Phanerozoic Diversity and Mass
Extinctions
Measuring Diversity
•
John Phillips produced
the first estimates of
Phanerozoic diversity in
1860, based on the British
fossil record
•
Intuitively it seems simple
to count the number of
taxa, but there are
actually quite a few
complications
Measuring Diversity
•
Diversity was revisited with a more quantitative approach
in the late 1970s and 1980s
•
The number of described species had increased ~25
times, providing much more data for analysis
New
analyses
Phillips, 1860
Measuring Diversity
•
Jack Sepkoski spent 15 years
compiling a database to
produce the classic diversity
curve
Phanerozoic Diversity
•
The Sepkoski curve revealed a number of interesting
patterns in the history of animal diversity
“Paleozoic Plateau”
Meso-Cenozoic
Diversification
Ordovician
Radiation
Cambrian
Explosion
Coherent Patterns?
•
Sepkoski wanted to analyze
fossil record for patterns
•
Are there groups of taxa with
coherent diversity trends?
Time Intervals
# Families per class
Classes
A
B
C
1
6
1
3
2
7
2
5
3
4
5
6
4
2
8
2
Used factor analysis to extract
variables that explain variation in
original data set
Factors are termed “evolutionary faunas”
Primarily controlled by changes in the diversity of four groups
Brachiopods w /
hinged shells
Evolutionary Faunas
• Sepkoski grouped clades
into “evolutionary faunas”
based on the times
during which they were
most diverse
• EFs are not coherent
biological units, but rather
are composed of different
taxa that happen to share
similar diversity patterns
Bias and Diversity Patterns
•
How closely does this curve actually match the true
history of Phanerozoic biodiversity?
What if fossil
preservation or the extent
of sedimentary rocks was
lower in the past?
Rock Volume Bias
•
There is a broad correspondence between diversity
and the exposed area of sedimentary rocks
More
sampling =
more taxa!
Rock Volume Bias
•
Changes in diversity correlate strongly with changes in
the number of named geological formations (in the USA)
# Formations
Diversity
“Pull of the Recent”
Because the living fauna is so well known, the ranges of
many taxa are artificially “pulled” to the Recent
Diversity
•
Diversity is inflated in
younger time intervals
Time
Standardization with the Paleobiology Database
www.paleodb.org
Standardization with the Paleobiology
Database
1. Sampling standardization accounts for variable rock area
2. Excluding unlithified sediments (Neogene) compensates for
“Pull of the Recent”
Cenozoic only slight
No “Paleozoic Plateau”
more diverse than
Paleozoic
Mass Extinctions
•
Raup, Sepkoski, and others realized that extinction and origination
rates could be calculated from diversity compilations
•
Five major mass extinctions have been recognized: Late Ordovician,
Late Devonian, End-Permian, End-Triassic, End-Cretaceous
Ecological Effects
• In addition to reduced diversity and taxonomic losses, mass
extinctions alter community and/or ecosystem structure
Ecological impacts not always predictable from taxonomic severity
Tempo of Extinction
• Were mass extinctions
gradual or abrupt?
• Signor-Lipps Effect
Because of the imperfect
fossil record we never
find the true last
occurrence of a
species
Extinctions are
“smeared” backwards
and appear to be more
gradual than they were
K-Pg ammonite ranges with
confidence intervals – abrupt
extinction
Abrupt Extinction – End Cretaceous
Abrupt Extinction – End Permian
• Timing: Very abrupt (<<100 kyr) single event just before
P/T boundary
Gradual Extinction – Late Devonian
• Timing: At least three major events in the Givetian,
Frasnian, and Famennian stages
Hangenberg Event
Kellwasser Event
Taghanic Event
Kellwasser event was the
most severe and virtually
eliminated stromatoporoid
reef ecosystems
Post-Extinction Recovery
• Biotic recovery after extinction can be divided into
“survival” and “recovery” phases
Survival phase:
1) Possible “dead zone”
2) Early crisis interval with extinction
survivors and “disaster” taxa
3) Later interval with opportunists +
some “Lazarus” taxa
Recovery phase:
1) Reappearance of many Lazarus
taxa
2) More rapid diversity increase,
radiation of new lineages
Disaster Taxa
Specific type of opportunistic organisms that expand to become
abundant in a wider range of habitats after a biotic crisis
Early Triassic disaster taxa:
Microbes
Lingulid
Brachiopods
Post-Extinction Recovery
Post-extinction
communities often exhibit
signs of stress
1. Fewer individuals,
dominance by
opportunists or “disaster
taxa”
2. Increase in generalists
(deposit feeders, etc.)
3. Reduction in body size
Typically lasts <500 ka
Delayed Early Triassic Recovery
4-5 Myr long
Reduced
bioturbation and
tiering, plus
disaster taxa
and opportunists
Causes of Extinctions
• End-Cretaceous: Largest impact of the Phanerozoic
(180 km-diameter Chicxulub crater)
• Contributions from environmental stress from Deccan
trap volcanism?
Impacts and Extinctions
• “Kill curve” to predict
impact-caused extinction
• But Chicxulub is only
impact associated with
extinction
Manicouagan, 214 Ma
Flood Basalt Volcanism
• Mass extinctions occurred during times of major flood
basalt eruptions
Flood Basalt Volcanism
• Three largest flood basalts associated with three largest
extinctions, but no correlation beyond that level
• CAMP was probably larger than Siberian Traps, but P/T
extinction is much more severe than T/J
Siberian Traps – P/T extinction
CAMP – T/J extinction
Importance of Target Rock
• Chicxulub hit carbonate platform with abundant sulfates
• Siberian traps erupted into West Siberian high-sulfur coal basin
 Additional CO2 , SO2 (+other toxic chemicals) more important
than actual impact or volcanism
• Volcanism (effects of reducing molecules like SO2 ) leads to
anoxia + euxinia (H2 S) in surface ocean, ocean acidification
• Devonian extinctions also
coincide with marine anoxia
• May actually be triggered by
land plant evolution, not by flood
basalt volcanism
• Climate cooling also
hypothesized for Devonian
crises (Hangenberg Event)
Kellwasser Event: Frasnian
Taghanic Event: Givetian
Late Ordovician – Glaciation
• Timing: Two short-lived events in the latest Ordovician
• Probable Cause: Climate
change, short-lived
Hirnantian glaciation
• Contrasts with extreme
climate warming, caused
by flood basalt eruptions,
during T/J and probably
P/T extinctions
“Hirnantia fauna”
Extinction Causes
Extinction
Climate
Change?
Late
Ordovician
COOLING /
WARMING
Late
Devonian
COOLING
YES
?
End Permian
WARMING
YES
YES
End Triassic
WARMING
YES
YES
End
Cretaceous
Anoxia?
Large Igneous
Province?
Impact?
?
Yes?
YES
Diversity / Extinction Summary
1. Cenozoic diversity is greater than in the Paleozoic, but
not as much as previously thought
2. Diversity compilations allow recognition of five major
mass extinctions and many smaller extinction events
3. The “Big Five” had differing taxonomic and ecological
consequences (but why is not clear)
4. Only the end Cretaceous was associated with an impact,
but the correlations between anoxia and extinctions, and
large igneous provinces and extinctions, are strong