Lecture Outlines
Natural Disasters, 7th edition
Patrick L. Abbott
The Great Dyings
Natural Disasters, 7th edition, Chapter 16
The Great Dyings
• Massive human loss by natural disasters is insignificant
when compared to great dyings in fossil record
• Entire species, millions of entire species  mass
extinctions
• Knowledge comes through fossil record
Fossils
• Evidence of former life
• Requisites for fossilization:
– Possession of hard parts – shells, bones, teeth
– Rapid burial, protecting from scavenging or disintegration
• Dinosaurs known by fossil bones, teeth and footprints
– Abundant footprints show that some species lived in herds
Figure 16.5
Figure 16.6
Fossils
Examples:
• Fossilized tree sap  amber
• Burial in oxygen-poor swamps or peat bogs preserves organisms,
like tanning
• Frozen animals (like mammoths) exposed as glaciers melt
• Celtic miner’s body preserved for 2,300 years in salt mine collapse
Figure 16.8
Early Understanding of Extinctions
and Geologic Time
• 1786, French paleontologist Georges Cuvier: proved that
extinction of species had occurred
– Skeletons of mammoths demonstrably different from skeletons
of elephants  mammoths had gone extinct
• Observed profound changes in
sedimentary record
– Abrupt first appearances of fossils
– Abundant fossils in overlying layers
– Absence in higher overlying layers
• Steno’s 1669 law of superposition:
– Younger layers of sediment are
deposited on top of older layers
Figure 16.10
Early Understanding of Extinctions
and Geologic Time
• William Smith, 1799: law of faunal assemblages
– Strata of same age can be recognized by same fossils
– Law of superposition + law of faunal assemblage  law of
faunal succession:
• Fossils from older (lower) rock layers are older and more
different from present-day organisms than fossils from
younger (higher) rock layers  old forms of life have died
out and new forms of life have developed
– Geologic maps of England and Wales published in 1815
Early Understanding of Extinctions
and Geologic Time
First geologic maps led to global movement in geology
• Sedimentary strata from around world classified and
subdivided on basis of fossil assemblages
• Same fossils found in rocks in different areas  same
age rocks
• 1841: standard geologic column based on fossil
succession, relative ages
Early Understanding of Extinctions
and Geologic Time
• 20th century:
geologic column
refined with
absolute ages
from radioactive
dating 
geologic
timescale
Figure 16.11
Early Understanding of Extinctions
and Geologic Time
Brief History of Life
• 3.85 billion years ago: archaea
– Found today at mid-ocean ridges, killed by oxygen
– Three branches of life – archaea, bacteria, eukarya (plants,
animals)
• 3.5 billion year ago: photosynthetic bacteria removing
CO2 from atmosphere, adding O2
• 1 billion years ago: single cell division  sexual reproduction
• 620 million years ago: multicellular animal life
Early Understanding of Extinctions
and Geologic Time
Brief History of Life
• 543 million years
ago: life began 40million-year-long burst
of evolutionary change
– First hard parts
(shells, etc.) that
preserve as fossils
• Evolution,
increasing diversity
and extinctions
continue
Figure 16.12
Species and the Fossil Record
Swedish botanist Linnaeus laid out basic terminology of
divisions of life into kingdoms down to species
Species:
• Population of organisms so similar in life habits and
functions that they can breed together and produce
reproductively viable offspring
• Reproductively isolated by differences from other species
• Share common pool of genetic material (genome)
• May migrate over broad area  mutation of genes may
cause reproductive isolation between local populations 
evolution (Darwin: descent with modification)
Species and the Fossil Record
• Extinction caused by inability to adapt to changes in
physical, chemical, biological conditions
• Background level of extinctions – always occurring
• Present species diversity: 40 to 80 million species
– 0.1% of species that have existed in Earth history
– 99.9% of species that have existed in Earth history are extinct
• Extinctions clear out niches  opportunity for new
organisms to evolve to occupy habitats
• Mass extinctions open up many new niches  burst of
evolution
The Tropical Reef Example
• Reefs: porous, wave-resistant
frameworks built by organisms
such as corals, clams, etc., and
used by organisms such as red
algae, worms, bryozoa, etc.
• Periods of Earth history with
and without reefs
• Each time reefs reappeared,
built by different organisms
Figure 16.15
Mass Extinctions During Phanerozoic Time
• Average life span of species: 4 million years
• Plot extinctions of genera (above species) against time
– % extinction =
number of generic extinctions
number of genera alive at that time
– Background
extinctions have
declined from
about 50% in
Cambrian time to
about 5-10%
recently
– Spikes in
extinctions 
mass extinctions
Figure 16.16
Mass Extinctions During Phanerozoic Time
• Extinction-frequency curve:
number of extinctions plotted
against recurrence interval 
estimates of how often given
size extinction might occur
Figure 16.17
Figure 16.18
Possible Causes of Mass Extinctions
Plate Tectonic Causes
• Today: oceans cover ~71% of Earth’s surface, continents ~29%
• Sea level drop:
continents could
cover up to 40%
• Sea level rise:
continents could
cover only 17%
Figure 16.19
Possible Causes of Mass Extinctions
Changes in Seafloor Spreading Rates
• More rapid spreading  spreading ridges increase in mass, volume
 sea level rises
• Mid-Cretaceous (110 to 85 million years ago): faster seafloor
spreading  global sea level 200 m higher than today
Figure 16.21
– Double area of shallow seas  warmer climate
– Severely reduced area of exposed land
Figure 16.20
Possible Causes of Mass Extinctions
Sea-Level Changes
• Bigger glaciers  lower sea level
• Most recent expansion of glaciers (20,000 years ago): sea level 140
m lower than today
• If all glaciers melted: sea level 70 m higher than today
• Sea level also rises or falls by changes in seafloor spreading rates –
can combine with each other or cancel each other out
Figure 16.22
Possible Causes of Mass Extinctions
Numbers and Sizes of Continents
• Late Permian to early Triassic (260 – 240 million years
ago): supercontinent Pangaea
• 200 million years ago: Pangaea began to be rifted apart
into today’s continents
– Greatly lengthened world’s shorelines
– Reduced areas of climatically harsh continental interiors
– Numerous habitat changes
• Large, combined landmass  fewer number of species
• Smaller, isolated landmasses  larger number of species
Possible Causes of Mass Extinctions
Continental Position and Glaciation
• Large landmasses at poles necessary to capture enough
snow to create massive ice sheets that cause Ice Age
• Ice Age: climatic extremes of glacial advances and
retreats  stresses on species  extinctions
Possible Causes of Mass Extinctions
Volcanic Causes
• Flood basalt: immense volumes of basaltic lava erupted in
geologically short time period, covering millions of square
kilometers of Earth
– Ontong Java flood basalt plateau created 120 million years ago
• 36 million km3 of lava erupted in less than 3 million years
• World sea level rose 10 m as ocean displaced by lava
Possible Causes of Mass Extinctions
Changes in Atmospheric Composition
• Flood basalt eruption  emission of massive volume of gases
• Sub-sea eruptions: oceans absorb and dilute some gas, ocean-water
acidity and oxygen concentrations change
• Continental eruptions: gas goes directly into atmosphere, enhancing
greenhouse effect, warming climate
• Eruption of Ontong Java plateau may have raised global
temperatures up to 13oC
Possible Causes of Mass Extinctions
Climate Change Causes
• Climate change: complex network of positive and negative
feedback responses
• Volcanism:
– Emits tremendous volume of gases
– Composition of atmosphere changes
– Earth’s heat balance changes via greenhouse effect
– Global climate changes
Possible Causes of Mass Extinctions
Ocean Composition Causes
• Ocean is chemically connected to dissolved salts, bottom
sediments, continents, atmosphere
• Equilibrium maintained by negative feedback buffers,
– Occasionally overcome  lethal disequilbrium
• Today: oceans ‘stirred’ by currents between different density layers
– Deep waters well oxygenated, rich with life
• Warm climate intervals: inadequate ocean circulation
– Polar waters too warm to sink
– Organic decay at ocean bottoms used up oxygen  anoxic
waters caused sea life extinctions
– Melting of glaciers flooded surfaces of oceans with fresh water
 lethal to sea life species
Possible Causes of Mass Extinctions
Extraterrestrial
Causes
• When 10 km
diameter object
hits Earth:
– Wildfires, acid
rain, tsunami,
dust cloud 
weeks of dark
winter, gases 
greenhouse
effect
temperature rise
Figure 16.23
Possible Causes of Mass Extinctions
Extraterrestrial Causes
• Bombardment of cosmic rays increases if supernova
nearby
• Bombardment of subatomic particles from Sun increases
when Sun’s intensity changes
– Both increase during periods of weakened magnetic
field
– (No correlation to fossil record found)
Possible Causes of Mass Extinctions
Biologic Causes
• Species-Area Effects
– Smaller area  fewer species
Figure 16.24
Possible Causes of Mass Extinctions
Biologic Causes
• Random Extinction
– Number of individuals of species goes up and down randomly
(random walk)
– Randomness guarantees that number of individuals will
eventually hit zero  no recovery (absorbing boundary)
Figure 16.25
Possible Causes of Mass Extinctions
Biologic Causes
Predation and Epidemic Disease
• Excessive predation can drive number of individuals of
species low enough for random extinction to finish the job
• Large carnivores or epidemic disease
• Today: homo sapiens biggest predator
Multiple Causes of Mass Extinction
• Any one factor alone can cause local stress to drive a few
species to extinction
• Extinction of numerous species around world probably
requires two or more causes
Examples of Mass Extinctions
Closing of Permian Time (Ended 253 Million Years Ago)
• On land: ¼ amphibian orders, 1/50 reptile genera survived
• Oceans: 80% or more species went extinct
• Formation of Supercontinent Pangaea
– Uniting of continents into supercontinent closed equatorial sea
 reduced shallow seas  triggered extinctions
• Sea-Level Fall
– Slower seafloor spreading shrank mid-ocean ridges, lowering
sea level 200 m
– Reduced area of shallow seas  triggered extinctions
Examples of Mass Extinctions
Closing of Permian Time (Ended 253 Million Years Ago)
• Climate Changes
– Supercontinent  less shoreline  greater percentage of land
away from climate-moderating effects of ocean
– Drier, more severe climate in interior of landmass
• Ocean Composition Changes
– End of long Ice Age: disappearance of cold polar waters may
have slowed ocean circulation and led to anoxic bottom water,
killing deep-water organisms
– Turnover of stratified layers may kill surface organisms
Examples of Mass Extinctions
Closing of Permian Time (Ended 253 Million Years Ago)
• Siberian Traps Flood Basalt
– 3 million km3 of lava erupted within 1 million years, emitted
huge volume of gases (CO2)
– Heated permafrost to release water vapor, methane from
hydrates
– Greenhouse gases raised global temperatures, acid rain
• Duration of the Extinction Events
– Took less than 1 million years
– Change in carbon isotopes (from collapse in biological
productivity) in maybe less than 30,000 years
Examples of Mass Extinctions
Closing of Permian Time (Ended 253 Million Years Ago)
• Life at the End of Permian Time
– Tropical seas virtually eliminated
– One major landmass placed species-area effect pressure on
terrestrial life
– Land covered in desert
– Deep-ocean water became anoxic, CO2-rich
– Climate warmed
– Extinction of Permian species allowed Mesozoic reptiles
(dinosaurs) to take over
Examples of Mass Extinctions
Close of Cretaceous Time (Ended 65 Million Years Ago)
• Late Cretaceous: North American heartland covered with
herds of dinosaurs and flowering plants
• Slow-acting changes elevated background level of
extinctions, followed by deadly volcanism and asteroid
impact – slow decline abruptly terminated
• Over 35% of genera and 65% of species went extinct
Examples of Mass Extinctions
Close of Cretaceous Time (Ended 65 Million Years Ago)
• Sea Level Fall
– During final 18 million years of Cretaceous, global sea levels
went from high to low, climate cooled
• Deccan Traps Flood Basalt
– ‘Deccan’ Sanskrit for southern, ‘trap’ Dutch for staircase: thick
piles of basaltic rock
– Over 2 million km3 erupted in less than 1 million years,
beginning 65.5 million years ago
– Worldwide climatic effects, like Permian Siberian traps flood
basalt
Examples of Mass Extinctions
Close of Cretaceous Time (Ended 65 Million Years Ago)
• Chicxulub Impact
– Yucatan peninsula, Mexico: rings (180 km, 300 km diameter) of
shattered rock  asteroid impact ~65 million years ago
– Worst-case scenario:
• 10 km asteroid plunges through atmosphere at 10 km/sec
• Fireball with 2,000 km diameter
• Searing hot winds ignite wildfires throughout North America
• Earthquake with magnitude greater than 11
Examples of Mass Extinctions
Close of Cretaceous Time (Ended 65 Million Years Ago)
• Chicxulub Impact
– Worst-case scenario:
• Tsunami 2 to 3 km high
• Blasted hole up to 60 km deep, shot plume of vaporized
water and rock into stratosphere
• Acid rain (from vaporized rock) killed ocean organisms
• Dust blocked sunlight  stopped photosynthesis for months
• Greenhouse gases remained aloft, raised temperatures
• Mass extinctions led to opportunities for surviving
organisms (including mammals)
Living Fossils
Not all species went extinct at Permian/Triassic and
Cretaceous/Tertiary mass extinctions:
• Horseshoe crabs have survived for last 450 million years
• Sharks have been successful predators for last 350 million
years – biggest threat of extinction is today, from humans
• Conifers, ferns, horsetail and scouring rushes have existed
since 275 million years ago
• Norfolk pine, gingko biloba, metasequoia, and sago palm
have existed since 235 million years ago
Living Fossils
Quaternary Extinctions
• Significant extinctions of large-bodied mammals in last
1.5 million years, during glacial advances and retreats
• Concentrations of extinctions  multiple causes
• Suspected additional cause of many extinctions  Homo
sapiens
• Large animals decimated in Americas and Australia
• Large animals fared best in Africa where humans evolved
– co-existed for thousands of generations
• Wherever humans went, extinctions followed
Quaternary Extinctions
• Arguments against climate change as cause:
– More large-animal extinctions than plant extinctions
– Large mammals not affected by climate change
– Increase in habitable land from retreat of glaciers should cause
increase in species, not decline
– No equivalent extinctions earlier during present Ice Age
Australia
• Humans arriving in Australia 56,000 years ago found 24
genera of large-bodied animals  1,000 years later, 23 of
those were extinct
– Regional, not global event  not climate-caused
Quaternary Extinctions
Madagascar and New Zealand
• When humans arrived, largest animals were flightless
birds – elephant birds in Madagascar and moas in New
Zealand
• Humans killed birds and stole eggs until populations were
low enough that random extinction finished them off
• Rate of human-induced or –related extinctions increased
in last 12,000 years, increased even more in last 200 years
Quaternary Extinctions
Figure 16.31
Quaternary Extinctions
In Greater Depth: La Brea Tar Pits,
Metropolitan Los Angeles
• One of most spectacular fossil localities in world
• Oil from underground reservoirs seeped upward to
surface where natural gas and lighter-weight oils
evaporated, leaving sticky, high-viscosity tar in pools
• Over last 40,000 years, more than 660 species of
organisms trapped and entombed (59 mammal species)
• Escape very difficult for four-legged, heavy animal
• Distress cries brought carnivores and scavengers  85%
of larger-bodied victims
• Also one 9,000 year old partial human skeleton –
indicates human presence during extinctions
Side Note: The Rewilding of North America
• North America lost many large-bodied vertebrate species
around 13,000 years ago
• Plan to restore large populations of vertebrates with
similar species from elsewhere
–
–
–
–
Critically endangered 50 kg Bolson tortoise (Mexico)
Horses and camels, which originated in North America
Elephants to replace mammoths, mastodons, gomphotheres
In fenced reserves: cheetahs, lions
• Pros: ties in with bison and wolf ranges underway, undo human
harm, save species from extinction, enhance biodiversity and
evolutionary potential, ecotourism
• Cons: not genetically identical to extinct species, habitats have
changed, possible disease transmission, unexpected consequences
End of Chapter 16