meter. Human size millipede/centipede-like insects were crawling under bushes preying on other insects. It was indeed the heaven of insects.
The widespread swampy conditions had also supported the diversification of amphibians. In the previous period tetrapods have made ascend on the land. From there on they diversified into many groups of stem amphibians and true amphibians. Stem animals predate the main species, so you can call them transition species of two connecting groups. Amphibians were dominant land vertebrates on land. Swampy conditions had promoted their kind. On top of that, big insects were an ample diet for bigger amphibians.
The Carboniferous period is divided into two subgroups that are in use globally, Mississippian and Pennsylvanian. The early carboniferous mean Mississippian sub-period was wet and tropical. However, as we moved towards a later carboniferous climate, it became dry and arid. At the end of the carboniferous continents came together as Pangaea.
The rise of ammonites
At the end of carboniferous when Pangea formed ammonites mainly reptiles became dominant species. The newly formed Pangea had enough dry inland with scarce water. The ammonites have an ammonitic sac that nourishes the young ones in the absence of water. This evolutionary advantage had given reptiles an upper edge to survive in dry inland. With the Pangea assemblage and many million subsequent years dry conditions persisted. This environment had promoted reptilian life to the fullest until the end of cretaceous extinction after mammals took over the world.
In early carboniferous, the comparatively smaller size of continents had allowed clouds to carry moisture even to the interiors. These wet conditions had provided ample niches for amphibians that depended on water to complete their life cycle. However, amphibian’s dependency on the water turned out to be a bad adaptation in Pangaea which was the total opposite of carboniferous with dry and arid conditions. This is the sole reason that in the Permian and subsequent Mesozoic era reptiles ruled over the world.
Early carboniferous was hot and tropical. The average temperature was around 20–25 degrees C. Across the earth we had tropical conditions and that’s why trees had no growth rings. In the middle carboniferous everything started to change as glaciers began to form on Gondwana. The temperature on earth dropped and higher latitudes had more of a temperate climate. The growth of glaciers had sucked ocean water and sea level dropped.
Read the rainforest collapse of carboniferous and how it changed the world by Conversation.com.
We say that carboniferous rainforests collapse was the tragedy of plants. However, it was a pretty bad time to live as animals especially those who directly depended on forests.
The middle carboniferous was not a good time for shallow ocean dwellers. Corals (horn and rogues) and other filter feeders like brachiopods and bryozoans, suffered during this time. As the continents were coming together and colliding, mountain ridges formed. There was high erosion due to the constant wrestling of landmasses. The erosion had made shallow coastal waters murky for days. Opaque conditions proved to be fatal for filter feeders and in return, their numbers declined.
The carboniferous stands for coal-bearing. The immense coal deposits of carboniferous formed because due to the collapse millions of trees were buried under soil without being decomposed. No insects lived during that time to digest the hard lignin material and that’s why it was left to fossilized.
A new land
The collapse happened gradually instead of instantly. As landmasses came together forests became fragmented and isolated patches. This also had induced more endemism of plant and animal life. In more than one way endemic species are more susceptible to any crisis. They need to the specific environment to live and minute variations can lead to death. Some research points towards more connectivity instead of endemism. Research has shown that when forests became fragmented and the area opened up animals were able to move vast lands. It was a golden situation for reptiles as they can raise they don’t need water for dependency. Moreover, because of hard-skinned covering, they were able to conserve water in the new dry world. There is a famous research paper on Rise of tetrapods following Carboniferous rainforest collapse. do read it.
The stages of collapse
The rainforests’ collapse happened in multiple stages. In the presence of trees erosions and water levels were in check. Trees have a complex root system that holds the soil and doesn’t let fertile layers wash away with water. They increase soil health and provide better conditions for soil organisms. In return, the soil holds water and lets it seeps inside to the water table. Forests provided directions to the rivers and shape their flow. These cycles got disrupted with the rainforest’s collapse.
When forests started to disappear the stability of river systems decreased. Imagine the Amazon river flowing through the rainforests. It can not flow in any direction as there are trees, debris, and wood jams on the way to direct its motion. In the late Carboniferous with no forest blocking the way, rivers flew in random directions. It had increased soil erosion and reduced the water carrying capacity of the land.
Rise of ferns
trees became separated into patches and were only able to grow in patches where sufficient water was available. until the later Carboniferous forest transitioned from lycopsids dominated to fern dominated.
As dominant trees were unable to handle conditions of later Carboniferous ferns replaced them. Ferns are like weeds that can grow anywhere with fewer resources. Ferns need less water to sustain themselves as they are not woody and tall. They have taken advantage of suddenly open niches. fossil finds belonging to later carboniferous have fewer log jams(tree trunk) and it is a telltale sign of disappearing forests.
Seasonality and onset of colder climate
The changing climate had introduced seasonality, especially into the higher latitudes. Seasonality means the onset of the season. Except for the tropics, every other biome experiences some kind of seasonality. In the tropics the intensity and availability of light are optimum. Plants always get growth opportunities as there are no lean seasons that’s why they don’t have growth rings. On the other hand in temperate regions, both light incidence and temperature are low. Temperate trees show growth in summer and in winter months they just survive.
As the world moved from tropical to temperate conditions the plant adapted to cold survived and others perished. Gymnosperms did fairly well in dry conditions of Pangea as they were more suited to seasonality. Animals like reptilian and subsequently birds, mammals made better use of this new world.
Why did the rainforest collapse happen?
The earlier Carboniferous period was wet and tropical with large trees. As the period came to an end, Pangea had already congregated. The earlier carboniferous of the Mississippian subperiod was vastly different from the later Pennsylvanian subperiod. You see, the difference was so huge that it is sometimes hard to imagine. It is easy to carry moisture far inland if land masses are smaller. This is what happened in earlier carboniferous times when rainforests had grown in far inland areas.
In the later carboniferous the dynamics had changed. Pangea was huge and major areas were inhospitable except for some hardy reptilian ancestors. There are many reasons responsible for this tragedy and we shall discuss them one by one
The plants are the culprit…..
After the late Devonian extinction we are again putting the blame on plants but bear with it. Things are about to get interesting.
Carboniferous had the highest oxygen concentration among all the geologic periods. It was as high as 35% in comparison to the present level of 21%. That’s a lot of oxygen andThe reason behind it is quite understandable. Because the climate was so welcoming for lush tropical vegetation they spread like wildfire through all the landmasses available. A lot of trees mean high oxygen. oxygen levels roared crazily in the early Carboniferous period. Oxygen was so available that arthropods became huge. They were larger than the recent ones. Many of you must have seen millipede/centipede and many of you might dislike it. Now imagine a 7-foot centipede slithering on the floor. What a sight to behold. Large insects including meganeura were crawling under bushes of large Lycopodiopsida trees. Amphibians were the dominant land vertebrates and thrived in swampy conditions.
oxygen and ice cycle
As you know plants do photosynthesis and in the process, they absorb carbon dioxide while releasing oxygen. When lush three-storeyed forests covered the majority of continents they absorbed more carbon dioxide. Constant absorption had lowered the percentage of carbon in the atmosphere. On top of that trees release oxygen replacing carbon dioxide. We all know that carbon dioxide is a major greenhouse gas. When its percentage dropped, the temperature also plunged. Lower temperatures had brought the ice age to the southern hemisphere. We have proof of glaciers in South America as well as Antarctica.
Once the ice building started it became harsher with subsequent years. The weak greenhouse was unable to melt it and boom glaciers spread all over the south pole. It was no longer a warm topic planet. Except for the tropics, higher altitudes were affected by this change in temperature. Lush dense forests were replaced by more seasonal vegetation like gymnosperms and ferns.
Fall of sea level
In this way plants were themselves responsible for their demise. Carboniferous rainforest collapse is the second major plant extinction. When glaciers formed, ocean water was locked in them and it lowered the sea levels.
Coastal regions as well as far inland that were always submerged suddenly became dry. Lepidendron and sister species needed a lot of water to sustain themselves. In the absence of water, they died out. The amphibians and other swamp-based species needed water to survive. They were unable to cope with dry conditions. As swamp and rainforests shrank in size, so do their dependents. The vast dry land had favored different types of species(reptiles) that adjusted to dry land.
The role of plate tectonics…
Plate tectonics has always been the force behind changing the face of the earth. The constant motion of plates has moved continents across the globe. When a landmass moves to a certain latitude its climate and environment changes. Therefore, it can no longer sustain the previous life. For example, if a landmass moves from the tropics to the temperate region, temperature and sunlight availability change. Temperate regions have lower temperatures and less sunlight. So, it is easy to understand that tropical life couldn’t cope with the changes and it is being replaced by cold-bearing species. Through ages, we have found fossil evidence of tropical Antarctica when it was near the tropics. The same is true with fossil finds of every continent.
Pangaea assemblage and new environment
Carboniferous rainforest collapse may partially have happened due to the continental movement. In the early Carboniferous period, the earth still had fragmented landmasses scattered near the tropics. Sufficient sunlight and high sea level had promoted rainforest growth. In the middle and later Carboniferous period, landmasses were aggregating into Pangea. North America was in contact with Europe. South America was joined with Africa. Just remove the Atlantic ocean and imagine all of them together. Asia was situated on the eastern side. Australia and Antarctica were in the southern region of Pangea. India, then part of Gondwana land, was just next to southern Africa.
Pangea was surrounded by Panthalassa. Imagine how clouds can move inland in such a vast land. Consequently, Pangea only had rainfall near the coastlines. It had transformed the tropical island condition into large dry land. In the vast Pangea, the moisture from the ocean could not move inland due to the shrinking rainforests. Only coastal regions that were able to hold water maintained decent rainforests. At the end of the Carboniferous period, glacier formation had lowered the sea level more and it had worsened the collapse.
volcanoes and asteroids?
In conclusion, we can say that climate change, glacier incidence, and plate tectonics are the main reasons for rainforests’ collapse. However, there are other possible culprits like volcanic eruptions and asteroid impacts. Earth was reshaping itself at the end of carboniferous. Big chunks of land were colliding so it is not a surprise that volcanoes erupted. We are still unsure how much it is responsible for the extinction.
- The biggest inspiration and primary information of this post have come from The Carboniferous rainforest collapse of The Common decent podcast.
- You can watch this video for the most basic explanation of the Carboniferous period.
- In case you haven’t read the above-mentioned research paper here is another link. Read The tetrapod diversity flow after the rainforest collapse.
- Watch this video on the carboniferous periods’ plants and their distinctive characteristic.
- Read the post on Rainforest collapse by BBC.
We mostly look at animal extinction with little regard for plant life. It is true that plant fossils are rare as they don’t fossilize often. We have to understand here that animals and plants work in harmony. The demise of one can very well be the demise of another. End carboniferous and subsequent Permian period were the worse time to be a plant. It was quite challenging for animals too. It made us think that maybe being large is not always good. The next posts will take us onto Permian Pangaea and the worst crisis on earth, the great dying.
Originally published at https://www.thescientificrevelation.com on July 20, 2021.