Until now: Massive photosynthesis combined with the freezing of the oceans depleted carbon dioxide from the atmosphere. As the late Devonian mass extinction occurred 375 mya, over a period of 25 million years and killed over 50% of all life in water. Plants on the newly colonised land were barely affected. This second of the five major mass extinctions was probably brought about as a combination of fall in temperatures due to both a Milankovitch cycle as well as plants. There were two giant continents on earth, Laurasia and Gondwana.
Fossil fuel is formed from fossils, as the name suggests. When we hear the word “fossil”, we think of dinosaurs. Unambiguously, those are the biggest fossils we’ve found. However, the fuel that powers our life in the form of coal, petroleum, and natural gas comes from mostly dead plants, both from land and water. We burned through the stuff of dinosaurs several decades ago. What we burn now comes from the slice of time that started since the Late Devonian mass extinction.
As marine plant and animal life died, they sank down to the depths of the ocean. Since the oceans were anoxic (i.e. bereft of oxygen), dead matter did not decay. Instead, it was compressed by the weight of the water above it. Further, the sea bed does not remain unchanging. Geological processes cause a constant shift in the contents of Earth’s crust. The surface of land and the ocean floor change fully every 200 million years due to volcanism and surface upheaval. As plants and animals die and settle on the ocean floor, mud and sand pile up over this dead life, eventually forming rock. Rock is physically porous in nature. Thus, as dead things were crushed, oil was absorbed and held by these rocks.
A similar thing happened to plant matter dying on land, becoming compressed into carbon and covered by rock. Over millions of years, this gave rise to coal. Plants had found stable footing on the ground in the Devonian. Even though the mass extinction did not affect as much terrestrial plant life as it did marine life, vast coal reserves from this time are still found today, especially in China, indicative of the sheer magnitude of the mass extinction.
The period that followed the Devonian is called the Carboniferous. It began 360 mya. Life recovered from the mass extinctions in this period, and evolved quickly to fill out the ecological niches that were occupied by creatures before the extinction. The word “carboniferous” in Latin means “coal bearing”. Nearly all of our coal reserves today come from this period, from both land and water.
Plants had truly grown and land was covered with forests, dominated by ferns. There was green everywhere there could be. There were horsetails and mosses and primitive conifers. The plant life during the Carboniferous was healthy, profuse and luxuriant. It stretched out from the shores of land into the vast interior of the continents. This massive, nearly involuntary growth and colonisation of land by plants had a major effect on Earth’s atmosphere.
The exponential increase in photosynthesis caused the atmospheric oxygen levels to climb steeply. Today, oxygen comprises 21% of the atmosphere. Back in the Carboniferous, oxygen took up nearly 35%. This surplus of oxygen meant more growing power. On average, life was larger in size. Plants grew up to 20 meters tall, fish in the water became as big as today’s sharks, amphibians grew big as soon as they started to exist, and insects the size of small cats flew in the air. No wonder then that so much coal formed from life that existed in this period.
In the waters, sharks prospered. This period saw the most diversification of sharks, in form as well as size. Because the previous mass extinction had killed off major predators in the oceans, sharks grew to fill in these ecological niches. They adapted swiftly and in bizarre ways. Sharks from the Cambrian had unusual body features. Palaeoxyris had a beak on its face; Orthacanthus was slim, long and looked like an eel; Akmonistion had a distinctive anvil shaped fin; Cladoselachidae’s spine split into two and extended out into its fins; Thrinacoselache was elongated like a snake or a worm; Xenacanthida had hooks in its mouth; and Cobelodus had tentacles.
Another dominating group of sea creatures were echinoderms, characterised by their symmetrical form. Some examples include starfish, sea urchins and sea cucumbers. In freshwater, large fishes that had powerful jaws seemed to have dominated. Moreover, some fish climbed out of water with the newly developed strength from their lobe fins, which would later evolve into legs. Once the amphibian seed was planted, they sprouted aplenty. In fact, as they were trying to evolve to find their footing, amphibians from this period were more diverse than they are today.
A very sharp period of transition in the Carboniferous was the quite sudden appearance of winged insects. We still do not know which ones were the first to fly or why. But in less than 10 million years, they multiplied; they grew large and fast, and cloyed the air above swamps. Due to the sheer quantity of plant life, there were a lot of swamps on land. The oxygen made plants and animals grow bigger and taller. Trees had reached heights of 30 metres and the dragonflies that flew among them had a wingspan of nearly 0.7 metres.
Toward the middle of the Carboniferous, there was a short extinction event, the causes of which are still unclear. About 300 mya, life on land suddenly died and dwindled for a short burst. The loss of large amounts of plant life formed coal beds even as they also cleared the swamps and made way for deserts. This event is known as the Carboniferous rainforest collapse, and it significantly affected the planet’s climate.
Within barely a couple million years after swamps cleared and arid environs set in, the reptiles evolved. Perhaps the biggest impetus for life in the Carboniferous was the full development of the amniotic egg, the kind of egg laid by animals around the world today. The land was aswarm with creepy crawlies: millipedes, snails, scorpions, spiders. The first cockroaches appeared and quickly multiplied into nearly 200 types, scuttling into the vast expanses of land. Invertebrates were everywhere.
Meanwhile, active volcanism resumed. The two supercontinents, Laurasia and Gondwana, were on the move still, slowly, almost imperceptibly getting closer to each other. In the Carboniferous, as life grew taller and bigger, so did the mountains. Gondwana’s Africa collided with Eurameria forming a part of the the Appalachian mountain range. It was a period of mountain forming. As Baltica closed in to form Laurasia, the Ural mountain ranges formed. South America collided with Laurasia to form the Ouachita mountains, now in central America. India and China were at the southernmost part of Gondwana, closer to the south pole and covered in sheets of ice.
All the landmasses finally came together approximately 300 mya, coinciding with the Carboniferous rainforest collapse. This giant supercontinent, called Pangea, straddled the equator and was the last known supercontinent. It was shaped like an ‘O’, with a small ocean enclosed within the landmasses.
The Permian period began some 298 mya and was characterised by the growth and diversification of reptiles on desert lands. These became the first true large vertebrates on land. They flourished throughout the Permian, eventually splitting into two: the ancestors of the dinosaurs, birds and crocodiles, and the ancestors of lizards and snakes today. The very first direct ancestors of dinosaurs appeared in this period – as did the ancestors of mammals. The cockroach’s ancestors truly dominated the land, though. They had diversified so intensely that nearly 90% of all insects on land were some form of cockroach. And even as beetles and bugs came to life, the dragonfly’s ancestors emerged as the dominant winged insects.
The first true trees appeared on land in the Permian, marking the first ever appearance of wood. Seeds began to take prominence over spores. Conifers (seeds held in cones, such as pine) appeared and diversified. These plants and trees were fed upon by the first large herbivores, which in turn fed large carnivores. The Permian saw the first appearance of megafauna – large animals. These were typically animals weighing over 40 kg on average.
As life flourished on land and in the seas, Earth was stirring again. The Permian also saw an increase in volcanic activity, particularly a type of volcanism known as a flood basalt event. This is a type of volcanism where land and ocean floor get coated slowly with basalt lava from volcanoes. A significant portion of central India, near the states of Maharashtra and Madhya Pradesh, is comprised of basaltic rock from flood basalt events caused by a series of volcanoes, and is known as all together known as the Deccan Traps.
Another infamous historic flood basalt event took place towards the end of Permian in a region called the Siberian Traps. And when the Siberian Traps erupted at the end of the Permian…
Next: The restructuring of Pangea and the largest mass extinction in history, the Permian-Triassic event.
Sandhya Ramesh is a science writer focusing on astronomy and earth science.