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