lecture 17 - diversity and extinctions (comp)pkoch/eart_110a... · abrupt extinction – end...
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Phanerozoic Diversity and MassExtinctions
• John Phillips producedthe first estimates ofPhanerozoic diversity in1860, based on the Britishfossil record
• Intuitively it seems simpleto count the number oftaxa, but there areactually quite a fewcomplications
Measuring Diversity
Phillips, 1860
Measuring Diversity
• Diversity was revisited with a more quantitative approachin the late 1970s and 1980s
• The number of described species had increased ~25times, providing much more data for analysis
Newanalyses
Measuring Diversity
• Jack Sepkoski spent 15 yearscompiling a database toproduce the classic diversitycurve
Phanerozoic Diversity
OrdovicianRadiation
CambrianExplosion
“Paleozoic Plateau”Meso-CenozoicDiversification
• The Sepkoski curve revealed a number of interestingpatterns in the history of animal diversity
Coherent Patterns?
• Sepkoski wanted to analyzefossil record for patterns
• Are there groups of taxa withcoherent diversity trends?
A B C
1 6 1 3
2 7 2 5
3 4 5 6
4 2 8 2
Classes
Tim
e In
terv
als
Used factor analysis to extractvariables that explain variation inoriginal data set
# Families per class
Factors are termed “evolutionary faunas”
Primarily controlled by changes in the diversity of four groups
Brachiopods w /hinged shells
Evolutionary Faunas
• Sepkoski grouped cladesinto “evolutionary faunas”based on the timesduring which they weremost diverse
• EFs are not coherentbiological units, but ratherare composed of differenttaxa that happen to sharesimilar diversity patterns
Bias and Diversity Patterns
• How closely does this curve actually match the truehistory of Phanerozoic biodiversity?
What if fossilpreservation or the extentof sedimentary rocks waslower in the past?
Rock Volume Bias
• There is a broad correspondence between diversityand the exposed area of sedimentary rocks
Moresampling =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 ofmany taxa are artificially “pulled” to the Recent
Time
Div
ersi
ty
Diversity is inflated inyounger time intervals
Standardization with the Paleobiology Databasewww.paleodb.org
Standardization with the PaleobiologyDatabase
1. Sampling standardization accounts for variable rock area
2. Excluding unlithified sediments (Neogene) compensates for“Pull of the Recent”
No “Paleozoic Plateau”
Cenozoic only slightmore diverse thanPaleozoic
Mass Extinctions
• Raup, Sepkoski, and others realized that extinction and originationrates 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, massextinctions alter community and/or ecosystem structure
Ecological impacts not always predictable from taxonomic severity
Tempo of Extinction
• Were mass extinctionsgradual or abrupt?
• Signor-Lipps Effect
Because of the imperfectfossil record we neverfind the true lastoccurrence of aspecies
Extinctions are“smeared” backwardsand appear to be moregradual than they were
K-Pg ammonite ranges withconfidence intervals – abruptextinction
Abrupt Extinction – End Cretaceous
Abrupt Extinction – End Permian
• Timing: Very abrupt (<<100 kyr) single event just beforeP/T boundary
Gradual Extinction – Late Devonian
• Timing: At least three major events in the Givetian,Frasnian, and Famennian stages
Taghanic Event
Kellwasser Event
Hangenberg Event
Kellwasser event was themost severe and virtuallyeliminated stromatoporoidreef ecosystems
Post-Extinction Recovery• Biotic recovery after extinction can be divided into
“survival” and “recovery” phasesSurvival phase:
1) Possible “dead zone”2) Early crisis interval with extinctionsurvivors and “disaster” taxa3) Later interval with opportunists +some “Lazarus” taxa
Recovery phase:
1) Reappearance of many Lazarustaxa2) More rapid diversity increase,radiation of new lineages
LingulidBrachiopods
Microbes
Specific type of opportunistic organisms that expand to becomeabundant in a wider range of habitats after a biotic crisis
Disaster Taxa
Early Triassic disaster taxa:
Post-extinctioncommunities often exhibitsigns of stress
1. Fewer individuals,dominance byopportunists or “disastertaxa”
2. Increase in generalists(deposit feeders, etc.)
3. Reduction in body size
Post-Extinction Recovery
Typically lasts <500 ka
Delayed Early Triassic Recovery
4-5 Myr long
Reducedbioturbation andtiering, plusdisaster taxaand opportunists
Causes of Extinctions
• End-Cretaceous: Largest impact of the Phanerozoic(180 km-diameter Chicxulub crater)
• Contributions from environmental stress from Deccantrap volcanism?
Impacts and Extinctions• “Kill curve” to predict
impact-caused extinction
• But Chicxulub is onlyimpact associated withextinction
Manicouagan, 214 Ma
• Mass extinctions occurred during times of major floodbasalt eruptions
Flood Basalt Volcanism
• Three largest flood basalts associated with three largestextinctions, but no correlation beyond that level
• CAMP was probably larger than Siberian Traps, but P/Textinction is much more severe than T/J
Flood Basalt Volcanism
Siberian Traps – P/T extinction
CAMP – T/J extinction
• Chicxulub hit carbonate platform with abundant sulfates
• Siberian traps erupted into West Siberian high-sulfur coal basin
Importance of Target Rock
Additional CO2, SO2 (+other toxic chemicals) more importantthan actual impact or volcanism
• Volcanism (effects of reducing molecules like SO2) leads toanoxia + euxinia (H2S) in surface ocean, ocean acidification
• Devonian extinctions alsocoincide with marine anoxia
• May actually be triggered byland plant evolution, not by floodbasalt volcanism
• Climate cooling alsohypothesized for Devoniancrises (Hangenberg Event)
Taghanic Event: Givetian
Kellwasser Event: Frasnian
Late Ordovician – Glaciation
• Timing: Two short-lived events in the latest Ordovician
“Hirnantia fauna”
• Probable Cause: Climatechange, short-livedHirnantian glaciation
• Contrasts with extremeclimate warming, causedby flood basalt eruptions,during T/J and probablyP/T extinctions
Extinction Causes
Extinction ClimateChange? Anoxia? Large Igneous
Province? Impact?
LateOrdovician
COOLING /WARMING
?
LateDevonian
COOLING YES ?
End Permian WARMING YES YES
End Triassic WARMING YES YES
EndCretaceous Yes? YES
Diversity / Extinction Summary
1. Cenozoic diversity is greater than in the Paleozoic, butnot as much as previously thought
2. Diversity compilations allow recognition of five majormass extinctions and many smaller extinction events
3. The “Big Five” had differing taxonomic and ecologicalconsequences (but why is not clear)
4. Only the end Cretaceous was associated with an impact,but the correlations between anoxia and extinctions, andlarge igneous provinces and extinctions, are strong