The Late Devonian extinction, occurring 375-360 million years ago, was a major event that reshaped life on Earth. This prolonged period of biodiversity loss affected both marine and terrestrial ecosystems, driven by factors like global cooling, sea level changes, and ocean anoxia.

Understanding this extinction provides insights into Earth's complex systems and life's vulnerability to environmental changes. It set the stage for the Carboniferous period, influencing the evolution of fish, early land animals, and plants, ultimately shaping the course of life on our planet.

Causes of Late Devonian extinction

• The Late Devonian extinction, which occurred approximately 375-360 million years ago, was a significant event in Earth's history that resulted in the loss of many marine and terrestrial species
• Multiple factors likely contributed to this extinction event, including global cooling, sea level changes, ocean anoxia, and possibly even extraterrestrial impacts
• Understanding the causes of this extinction provides valuable insights into the complex interplay between Earth's systems and the vulnerability of life to major environmental changes

Role of global cooling

• Global cooling during the Late Devonian period led to the formation of glaciers and ice sheets, particularly in the southern hemisphere (Gondwana)
• Cooler temperatures reduced evaporation rates and altered global precipitation patterns, affecting both marine and terrestrial ecosystems
• Cooling also caused a drop in sea levels, reducing the extent of shallow marine habitats that were crucial for many species
• The onset of glaciation likely triggered a positive feedback loop, further amplifying the cooling trend and its impact on biodiversity

Impact of sea level changes

• Sea level changes during the Late Devonian had a profound impact on marine ecosystems, particularly in shallow shelf areas
• Falling sea levels exposed previously submerged habitats, leading to the loss of critical marine environments such as coral reefs and coastal wetlands
• The reduction in shallow marine habitats increased competition among species and disrupted food webs, contributing to the extinction of many marine organisms
• Sea level changes also affected nutrient cycling and ocean circulation patterns, further stressing marine ecosystems

Influence of ocean anoxia

• Ocean anoxia, or the depletion of dissolved oxygen in seawater, was a significant factor in the Late Devonian extinction
• Anoxic conditions developed in many marine basins due to a combination of factors, including increased nutrient input from land, reduced ocean circulation, and warmer water temperatures
• Anoxia led to the formation of "dead zones" in the oceans, where most marine life could not survive due to the lack of oxygen
• The spread of anoxic conditions likely caused mass die-offs of marine species and contributed to the collapse of entire ecosystems

Possibility of extraterrestrial factors

• Some researchers have suggested that extraterrestrial factors, such as meteor impacts or cosmic radiation, may have played a role in the Late Devonian extinction
• Evidence for extraterrestrial involvement includes the presence of impact craters and unusual concentrations of iridium in Late Devonian sediments
• However, the role of extraterrestrial factors in this extinction event remains controversial and requires further research to confirm
• If extraterrestrial impacts did occur, they likely worked in conjunction with other environmental stressors to drive the extinction

Timing and duration of extinction

• The Late Devonian extinction is characterized by its prolonged duration and multiple pulses of species loss
• Unlike some other mass extinctions that occurred relatively rapidly, the Late Devonian event unfolded over several million years
• The timing and duration of the extinction provide insights into the complex interplay of environmental factors that led to the loss of biodiversity

Initial pulse of extinction

• The first pulse of the Late Devonian extinction occurred around 375 million years ago, marking the beginning of the event
• This initial pulse primarily affected marine species, particularly those living in shallow shelf environments
• The cause of this first pulse is still debated, but it may have been triggered by a combination of global cooling, sea level changes, and ocean anoxia

Second pulse of extinction

• A second, more severe pulse of extinction occurred approximately 360 million years ago, near the end of the Devonian period
• This pulse affected both marine and terrestrial species, leading to a significant loss of biodiversity across multiple ecosystems
• The second pulse may have been driven by the intensification of environmental stressors, such as further global cooling and the spread of anoxic conditions in the oceans

Overall timeline vs other extinctions

• The Late Devonian extinction occurred over a period of roughly 15-20 million years, making it a relatively prolonged event compared to some other mass extinctions
• In contrast, the end-Cretaceous extinction, which wiped out the dinosaurs, occurred over a much shorter timescale (thousands to tens of thousands of years)
• The prolonged nature of the Late Devonian extinction highlights the cumulative impact of multiple environmental stressors on biodiversity over an extended period

Marine life during Late Devonian

• The Late Devonian period was characterized by a diverse array of marine life, including reef-building organisms, brachiopods, trilobites, and early vertebrates
• The extinction event had a profound impact on marine ecosystems, leading to the loss of many species and the restructuring of communities
• Understanding the state of marine life before and during the extinction provides insights into the event's selectivity and long-term consequences

Diversity before the extinction

• Prior to the Late Devonian extinction, marine ecosystems were highly diverse, with a wide range of invertebrate and vertebrate species
• Reef ecosystems, in particular, were extensive and hosted a rich array of life, including corals, sponges, and various reef-dwelling organisms
• The high diversity of marine life during this time was supported by favorable environmental conditions, such as warm temperatures and high sea levels

Groups most impacted

• The Late Devonian extinction had a significant impact on several marine groups, including reef-building organisms, brachiopods, and trilobites
• Reef ecosystems were particularly hard hit, with many coral and sponge species going extinct due to changes in sea level, ocean chemistry, and water temperature
• Brachiopods, which were dominant shell-forming organisms in Devonian seas, experienced a significant decline in diversity and abundance
• Trilobites, which had been a successful group for millions of years, also suffered major losses during the extinction

Reef ecosystems collapse

• The collapse of reef ecosystems was one of the most significant consequences of the Late Devonian extinction
• Reefs are complex, biodiverse habitats that support a wide range of marine life and play critical roles in nutrient cycling and coastal protection
• The loss of reef-building organisms, such as corals and sponges, led to the breakdown of these ecosystems and the extinction of many reef-dwelling species
• The collapse of reefs likely had far-reaching effects on marine food webs and ocean chemistry, contributing to the overall impact of the extinction event

Terrestrial life during Late Devonian

• The Late Devonian period saw significant changes in terrestrial ecosystems, with the spread of vascular plants and the emergence of early tetrapods
• Although the impact of the Late Devonian extinction on terrestrial life is less well-understood than its effects on marine ecosystems, it likely played a role in shaping the evolution of land plants and animals

Spread of vascular plants

• During the Late Devonian, vascular plants, which have specialized tissues for water and nutrient transport, became increasingly widespread on land
• The spread of vascular plants, such as ferns and early seed plants, led to the development of more complex terrestrial ecosystems
• Vascular plants played a crucial role in soil formation, nutrient cycling, and the creation of new habitats for terrestrial organisms

Early tetrapods emergence

• The Late Devonian period witnessed the emergence of early tetrapods, which were the first vertebrates to venture onto land
• Tetrapods evolved from lobe-finned fish and developed adaptations for terrestrial life, such as weight-bearing limbs and air-breathing capabilities
• The earliest tetrapods, such as Acanthostega and Ichthyostega, likely lived in shallow, swampy environments and represented an important step in the colonization of land by vertebrates

Impact of extinction on land

• While the Late Devonian extinction is primarily associated with marine ecosystems, it also had an impact on terrestrial life
• The extinction event may have affected early tetrapods and other terrestrial organisms through changes in climate, habitat availability, and food web dynamics
• However, the extent and nature of the extinction's impact on terrestrial ecosystems remain less clear than its effects on marine life, and further research is needed to fully understand its consequences for land-dwelling organisms

Recovery after the extinction

• Following the Late Devonian extinction, marine and terrestrial ecosystems underwent a period of recovery and restructuring
• The recovery process involved the diversification of surviving lineages, the evolution of new species, and the establishment of novel ecological relationships
• Understanding the recovery patterns after the extinction provides insights into the resilience and adaptability of life in the face of major environmental perturbations

Restructuring of marine ecosystems

• In the aftermath of the Late Devonian extinction, marine ecosystems underwent significant restructuring
• The loss of reef-building organisms and other dominant groups created opportunities for new species to evolve and fill vacant ecological niches
• The recovery of marine ecosystems was gradual and involved the establishment of new community structures and food web dynamics

Diversification of fish

• Fish, which had already begun to diversify during the Devonian period, continued to evolve and radiate in the wake of the extinction
• The post-extinction recovery saw the rise of new fish lineages, such as ray-finned fishes (Actinopterygii), which would go on to dominate marine and freshwater ecosystems
• The diversification of fish likely played a crucial role in the restructuring of marine food webs and the recovery of ecosystems

Rise of sharks and rays

• Sharks and rays (Chondrichthyes) experienced a significant evolutionary radiation following the Late Devonian extinction
• The extinction event may have created new ecological opportunities for these cartilaginous fishes, allowing them to diversify and occupy new niches
• The rise of sharks and rays likely had important implications for marine ecosystem dynamics, as these predators played key roles in structuring communities and regulating populations of other species

Colonization of land by tetrapods

• The recovery period following the Late Devonian extinction saw the continued colonization of land by tetrapods
• Early tetrapods, which had emerged during the Late Devonian, continued to evolve and adapt to terrestrial life in the aftermath of the extinction
• The colonization of land by tetrapods set the stage for the evolution of diverse terrestrial vertebrate lineages, including amphibians, reptiles, and eventually, mammals and birds

Comparison to other mass extinctions

• The Late Devonian extinction is one of the five major mass extinctions in Earth's history, each of which had unique characteristics and consequences
• Comparing the Late Devonian event to other mass extinctions helps to contextualize its severity, selectivity, and role in shaping the evolution of life on Earth

Severity vs other extinctions

• The Late Devonian extinction is considered to be less severe than some other mass extinctions, such as the end-Permian (252 million years ago) or end-Cretaceous (66 million years ago) events
• Estimates suggest that the Late Devonian extinction resulted in the loss of around 50-60% of marine species, compared to over 90% in the end-Permian extinction
• However, the prolonged duration of the Late Devonian event and its impact on both marine and terrestrial ecosystems make it a significant event in Earth's history

Selectivity of Late Devonian extinction

• The Late Devonian extinction exhibited a degree of selectivity, with certain groups of organisms being more severely affected than others
• Reef-building organisms, brachiopods, and trilobites were among the most heavily impacted groups, while other lineages, such as fish and tetrapods, survived and diversified in the aftermath
• The selectivity of the extinction may reflect the differential vulnerability of organisms to the specific environmental stressors that triggered the event, such as changes in sea level or ocean chemistry

Role in paving way for Carboniferous period

• The Late Devonian extinction played a significant role in setting the stage for the subsequent Carboniferous period (359-299 million years ago)
• The extinction event and its aftermath reshuffled ecosystems and created new ecological opportunities, allowing for the diversification of groups such as sharks, ray-finned fishes, and early tetrapods
• The spread of vascular plants during the Late Devonian also paved the way for the development of extensive coal swamps during the Carboniferous, which would become a defining feature of the period and a major source of fossil fuels
• In this sense, the Late Devonian extinction can be seen as a pivotal event that helped to shape the evolution of life and the Earth system in the millions of years that followed