🦕Paleontology Unit 11 – Mass extinctions and their causes

What Are Mass Extinctions?

• Catastrophic events leading to a significant loss of biodiversity over a geologically short period of time
• Characterized by the extinction of a large percentage of species across multiple taxonomic groups and habitats
• Typically defined as the loss of at least 75% of species within a geologically brief interval (less than 2.8 million years)
• Differ from background extinctions, which occur at a slower rate and affect fewer species
  • Background extinction rates estimated at 0.1 to 1 species per million species per year
• Can be caused by a variety of factors, including global climate change, volcanic eruptions, and extraterrestrial impacts
• Have occurred periodically throughout Earth's history, with five major events identified in the fossil record
• Followed by periods of recovery and diversification, as surviving species adapt to new ecological niches

Major Mass Extinction Events

• End-Ordovician (444 million years ago)
  • 85% of marine species extinct, caused by global cooling and sea level drop
• Late Devonian (375-360 million years ago)
  • 75% of species extinct, caused by global cooling and ocean anoxia
• End-Permian (252 million years ago)
  • 96% of marine species and 70% of terrestrial vertebrate species extinct, caused by Siberian Traps volcanism and global warming
• End-Triassic (201 million years ago)
  • 80% of species extinct, caused by Central Atlantic Magmatic Province volcanism and ocean acidification
• End-Cretaceous (66 million years ago)
  • 76% of species extinct, including non-avian dinosaurs, caused by Chicxulub asteroid impact and Deccan Traps volcanism
• Other notable extinction events include the End-Capitanian (260 million years ago) and the Eocene-Oligocene (34 million years ago)

Causes of Mass Extinctions

• Volcanic eruptions release greenhouse gases and toxic chemicals, leading to global climate change and ocean acidification
  • Siberian Traps (End-Permian) and Deccan Traps (End-Cretaceous) are examples of large igneous provinces implicated in mass extinctions
• Asteroid and comet impacts can cause global cooling, wildfires, and acid rain
  • Chicxulub impact (End-Cretaceous) created a crater 180 km in diameter and led to the extinction of non-avian dinosaurs
• Sea level changes can disrupt marine ecosystems and alter global climate patterns
  • End-Ordovician extinction coincided with a major sea level drop, reducing shallow marine habitats
• Ocean anoxia occurs when oxygen levels in the water become too low to support marine life
  • Caused by a combination of factors, including global warming, increased nutrient runoff, and reduced ocean circulation
• Climate change, both warming and cooling, can exceed species' tolerance limits and alter ecosystem dynamics
  • End-Permian extinction coincided with a rapid increase in global temperatures, while the Late Devonian and End-Ordovician events were associated with cooling
• Combinations of multiple factors often contribute to the severity and duration of mass extinction events

Geological Evidence

• Fossil record provides direct evidence of species extinctions and changes in biodiversity over time
  • Absence of certain species or groups in younger rock layers indicates their extinction
• Geochemical signatures in sedimentary rocks can indicate environmental changes associated with mass extinctions
  • Carbon isotope excursions suggest disruptions to the global carbon cycle, such as increased volcanic activity or methane release
  • Iridium anomalies in sedimentary layers provide evidence for extraterrestrial impacts, as iridium is rare in Earth's crust but abundant in asteroids
• Changes in sedimentary rock types and depositional environments can reflect sea level changes and altered climate patterns
  • End-Ordovician glacial deposits indicate a period of global cooling and ice sheet growth
• Fossil malformations and abnormalities may suggest environmental stress or mutagenic effects during extinction events
• Reduced diversity and abundance of fossils in post-extinction layers reveal the magnitude of species loss
• Gradual recovery of biodiversity in subsequent rock layers demonstrates the process of ecosystem regeneration and adaptive radiation

Impact on Evolution

• Mass extinctions can alter the course of evolution by removing dominant species and creating opportunities for new lineages to diversify
  • Extinction of non-avian dinosaurs at the End-Cretaceous allowed mammals to radiate and occupy new ecological niches
• Surviving species may possess adaptations that provide advantages in post-extinction environments
  • Small, generalist mammals with higher reproductive rates were better equipped to survive and diversify after the End-Cretaceous extinction
• Removal of competitive exclusion enables rapid diversification and adaptive radiation of surviving lineages
• Mass extinctions can lead to the emergence of entirely new body plans and functional adaptations
  • Evolution of birds from theropod dinosaurs was likely accelerated by the End-Cretaceous extinction
• Ecosystem restructuring following mass extinctions can lead to novel species interactions and coevolutionary relationships
• Extinctions can create "bottlenecks" in evolutionary lineages, reducing genetic diversity and potentially influencing future evolutionary trajectories
• Long-term evolutionary trends, such as increased body size (Cope's Rule) or specialization, may be reset or altered by mass extinction events

Recovery and Adaptive Radiations

• Surviving species recolonize vacant ecological niches and adapt to new environmental conditions
  • Mammals underwent a rapid adaptive radiation following the End-Cretaceous extinction, giving rise to diverse lineages (bats, whales, primates)
• Recovery typically begins with opportunistic, generalist species that can quickly exploit available resources
  • Pioneer species, such as ferns, often dominate post-extinction floras
• Specialist species and more complex ecosystems gradually reestablish over time as competition and niche partitioning increase
• Duration of recovery varies depending on the severity of the extinction and the resilience of surviving biota
  • End-Permian recovery took up to 10 million years, while End-Cretaceous recovery was relatively rapid (1-2 million years)
• Evolutionary innovations, such as new body plans or functional adaptations, can emerge during the recovery phase
  • Evolution of echolocation in bats and baleen feeding in whales occurred during the post-Cretaceous adaptive radiation of mammals
• Biodiversity may exceed pre-extinction levels as new species evolve to fill available niches
• Ecosystem structure and function may differ from pre-extinction states, reflecting the contingent nature of evolutionary processes

Current Extinction Crisis

• Anthropocene extinction, or Sixth Mass Extinction, is an ongoing biodiversity crisis caused by human activities
  • Current extinction rates are estimated to be 100 to 1000 times higher than background rates
• Habitat loss and fragmentation due to land-use change (deforestation, urbanization) is a primary driver of species extinctions
  • Destruction of tropical rainforests threatens numerous endemic species
• Climate change, driven by anthropogenic greenhouse gas emissions, is altering species' ranges and phenology
  • Coral bleaching events, caused by rising ocean temperatures, have led to widespread reef degradation
• Overexploitation of species for food, medicine, and other resources can lead to population collapses
  • Overfishing has caused declines in numerous shark and tuna species
• Invasive species, introduced by human activities, can disrupt ecosystems and outcompete native species
  • Introduction of rats and cats to islands has caused the extinction of many endemic bird species
• Pollution, including plastic waste, toxic chemicals, and nutrient runoff, can have detrimental effects on species and ecosystems
• Synergistic effects of multiple stressors can exacerbate the impact of the current extinction crisis
• Conservation efforts, such as habitat protection, captive breeding, and reintroduction programs, aim to mitigate species losses

Research Methods in Extinction Studies

• Paleontological fieldwork and fossil collection provide direct evidence of past extinctions and biodiversity changes
  • Stratigraphic analysis of fossil-bearing rock layers helps establish the timing and sequence of extinction events
• Geochemical analysis of sedimentary rocks can reveal environmental conditions associated with mass extinctions
  • Stable isotope ratios (carbon, oxygen) provide insights into global climate and carbon cycle perturbations
• Radiometric dating techniques (uranium-lead, potassium-argon) allow absolute age determination of extinction horizons
• Phylogenetic analysis and molecular clock dating help reconstruct evolutionary relationships and divergence times of lineages affected by extinctions
  • Comparative analysis of DNA from extant species can reveal population bottlenecks and post-extinction diversification patterns
• Ecological modeling and computer simulations can test hypotheses about extinction mechanisms and ecosystem dynamics
• Taphonomic studies investigate the processes of fossil preservation and potential biases in the fossil record
  • Understanding preservational biases is crucial for accurately interpreting patterns of extinction and recovery
• Interdisciplinary collaborations, integrating data from paleontology, geology, geochemistry, and biology, provide a comprehensive understanding of extinction events
• Studies of modern species declines and ecosystem disturbances offer insights into potential drivers and consequences of ongoing extinctions