Understanding deep time: Volume II
A simplified geological history of the Earth, covering the entirety of the Paleozoic Era.
Welcome back to our in-depth exploration of Earth’s deep history, where we will continue our journey through the tapestry of Geological time to illuminate the secrets of our planet’s past. In our earlier installment in the series, we ventured through the ancient eons and epochs, from the fiery origins of the Hadean to the closing moments of the Neoproterozoic Era, setting the stage for a new era of life and geological transformation.
As we depart on the second part of our journey, we invite you to accompany us through the dramatic events of the Paleozoic Era, beginning with the extraordinary Cambrian Explosion and culminating at the dawn of the Mesozoic.
In this volume, we will delve into the complex interplay of paleobiology, tectonics, and climate that shaped the Earth during this transformative epoch.
Together, we will investigate the unparalleled diversification of life during the Cambrian Explosion, trace the remarkable milestones that define the Paleozoic Era, and examine the intricate dance of geological forces that molded Earth’s continents, oceans, and climate.
Without further preamble, Let’s jump into it!
Vol I — The Pre-Cambrian Period
Vol II — The Paleozoic Era
Vol III — The Mesozoic Era
Vol IV — The Cenozoic Era
Contents of this post:
- The Phanerozoic Eon (538.8Mya — Present)
- The Paleozoic Era (538.8–251.9Mya)
— The Cambrian Period (538.8–485.4Mya)
— The Cambrian Explosion
— Evolution of the first complex land animals - The Ordovician period (485.4–443.8Mya)
— Marine biodiversity during the Ordovician
— First land plants in the Ordovician period
— The end-Ordovician extinction event - The Silurian period (443.8–419.2Mya)
— The evolution of jawed fish
— The evolution of land plants
— The development of terrestrial ecosystems - The Devonian Period (419.2–358.9Mya)
— The Age of Fishes
— First forests and insects
— The late-Devonian extinction event - The Carboniferous period (358.9–298.9Mya)
— The formation of Coal deposits
— The evolution of amphibians and reptiles - The Permian Period (298.9–251.9Mya)
— The formation of Pangea
— Evolution of Reptiles and Synapsids
— The Permian Mass extinction event - Conclusion
The Phanerozoic Eon (538.8 million years ago — present)
The Phanerozoic Eon, encompassing the last 538.8 million years of Earth’s history, marks a transformative period in the development of life on our planet, and is characterized by the rise of complex, diverse ecosystems and the appearance of easily visible life forms, as opposed to the microscopic life that dominated the earlier Proterozoic.
The Phanerozoic Eon is divided into three distinct eras: the Paleozoic (538.8–251.9Mya, 289 Million years total), Mesozoic (251.9–66Mya, 186 Million years total), and Cenozoic (66Mya — Present, 66Million years total so far), each of which boasts its own unique set of organisms, ecosystems, and geological events.
Among the others, the Phanerozoic Eon is particularly significant because it witnessed the rapid diversification and evolution of life, ultimately leading to the world we know today, and the period saw the emergence of not only the first animals with hard shells, but also the rise and fall of various ancient ecosystems, and the eventual appearance of mammals, birds, and flowering plants. Several major mass extinction events also occurred during the Phanerozoic, reshaping the course of evolution and altering the face of the planet, all of which we will look at in passing, and will later be covered in greater depth.
To help us out, the geological record of the Phanerozoic is much more abundant and well-preserved compared to earlier eons, providing scientists, researchers, and educators with a wealth of information about the development of life and the Earth’s environment, and allowing us, in the sections that follow, to delve into the key events and breakthroughs that have characterized the Phanerozoic Eon, offering a detailed look at this fascinating period in Earth’s history.
The Paleozoic Era (538.8–251.9 million years ago)
The Paleozoic Era, spanning from 538.8 to 251.9 million years ago, marked a pivotal time in the history of life on Earth, and as the first of the three eras within the Phanerozoic Eon, witnessed the emergence of a diverse array of life forms, from the first complex organisms to the evolution of plants and animals capable of colonizing land.
This era is divided into six geological periods: the Cambrian, Ordovician, Silurian, Devonian, Carboniferous, and Permian, each characterized by its own unique evolutionary and geological milestones.
One of the most significant events of the Paleozoic Era, and its truly defining early moment, was the Cambrian Explosion, a rapid diversification of life that gave rise to the majority of modern animal phyla, and began the Eon itself. As the era progressed, organisms continued to evolve, leading to the establishment of the first coral reefs, the colonization of land by plants and arthropods, and the development of terrestrial ecosystems.
The Paleozoic also saw the formation of several major supercontinents, including Gondwana and Pangaea, which greatly influenced the Earth’s climate and sea levels. These geological changes, combined with the diversification of life, led to a complex interplay between ecosystems and their environments, setting the stage for the ecological dynamics we observe in our world today.
Throughout the Paleozoic Era, life experienced both remarkable advances and devastating setbacks, as the planet underwent multiple mass extinction events, but the most significant of these occurred at the end of the Permian period, marking the close of the Paleozoic and the beginning of the later Mesozoic.
The Cambrian Period (538.8–485.4 million years ago)
The Cambrian Period, spanning from 538.8 to 485.4 million years ago witnessed a truly remarkable evolutionary event known as the Cambrian Explosion, during which an unprecedented diversity of life forms rapidly emerged. This explosion of biodiversity led to the appearance of most major animal phyla that we see today, fundamentally shaping the course of life on our planet.
The Cambrian Period is characterized by its unique and diverse fossil record, which provides invaluable insights into the early evolution of complex animals. Fossil sites such as the Burgess Shale in Canada and the Chengjiang Lagerstätte in China showcase a captivating array of early life forms, many of which have no modern analogs, and these ancient creatures give us a glimpse into the early stages of animal evolution and help us understand the origins of modern ecosystems.
From the first appearance of complex animals to the establishment of new ecological roles, the Cambrian Period laid the groundwork for the continued diversification and evolution of life throughout Earth’s history, and its significance as a transformative epoch cannot be overstated, making it a fascinating and vital subject for those interested in understanding the deep time of our planet, and it’s one that we will come back to, time and time again, as we progress through our exploration into deep time and evolution.
The Cambrian Explosion
The Cambrian Explosion, a massive event in the history of life on Earth, occurred approximately 538 million years ago and lasted for a relatively short period of about 20–25 million years. This event is characterized by the rapid diversification and emergence of a wide array of multicellular life forms, particularly among animals. In fact, as we have stated, the Cambrian Explosion marked the appearance of most major animal phyla that we see today, as well as several unique organisms that have no modern counterpart.
One of the key factors driving this sudden diversification was the development of new genetic and developmental pathways that allowed for the evolution of complex body plans and specialized structures. The emergence of the Hox gene family, a group of regulatory genes responsible for orchestrating the development of animal body plans, played a crucial role in the Cambrian Explosion by enabling the formation of diverse body structures and functions.
Another important factor was the establishment of complex ecosystems with new ecological niches. Essentially, as animals evolved, they began to exploit different food sources and adapt to various environments, which in turn fueled further diversification. To accompany this, the rise of predation also played a significant role, as it drove an evolutionary arms race between predators and prey, leading to the development of various defensive and offensive adaptations.
During the Cambrian Explosion, some of the most iconic early animals emerged, such as Anomalocaris, a large predator with grasping appendages and compound eyes, and Opabinia, a peculiar creature with five eyes and a proboscis-like structure. Additionally, the first arthropods, such as trilobites, made their debut during this time, eventually becoming one of the most successful and diverse groups of animals in Earth’s history.
The Cambrian Explosion also saw the emergence of the first animals with hard shells, such as mollusks and brachiopods, and this development led to an increasingly detailed fossil record, as hard-shelled organisms were more likely to be preserved than their soft-bodied counterparts.
The causes of the Cambrian Explosion are still a subject of scientific debate, with factors such as increased oxygen levels, changes in ocean chemistry, and the availability of key nutrients potentially playing a role. Regardless of the exact cause, the Cambrian Explosion remains a crucial event in the history of life on Earth, marking the beginning of the modern era of complex animal diversity.
Ediacaran Biota: The appearance of complex animals during the Cambrian can be traced back to the Ediacaran biota, which existed during the late Precambrian. The enigmatic Ediacaran organisms, such as Dickinsonia and Spriggina, likely represent early forms of multicellular life, although their exact phylogenetic relationships remain unclear.
The transition from the Ediacaran biota to the Cambrian fauna is marked by an increase in the complexity of body plans, skeletal structures, and ecological interactions.
The evolution of these early complex animals during the Cambrian Period set the stage for the remarkable diversity of life that would continue to unfold throughout the remainder of the Phanerozoic Eon.
Evolution of the first Complex Animals
During the Cambrian Period, the evolution of the first complex animals marked a significant milestone in the history of life on Earth. As we have stated, many of the major animal groups that exist today first appeared during this time, including arthropods, mollusks, echinoderms, and also the early ancestors of vertebrates.
Arthropods: The emergence of arthropods, such as trilobites, during the period was a major, and critical development. Arthropods are characterized by their exoskeletons, segmented bodies, and jointed appendages.
Trilobites became one of the most abundant and diverse groups of organisms during the Paleozoic Era, and their success can be attributed to their adaptable body plan, which allowed for specialization in various ecological niches, as well as their well-developed visual systems.
Mollusks: Another major group that emerged during the Cambrian Period was the mollusks, which include modern-day snails, clams, and cephalopods. Early mollusks, such as Kimberella and Odontogriphus, displayed a variety of body plans, showcasing the versatility of this group, and the development of the shell provided protection and allowed mollusks to thrive in a variety of environments, pushing the evolutionary arms race ever further, and driving innovation within the burgeoning Animalia.
Echinoderms: Echinoderms, including early sea stars, sea urchins, and crinoids, also made their first appearance during the Cambrian.
These marine invertebrates are characterized by their radial symmetry and water vascular system, which enables them to move, feed, and respire, and they quickly began to diversify during the Cambrian and continued to evolve, occupying various niches in the oceanic ecosystem.
Ancestors of Vertebrates: The first chordates, which include the ancestors of vertebrates, also began to evolve during the Cambrian Period.
The discovery of early chordates, such as Pikaia and Haikouichthys, provided valuable insights into the early stages of vertebrate evolution, with these primitive chordates displaying a notochord, a flexible rod-like structure that would later give rise to the vertebral column in more advanced animals.
Ordovician Period (485.4–443.8 million years ago)
The Ordovician Period, spanning from approximately 485.4 to 443.8 million years ago, was another decisive time in Earth’s history. Following the Cambrian Explosion, the Ordovician witnessed the continued diversification of life and the establishment of complex ecosystems, and this period is characterized by significant geological and climatic changes, as well as important evolutionary events that would ultimately shape the course of life on Earth.
During the Ordovician, marine ecosystems thrived and diversified, with new animal groups such as corals, bryozoans, and brachiopods emerging. These organisms played crucial roles in building the first reef systems and shaping the ocean’s structure. In addition, the first evidence of life on land — in the form of primitive plants, fungi, and arthropods — began to appear, and would set the stage for the future of the planet.
The Ordovician Period was also marked by dramatic changes in the Earth’s geography, with the breakup of the supercontinent Gondwana and the formation of new continents and ocean basins. These tectonic shifts led to fluctuations in sea levels, influencing the distribution and diversification of marine life.
Moreover, the end of the Ordovician saw one of the most severe mass extinction events in Earth’s history, resulting in the loss of up to 85% of all species. This extinction event, likely driven by a combination of factors such as rapid climate change and shifting ocean chemistry, paved the way for new ecosystems and evolutionary paths in the following periods.
Marine Biodiversity in the Ordovician Period
The Ordovician Period was a time of wide diversification and expansion of marine life. During this period, the ocean’s biodiversity increased dramatically, with the emergence of new animal groups and the continued evolution of existing ones. The following are some of the significant developments in marine biodiversity that took place during the Ordovician:
Invertebrates: As we have mentioned, the Ordovician saw the diversification of invertebrate groups such as mollusks, echinoderms, and arthropods. Among mollusks, the cephalopods, which include the ancestors of modern-day octopuses and squids, experienced a significant radiation. Echinoderms, like crinoids (sea lilies) and blastoids, flourished in the shallow seas, while trilobites, a group of arthropods, reached their peak diversity during this time.
Brachiopods and Bryozoans: The Ordovician was a golden age for brachiopods and bryozoans, two groups of marine invertebrates that played crucial roles in building complex reef ecosystems. These filter-feeding animals contributed to the formation of extensive carbonate platforms, which provided habitats for other organisms.
Corals: The first true corals appeared during the Ordovician Period, with the emergence of both rugose and tabulate corals. These early corals contributed to the formation of the first reefs and supported a diverse range of marine life.
Graptolites: Graptolites, colonial marine animals, were particularly abundant during the Ordovician. These planktonic organisms were essential for reconstructing the period’s paleogeography and understanding the evolution of the marine ecosystem.
Conodonts: Conodonts, small eel-like marine animals, also experienced significant diversification during the Ordovician. These organisms are especially important for understanding the period’s biostratigraphy due to their well-preserved microfossils.
Early Fish: The first primitive fish, such as ostracoderms and jawless fish, began to appear during the late Ordovician. Although their diversity was still relatively low, these early fish marked the beginning of the vertebrate lineage that would eventually give rise to all other vertebrates, including humans.
First Land Plants in the Ordovician Period
The Ordovician Period marked a crucial milestone in the history of life on Earth, as it saw the emergence of the first land plants.
This transition from aquatic to terrestrial environments set the stage for the diversification and evolution of plant life, which would eventually lead to the complex terrestrial ecosystems we see populating our world today.
Early Land Plants: The first land plants, known as bryophytes, appeared around 470 million years ago (Possibly earlier within the Viridiplantae lineage, then later with the emergence of green algae). These primitive plants were non-vascular, lacking the specialized tissues for transporting water and nutrients that modern vascular plants possess.
Bryophytes include mosses, liverworts, and hornworts, which share some common features such as a dominant haploid gametophyte stage in their life cycle and the lack of true roots, leaves, and stems.
Adaptations for Terrestrial Life: To colonize the land, early plants had to develop new strategies and adaptations to overcome the challenges posed by the terrestrial environment.
Some of these adaptations included a waxy cuticle to prevent water loss, specialized cells for gas exchange (stomata), and the ability to anchor themselves to surfaces using rhizoids, which are hair-like structures that function like roots in modern plants.
Symbiotic Relationships: Early land plants often formed symbiotic relationships with fungi, known as mycorrhizae, to enhance nutrient uptake. These relationships allowed plants to access vital nutrients from the soil and played a crucial role in their ability to colonize terrestrial environments.
Ecological Impact: The colonization of land by plants during the Ordovician had profound implications for Earth’s ecosystems. By stabilizing the soil, reducing erosion, and providing habitats for other organisms, plants played a vital role in shaping terrestrial landscapes.
Furthermore, their photosynthetic activity contributed to the planet’s oxygen levels and influenced global climate by removing carbon dioxide from the atmosphere.
The Evolutionary Significance: The appearance of land plants in the Ordovician set the stage for the evolution of more complex plant forms. Over time, plants developed vascular tissues, true roots, leaves, and seeds, which allowed them to diversify and conquer a wide range of terrestrial habitats.
End-Ordovician Extinction Event: A Turning Point in Earth’s History
The End-Ordovician extinction event, which occurred around 443.8 million years ago, is the first of the “Big Five” mass extinctions in Earth’s history, resulting in the loss of an estimated 60–85% of marine species, and reshaping the course of life on our planet. In this section we will delve into the causes, impacts, and significance of this pivotal extinction event.
Causes: The primary cause of the End-Ordovician extinction event was a series of drastic climate changes that affected marine ecosystems. The Ordovician Period experienced a major glaciation event, which led to the formation of a large ice sheet on the southern continent of Gondwana. This glaciation caused sea levels to drop significantly, resulting in habitat loss for many marine organisms.
The subsequent rapid deglaciation and sea level rise led to widespread anoxia (lack of oxygen) in the oceans, further contributing to the mass die-off of marine species.
Affected Species: The End-Ordovician extinction event primarily affected marine life, as terrestrial ecosystems were still in their early stages of development. Among the hardest-hit groups were brachiopods, trilobites, bryozoans, and some types of corals, however, some groups, such as nautiloids, managed to survive the extinction event and continued to diversify in the subsequent periods.
Recovery and Evolutionary Impact: Following the extinction event, ecosystems took millions of years to recover fully. The vacant ecological niches left by the extinct species provided new opportunities for surviving organisms to adapt and diversify. This led to the rise of new groups of marine animals, such as crinoids, bivalves, and various types of fish.
Significance for Earth’s History: As the first of the five major extinction events that have occurred throughout Earth’s history, the End-Ordovician marked a critical turning point in the evolution of life, highlighting the vulnerability of ecosystems to rapid environmental change.
Moreover, the event serves as a valuable case study for understanding the complex interplay between climate change, sea level fluctuations, and the biosphere.
The Silurian Period (443.7–419 million years ago): A Time of Recovery and Expansion
The Silurian Period, spanning from 443.7 to 419 million years ago, was a complex time in Earth’s history, following on from the catastrophic End-Ordovician extinction event. As the planet’s ecosystems began to recover and new life forms emerged, this period saw significant developments in the marine and terrestrial realms.
This brief introduction will touch upon the key aspects of the Silurian Period, providing a glimpse into its importance in the grand tapestry of Earth’s history.
Recovery and Diversification: The Silurian Period was marked by the recovery of marine ecosystems from the devastating effects of the End-Ordovician extinction event. New groups of organisms evolved and diversified, filling the ecological niches left vacant by the mass extinction. Notably, the Silurian saw the emergence of new coral reefs, the continued diversification of fish, and the rise of arthropods, such as eurypterids (sea scorpions).
Terrestrial Life: The Period was also a time of significant advancement in the colonization of land, with the first vascular plants, such as Cooksonia, appearing during this time, and providing the foundation for more complex terrestrial ecosystems. Alongside the evolution of plants, the Silurian saw the first true terrestrial arthropods, including millipedes and scorpions, which played vital roles in the establishment of land-based food webs.
Climate and Sea Level: The Silurian experienced a stable, warm climate, with the ice caps of the preceding Ordovician Period gradually receding. This shift in climate led to rising sea levels and the flooding of continental shelves, creating new marine habitats that promoted biodiversity.
With all of this considered, the Silurian Period holds a unique position in Earth’s history, acting as both a time of recovery from the devastation of the End-Ordovician extinction event and a period of significant advancements in the diversification of life on land.
The Evolution of Jawed fish
During this period, jawed fish (gnathostomes) experienced significant diversification and adaptive radiation, which set the stage for the emergence of various fish lineages that would come to dominate the aquatic realm.
In this section, we will delve into the evolution of jawed fish in the Silurian, exploring key innovations, major fossil discoveries, and the significance of these events in the broader context of vertebrate evolution.
Early Jawed Fish: Placoderms and Acanthodians
The first jawed fish are believed to have appeared in the Early Silurian, with two major groups of early jawed fish, the placoderms and acanthodians, dominating the Silurian seas. Placoderms were characterized by their armored plates, while acanthodians possessed spines on their fins.
Placoderms: Placoderms were among the earliest jawed fish, and they exhibited a diverse range of body forms, sizes, and ecological niches. Key features of placoderms included their bony exoskeleton, consisting of dermal plates that protected the head and thorax. The head and thoracic plates were hinged, allowing for improved mobility.
Placoderms possessed relatively simple jaws and teeth, which were derived from modified dermal plates.
Acanthodians: Acanthodians, or “spiny sharks,” were another group of early jawed fish characterized by their slender bodies and the presence of spines on their fins. While they were not true sharks, they shared certain features with modern sharks and bony fish, making them an important group for understanding the early evolution of jawed vertebrates.
Acanthodians had relatively large eyes, indicating that vision played a significant role in their ecology.
Innovations in Jaw and Gill Structures
The evolution of jawed fish in the Silurian marked a significant step forward in vertebrate development. The emergence of jaws allowed for the capture and processing of larger prey items, which in turn opened up new ecological opportunities for these early vertebrates.
The development of jaws was accompanied by the evolution of more complex gill structures, which improved respiration and facilitated the increased metabolic demands of these active predators.
Major Fossil Discoveries
Key Silurian fossil localities have provided critical insights into the evolution of jawed fish during this period. Notable sites include the Wenlock Limestone in England, the Ludlow Bone Bed in Wales, and the Eramosa Formation in Ontario, Canada.
These localities have yielded numerous well-preserved specimens of placoderms, acanthodians, and early chondrichthyans (cartilaginous fish), enhancing our understanding of the diversity and ecology of jawed fish in the Silurian.
The Evolution of Land plants
The Silurian Period (443.7 to 419 million years ago) witnessed the critical transition of life from water to land, with the emergence and diversification of the earliest terrestrial plants. This fundamental shift in the history of life on Earth transformed the landscape, creating new habitats and laying the groundwork for the future evolution of our terrestrial ecosystems.
This section will explore the evolution of land plants in the Silurian, examining many of the key innovations, major fossil discoveries, and the broader implications of these developments for the Earth’s environment and the diversification of life.
Early Terrestrial Plants: Non-Vascular and Simple Vascular Plants
The first land plants to colonize terrestrial environments were non-vascular, lacking specialized tissues for water and nutrient transport. These early land plants included the bryophytes, a group comprising liverworts, mosses, and hornworts.
By the Late Silurian however, simple vascular plants had also evolved, initiating the diversification of more complex terrestrial plants.
Non-vascular plants: The non-vascular plants relied on direct contact with water for their reproduction and nutrient uptake. As such, they were generally restricted to moist environments near bodies of water. These early land plants played a crucial role in stabilizing soils and creating microhabitats for other organisms, and we’re a driving factor in the terrestrial colonization to come.
Simple vascular plants: The evolution of vascular tissues allowed for the emergence of the first vascular plants, which possessed a basic system for transporting water and nutrients. The development of vascular tissues enabled these plants to grow larger and colonize a wider range of terrestrial environments.
Key Innovations in Plant Morphology and Reproduction
The transition from aquatic to terrestrial environments required several key innovations in plant morphology and reproduction.
These adaptations allowed early land plants to cope with the challenges posed by life on land, such as desiccation, gravity, and the need for sexual reproduction in the absence of water.
Cuticle: The development of a waxy cuticle on the surface of plant tissues provided a barrier to water loss, allowing plants to conserve moisture and thrive in drier environments.
Stomata: The evolution of stomata, microscopic pores on the surface of leaves, facilitated gas exchange while minimizing water loss.
Gametangia: Early land plants developed specialized structures called gametangia to protect their gametes (reproductive cells) from desiccation. This innovation was critical for sexual reproduction in terrestrial environments.
Sporangia: The emergence of sporangia, structures that produce and protect spores, enabled the dispersal of plant propagules without the need for direct contact with water.
Development of Terrestrial Ecosystems: The pioneers of Silurian Landscapes
During the Silurian, as the first land plants began to colonize Earth’s surface, the period saw the establishment of essential components that would shape future ecosystems, including complex interactions among plants, animals, and their environments.
The Role of Land Plants: The initial colonization of land by plants was a major driver in the development of terrestrial ecosystems. Early land plants, like Cooksonia, were simple and small, lacking roots, leaves, and complex vascular systems. However, their presence on land had profound implications. By stabilizing the soil, these pioneering plants reduced erosion and created habitats for other organisms. They also played a role in the actual formation of soils, which facilitated the development of more complex plant species that could harness greater amounts of sunlight and nutrients.
The First Terrestrial Animals: As land plants began to establish themselves, the first terrestrial animals also emerged. During the Silurian Period, arthropods, including early millipedes and arachnids, started to colonize the land.
These early land-dwelling arthropods were primarily detritivores, feeding on decaying plant material and contributing to the decomposition process. This established the first terrestrial food webs, paving the way for the evolution of more complex predator-prey relationships on land.
The Development of Terrestrial Ecosystems: With the increasing contact between the various organisms, the Silurian Period saw the gradual development of terrestrial ecosystems, with increasingly complex interactions among plants, animals, and their environments. The evolution of roots and leaves allowed plants to form more extensive networks for nutrient and water uptake, further enhancing terrestrial productivity.
Additionally, the increased availability of organic material led to the development of more diverse and complex soil systems, fostering the establishment of diverse communities of microorganisms, fungi, and invertebrates.
The Oxygenation of Earth’s Atmosphere: The spread of land plants during the Silurian Period also contributed to the oxygenation of Earth’s atmosphere. Through photosynthesis, these early plants produced oxygen as a byproduct, increasing atmospheric oxygen levels and supporting the diversification and evolution of aerobic organisms.
Devonian Period (419.2–358.9 million years ago): The Age of Fishes and the Emergence of Forests
The Devonian Period, spanning from 419.2 to 358.9 million years ago, was an iconic time in Earth’s history, characterized by significant evolutionary events and the further diversification of life both in the oceans and on land.
Often referred to as the “Age of Fishes,” the Devonian witnessed the rise and diversification of many fish groups, as well as the emergence of the first forests, which set the stage for the development of complex terrestrial ecosystems.
During this period, Earth’s climate was generally warm and stable, with continents clustered together in the supercontinent Gondwana, and smaller landmasses scattered across the globe. The warm, shallow seas of the Devonian provided ideal environments for marine life to flourish, leading to the rise of many new fish groups, including the first sharks and armored fish known as placoderms.
On land, plants continued to evolve, transitioning from small, simple organisms into more complex and diverse species, and the period saw the development of the first true trees, such as Archaeopteris, and the formation of the first forests, which significantly altered the landscape and the global carbon cycle.
The growth of these forests provided new habitats for a range of terrestrial organisms, including the first insects and tetrapods, which were the ancestors of all modern land-dwelling vertebrates.
Age of Fishes: The Devonian Period’s Diverse and Abundant Marine Life
As has been mentioned, the Devonian Period was marked by an explosion of fish diversity and abundance, where marine ecosystems flourished in the warm, shallow seas that covered much of the Earth, providing ideal conditions for various fish groups to evolve and diversify.
Jawless fish: Agnathans, or jawless fish, were among the earliest fish to evolve. During the Devonian, these primitive fish continued to diversify, giving rise to a variety of species, such as the ostracoderms, which as were covered in bony armor.
Placoderms: The Devonian also saw the rise of placoderms, a group of armored fish with bony plates covering their heads and thoraxes. Placoderms were the first jawed vertebrates and represented a major step in the evolution of fish. They occupied various ecological niches, from apex predators like Dunkleosteus, which could grow up to 20 feet long, to small, bottom-dwelling species.
Cartilaginous fish: Sharks and their relatives, the cartilaginous fish, made their first appearance during the Devonian. The early sharks were small, with simple teeth and a wide range of body shapes, reflecting their diverse feeding habits and ecological roles.
Ray-finned fish: The Devonian also marked the origin of ray-finned fish, or actinopterygians, the largest and most diverse group of fish today. These fish possessed a unique fin structure, with bony rays supporting their fins. Ray-finned fish diversified rapidly during the Devonian, filling various niches in the marine ecosystem.
Lobe-finned fish: Another significant group of fish that emerged during the Devonian were the lobe-finned fish, or sarcopterygians, which had fleshy, lobed fins supported by a bony internal skeleton. This group includes the coelacanths and lungfish, as well as the ancestors of tetrapods, which would eventually give rise to all land-dwelling vertebrates.
First Forests and Insects: The Emergence of Complex Terrestrial Ecosystems
The Devonian Period also marked a furhter significant shift in the Earth’s ecosystems, as the first forests and insects began to emerge. These new life forms played crucial roles in shaping the planet’s terrestrial habitats, and their evolutionary paths are still evident and obvious in today’s ecosystems.
First forests: This period saw the development of the first true forests, with large, complex plants transforming landscapes and laying the foundation for complex terrestrial ecosystems. Early land plants, such as the moss-like Cooksonia, evolved into more complex vascular plants, including the lycophytes, horsetails, and ferns. These plants developed specialized tissues for water and nutrient transport, allowing them to grow taller and more robust.
One of the most important plant groups during this time was the progymnosperms, which bore characteristics of both ferns and seed plants. The most famous progymnosperm, Archaeopteris, had a tree-like form with large, fern-like leaves, and its wood contained specialized tissues, similar to modern seed plants.
These early forests created new habitats, altered the climate, and increased the availability of resources for other organisms, helping to push the evolutionary race.
Evolution of insects: As we have stated, alongside the development of the first forests, the Devonian also saw the emergence of the first insects. These primitive insects are thought to have evolved from a common ancestor with crustaceans, adapting to terrestrial environments through a series of unique adaptations.
The earliest known insects were small, wingless arthropods that fed on decaying plant material. As plants diversified and forests expanded, insects evolved new strategies to exploit these rich environments, and eventually, by the end of the Devonian, the first winged insects appeared, representing a major evolutionary innovation that allowed them to colonize new habitats and escape predators.
Co-evolution of plants and insects: This emergence of insects and forests in the Devonian set the stage for a long and intricate history of co-evolution between these two groups. Insects played crucial roles in pollination and seed dispersal, while plants provided food and shelter for insects.
As plants evolved defenses against herbivory, such as tough leaves and chemical toxins, insects developed counter-adaptations, such as specialized mouthparts and detoxification enzymes, continuing the evolutionary struggle down all of its many pathways.
This co-evolutionary “arms race” between plants and insects has shaped the diversity and ecology of both groups, with profound impacts on Earth’s ecosystems throughout deep time.
Late Devonian Extinction Event: A Turning Point in Earth’s Biodiversity
The Late Devonian extinction event, which occurred approximately 358.9 million years ago, marked a significant turning point in Earth’s biodiversity. This extinction event, one of the “Big Five” mass extinctions, affected both marine and terrestrial ecosystems and ultimately shaped the course of life on Earth. Here, we will delve into the possible causes and impacts of the Late Devonian extinction event.
Possible causes:
Although the exact cause of the Late Devonian extinction remains unclear, several factors may have contributed to the event:
Climate change: During the Late Devonian, the Earth experienced significant climatic shifts, including cooling and fluctuations in sea levels. These changes may have disrupted habitats and led to the extinction of many species.
Ocean anoxia: Evidence suggests that widespread ocean anoxia (a lack of dissolved oxygen) played a major role in the extinction event. The anoxic conditions were likely caused by changes in ocean circulation and increased nutrient input from eroding land masses, leading to massive algal blooms and the depletion of oxygen in the ocean.
Bolide impact: Some researchers propose that an asteroid or comet impact may have contributed to the extinction event, as evidenced by impact craters and shocked quartz dating to the Late Devonian. However, this hypothesis remains contentious, and the role of impact events in the extinction is still debated.
Volcanic activity: Large-scale volcanic activity, such as the eruption of the Viluy Traps in Siberia, may have released large amounts of greenhouse gases and particulate matter into the atmosphere, disrupting global climate and causing extinctions.
Impacts on marine ecosystems
The Late Devonian extinction event had a significant impact on marine life, with an estimated 70–80% of all marine species going extinct.
The extinction event was particularly devastating for reef-building organisms, such as stromatoporoids, tabulate corals, and rugose corals. These organisms played a critical role in constructing complex reef systems, which served as vital habitats for a wide variety of marine life, and the loss of these key species led to the collapse of reef ecosystems, with long-lasting consequences for marine biodiversity.
Several other marine groups also experienced significant declines during the Late Devonian extinction, including Trilobites, which were once diverse and abundant, and who faced a dramatic reduction in their numbers, ultimately leading to their complete extinction in the subsequent Permian period. Brachiopods, which were previously common in marine environments, also suffered a severe loss of diversity, although some species managed to survive and continue to exist and thrive today. Ammonites, a group of marine mollusks with distinctive spiral shells, also experienced a significant decline in diversity and abundance during this time.
Impacts on terrestrial ecosystems
Although the extinction event is often considered primarily a marine crisis, terrestrial ecosystems were also affected.
Many early terrestrial plants experienced declines, and several early tetrapod lineages disappeared entirely, though, the extinction event may have paved the way for the diversification of seed plants and the evolution of new terrestrial ecosystems during the subsequent Carboniferous Period.
The Carboniferous Period (358.9–298.9 million years ago): The Age of Coal and Giant Insects
The Carboniferous Period, spanning from 358.9 to 298.9 million years ago, is often referred to as the “Age of Coal” due to the vast coal deposits that were formed during this time.
The Carboniferous Period is divided into two subperiods: the Mississippian (358.9–323.2 million years ago) and the Pennsylvanian (323.2–298.9 million years ago). These two subperiods are characterized by distinct climatic conditions, with the Mississippian experiencing warmer, more tropical climates and the Pennsylvanian marked by cooler temperatures and the formation of vast ice sheets.
During the Carboniferous Period, several crucial developments occurred, including:
- The formation of coal: As the name suggests, the Carboniferous Period is particularly known for the formation of extensive coal deposits. This was mainly due to the proliferation of swampy forests dominated by giant ferns, club mosses, and horsetails, which, upon their death and burial, eventually transformed into coal.
- Diversification of terrestrial life: The Carboniferous saw an explosion of terrestrial life forms, with the diversification of seed plants, insects, and many early tetrapods. These new species contributed to the development of complex terrestrial ecosystems.
- The rise of giant insects: The Earth’s atmosphere during the Carboniferous Period had a much higher oxygen content than it does today, allowing for the existence of gigantic insects and other arthropods. Fossils from this time period reveal dragonflies with wingspans of up to 70 cm and millipedes that were over 2 meters long. This has helped to engrain the carboniferous in the mind of many, and has been a common adoption by Hollywood.
- The assembly of Pangaea: The Carboniferous Period also marked significant tectonic shifts, with the continents coming together to form the supercontinent Pangaea, which would go on to play a key role in shaping the Earth’s climate, ecosystems, and the distribution of species.
The formation of Coal Deposits
During the Carboniferous Period, the formation of extensive coal deposits became one of the defining features of the era. Coal is a fossil fuel that forms from the remains of plants that once thrived in swampy, wetland environments.
The process of coal formation, known as “coalification,” took place over millions of years, as plant material accumulated and underwent various stages of transformation. In this section, we delve into the factors that contributed to the formation of these expansive coal deposits during the Carboniferous Period.
Proliferation of swampy forests: As mentioned, the Carboniferous Period was characterized by warm, humid climates, particularly during the Mississippian subperiod, and these conditions gave rise to vast swampy forests, dominated by primitive plants such as giant ferns, club mosses, and horsetails. These swampy forests provided an ideal environment for the accumulation of organic matter, which would later become coal.
High rate of plant growth and decay: The warm, wet climate of the Carboniferous Period facilitated rapid plant growth, leading to an abundance of organic material. When these plants died, they fell into the waterlogged swamps, where they decayed under anaerobic conditions. This slow decay process allowed for the preservation of plant material and the accumulation of peat, which would eventually transform into coal.
Tectonic activity and sedimentation: The Period was a time of significant tectonic activity, with continents moving together to form the supercontinent Pangaea. This activity led to the formation of large basins, where the swampy forests thrived, and, as tectonic forces continued to shape the Earth’s crust, sediments from nearby mountains and highlands were deposited over the swamps, burying the peat deposits. Over time, the weight of these sediments compressed the peat, forcing out water and other volatile substances.
Coalification process: The transformation of peat into coal occurred over millions of years and under immense heat and pressure. As the peat deposits became more deeply buried, they experienced increased pressure and temperature, leading to a series of chemical and physical changes.
These changes caused the peat to become progressively more carbon-rich, forming lignite, sub-bituminous coal, bituminous coal, and finally, anthracite, which is the highest grade of coal.
Evolution of Amphibians and Reptiles: The Rise of Terrestrial Vertebrates
The Carboniferous Period saw a significant milestone in the evolution of life on Earth, as amphibians and reptiles emerged and rapidly diversified.
These early tetrapods were some of the first vertebrates to adapt to a terrestrial lifestyle, paving the way for future terrestrial animal life, and in this section, we will explore the evolutionary paths and phylogenetic relationships of these early pioneering animals.
Amphibians
Amphibians evolved from lobe-finned fish, which had fleshy, paired fins that contained bones similar to those found in tetrapod limbs. These lobe-finned fish, part of the group Sarcopterygii, are the ancestors of all terrestrial vertebrates.
One key transitional fossil, known as Tiktaalik, displays both fish-like and tetrapod-like characteristics, providing important insights into the early evolution of amphibians.
The first true amphibians appeared around 370 million years ago, during the late Devonian Period, and diversified during the Carboniferous Period. These early amphibians, such as Ichthyostega and Acanthostega, had both aquatic and terrestrial adaptations, including lungs for breathing air and limbs for locomotion on land, and are believed to have spent much of their time in water, as their limbs were not yet fully adapted for efficient terrestrial movement.
Reptiles
The reptiles, which are part of the amniote group, evolved from early amphibian ancestors. The key innovation that set reptiles apart was the amniotic egg, which allowed embryos to develop in a protected, self-contained environment. This adaptation enabled reptiles to lay their eggs on land, reducing their dependence on water for reproduction and allowing them to colonize a wider range of habitats.
The first reptiles appeared around 310 million years ago, and again, diversified rapidly during the Carboniferous and Permian periods.
Phylogenetic relationships:
The phylogenetic relationships between early amphibians and reptiles can be complex and are still being refined with new discoveries. However, it is clear that both groups share a common ancestor with lobe-finned fish. Amphibians and reptiles are part of a larger clade called Tetrapoda, which also includes mammals and birds. Within this group, amphibians form a sister clade to the amniotes, which include reptiles, mammals, and birds.
The amniotes are further divided into two main groups: Synapsids, which include mammals and their extinct relatives, and Diapsids, which include reptiles and birds.
Diversification and adaptation:
Throughout the Carboniferous, both amphibians and reptiles continued to diversify and adapt to their changing environments. The rise of forests provided new niches and opportunities for these early tetrapods, leading to the evolution of various feeding strategies, body forms, and locomotion methods.
Some groups, such as the early reptiles called captorhinids, were successful herbivores, while others, like the amphibian-like temnospondyls, occupied various carnivorous roles in aquatic and terrestrial ecosystems.
The Permian Period (298.9–251.9 million years ago)
The Permian Period, spanning from approximately 298.9 to 251.9 million years ago, marks the sixth geological period within the Paleozoic Era. This time in Earth’s history was defined by significant geological, climatic, and evolutionary events, culminating in one of the most dramatic mass extinction events ever recorded.
During the Permian, the Earth underwent substantial shifts in its tectonic plates, and these movements led to the assembly of the supercontinent Pangaea, as well as the formation of the vast Panthalassa Ocean that surrounded it.
The immense landmass of Pangaea created a unique environment, characterized by a dry and arid climate with vast inland deserts, and these conditions played a significant role in shaping the flora and fauna that inhabited the region.
Life on Earth during the Permian Period was diverse and abundant, with a wide range of terrestrial and marine organisms thriving in various ecosystems. On land, early mammal-like reptiles, known as therapsids, and the ancestors of modern reptiles, called sauropsids, began to dominate, meanwhile, in the seas, diverse communities of invertebrates, fish, and marine reptiles flourished.
For all its glory, however, the Permian Period is perhaps best known for its dramatic finale: the Permian-Triassic Extinction Event, or “The Great Dying.” This catastrophic event, which occurred around 252 million years ago, resulted in the loss of nearly 90–96% of all marine species and around 70% of terrestrial species.
The causes of this mass extinction are still a subject of intense scientific debate, with hypotheses ranging from massive volcanic eruptions to climate change and ocean anoxia, but its undebated that the aftermath of this event would forever alter the course of life on Earth and pave the way for the rise of the dinosaurs in the subsequent Mesozoic Era.
The formation of Pangea
Pangaea, the supercontinent that formed during the late Paleozoic Era, plays a crucial role in our understanding of deep time and Earth’s history. Its assembly during the Permian Period had significant implications for global climate, ocean circulation, and the distribution of flora and fauna.
Tectonic Processes
The formation of Pangaea was driven by the continuous movement of Earth’s tectonic plates, which are vast slabs of rock that make up the Earth’s lithosphere.
These plates float on the partially molten asthenosphere below and move due to the processes of mantle convection and seafloor spreading. During the Paleozoic Era, the gradual convergence of several continents led to the assembly of Pangaea.
Continental Collisions
Throughout the late Paleozoic Era, a series of continental collisions occurred, which eventually led to the formation of Pangaea. These events included:
Gondwana and Laurussia: Gondwana, a large southern continent, collided with Laurussia, a northern continent, during the late Carboniferous Period. This collision formed the Appalachian Mountains in North America and the Variscan Mountains in Europe.
Siberia and Kazakhstania: Siberia, a separate continental landmass, collided with the small continent of Kazakhstania, which had already merged with Laurussia.
The final stages of Pangaea’s formation occurred during the early Permian Period when Siberia and Gondwana merged, closing the Paleo-Tethys Ocean.
The Evolution of Reptiles and Synapsids
The Permian Period was a pivotal time in the evolutionary history of terrestrial vertebrates, marked by the diversification and rise of reptiles and synapsids.
These two groups played a significant role in shaping the ecosystems of the time, and their evolutionary trajectories would have long-lasting implications for Earth’s biodiversity.
Origins of Reptiles and Synapsids
Reptiles and synapsids share a common ancestry with amphibians, evolving from a group of tetrapods known as amniotes, who were distinguished by the development of the amniotic egg, which provided a watertight environment for embryonic development.
This adaptation allowed these animals to thrive in the drier environments that characterized the Permian Period.
Additionally, the evolution of the amniotic egg facilitated the emergence of various reproductive strategies and enabled these creatures to colonize diverse habitats.
In this short section, we’ll take a brief look at the evolutionary paths of both.
Continued Evolution of Reptiles
The reptiles, or sauropsids, diversified into two major lineages during the Permian Period, the Anapsids, and Diapsids:
Anapsids: These early reptiles were characterized by a solid, unperforated skull. They were primarily small, lizard-like creatures that occupied various ecological niches. Some anapsids, such as the pareiasaurs, grew to become large herbivores, while others, like the aquatic mesosaurs, adapted to life in the water. Fossil evidence suggests that the anapsids had a diverse range of body forms and ecological roles.
Diapsids: Diapsids possessed two temporal openings in their skull, which allowed for greater jaw muscle attachment and more efficient biting. This group would eventually give rise to various reptilian lineages, including dinosaurs, marine reptiles, and modern reptiles such as lizards and snakes.
During the Permian, early diapsids like the araeoscelidians and younginiforms occupied niches as small insectivores and carnivores.
Evolution of Synapsids
Synapsids, also known as “mammal-like reptiles,” were a diverse group of amniotes that ultimately gave rise to modern mammals. They can be broadly divided into two groups:
Pelycosaurs: The earliest synapsids were the pelycosaurs, which included small insectivores and large, sail-backed herbivores and carnivores, such as Dimetrodon and Edaphosaurus. These creatures dominated the early Permian ecosystems, occupying a wide range of ecological roles, and exhibited a diverse array of body forms, from sprawling, lizard-like creatures to robust, powerful predators.
Therapsids: Therapsids, which evolved from the pelycosaurs during the Middle Permian, exhibited more mammal-like features. They had differentiated teeth and a more advanced jaw structure, and some may have had hair or whiskers. This group included herbivores, such as the dicynodonts, carnivores, such as the gorgonopsians, and omnivores like the therocephalians.
The therapsids’ mammal-like characteristics allowed them to exploit a broader range of resources and become successful competitors with reptiles.
Ecological Roles and Adaptations
Reptiles and synapsids occupied various ecological niches during the Permian Period, with some species filling the roles of predators, herbivores, and scavengers.
Throughout this time, they developed numerous adaptations that allowed them to exploit these roles, including:
Jaws and Teeth: The evolution of specialized jaws and teeth allowed reptiles and synapsids to consume a diverse array of food sources, from plants to insects and other vertebrates. Different tooth shapes and arrangements enabled efficient processing of various types of food, allowing these animals to adapt to changing environmental conditions and resource availability.
Limbs and Locomotion: The development of stronger, more upright limbs facilitated greater mobility and allowed these animals to traverse the diverse landscapes of the Permian Period. Improved limb structure and posture enabled more efficient walking, running, and climbing, which in turn allowed reptiles and synapsids to exploit a broader range of habitats and escape from predators or pursue prey more effectively.
Thermoregulation: Some synapsids, such as the sail-backed Dimetrodon, may have utilized specialized structures for thermoregulation, helping them maintain an optimal body temperature in the variable climates of the time. This adaptation would have provided a competitive advantage, as it enabled them to remain active and hunt or forage under a wider range of environmental conditions.
Reptiles, on the other hand, were ectothermic, relying on external sources of heat to regulate their body temperature.
Sensory Adaptations: Both reptiles and synapsids evolved various sensory adaptations that allowed them to better detect prey, avoid predators, and navigate their environments. Enhanced vision, hearing, and olfactory senses would have improved their ability to locate food, find mates, and avoid danger.
Social and Reproductive Behavior: The evolution of complex social and reproductive behaviors allowed reptiles and synapsids to maximize their chances of survival and reproduction in the challenging Permian environment, and parental care, territoriality, and cooperative hunting strategies may have played a role in the success of these animals during this time.
Permian Mass Extinction Event
The Permian Mass Extinction Event, also known as the Great Dying, marked the end of the Permian Period and remains the most severe extinction event in Earth’s history. Approximately 90–96% of all marine species and roughly 70% of terrestrial vertebrate species went extinct during this cataclysmic event, permanently altering the course of life on our planet.
As we stated at the beginning of this section, the precise cause of the Permian Mass Extinction Event remains a subject of ongoing scientific investigation, however, several lines of evidence point to a combination of factors, including:
Massive Volcanic Activity: The eruption of the Siberian Traps, a vast volcanic province in present-day Russia, released enormous quantities of lava, ash, and greenhouse gases, such as carbon dioxide and methane, into the atmosphere.
This led to rapid global warming, acid rain, and ocean acidification, which had devastating effects on marine and terrestrial ecosystems.
Formation of Pangaea: The assembly of the supercontinent Pangaea during the Permian Period altered global climate patterns and ocean circulation, leading to the development of vast deserts and disrupting the distribution of heat and nutrients. This may have contributed to widespread habitat loss and the decline of biodiversity.
Methane Hydrate Release: The destabilization and release of methane hydrates from ocean sediments, potentially triggered by volcanic activity or global warming, may have exacerbated climate change and created large-scale anoxic (oxygen-depleted) conditions in the oceans, leading to massive die-offs of marine life.
Impact Event: Although less widely supported, some researchers propose that an asteroid or comet impact may have played a role in the extinction event, similar to the one that contributed to the extinction of the dinosaurs at the end of the Cretaceous Period. Again, to reiterate… this is not widely supported (currently).
The Permian Mass Extinction Event unfolded over a relatively short geological timescale, with the most severe phase occurring approximately 251.9 million years ago. The extinction event is believed to have transpired in multiple pulses, with the most significant losses of biodiversity occurring within a span of less than 100,000 years.
This rapid pace of extinction further underscores the severity of the environmental changes that took place during this time.
Consequences of the Extinction
The Permian Mass Extinction Event had far-reaching and long-lasting consequences for life on Earth, including but not limited to:
Loss of Biodiversity: The extinction event decimated the planet’s biodiversity, eliminating numerous marine invertebrate groups, such as trilobites and rugose corals, as well as many terrestrial plants, insects, and vertebrates, including the majority of the synapsids.
Ecological Reorganization: The event led to a major reorganization of ecosystems, opening up new niches and paving the way for the rise of new groups of organisms. Among the survivors, the reptiles, particularly the diapsids, were well-positioned to capitalize on the available resources and diversify into the dominant groups of the Mesozoic Era, including of course, the dinosaurs.
Evolutionary Opportunities: The Permian extinction created new opportunities for the evolution of novel adaptations and ecological strategies. In the wake of the Great Dying, the surviving organisms were faced with a dramatically altered environment, which provided the impetus for the evolution of new forms and the emergence of new lineages. This has often been a common cause for evolutionary push, since the beginnings of deep time and emergence of single-celled life.
Conclusion
As we conclude this odyssey through the Phanerozoic Eon, from the enigmatic Cambrian Explosion to the very threshold of our next, the Mesozoic Era, we hope to have illuminated the intricate tapestry of geological and biological processes that have shaped our planet’s history.
In the third installment of this in-depth series, we will journey through the next chapters of Earth’s geological history, the Mesozoic era.
In the interim, we invite you to follow our blog and stay tuned for the third part of this series, as well as our other content on Earth’s history, geology, evolution, and the wonders of the natural world!
Resources
The Paleozoic Era — UCMP
Paleozoic Era — Smithsonian National Museum of Natural History
The Paleozoic Era — University of Tennessee, Knoxville
Paleozoic Fossil Plants, by Thomas N. Taylor and Edith L. Taylor
The Rise of Animals: Evolution and Diversification of the Kingdom Animalia by Mikhail A. Fedonkin and James G. Gehling
Paleozoic Era — Discovering Fossils
