Mass ExtinctionsMass ExtinctionsTim BradshawTim Bradshaw

December 7, 2005December 7, 2005

ExtinctionExtinction

““ceasing of existence of a species or ceasing of existence of a species or group of taxa” group of taxa”

Classic cause: failure to adapt to Classic cause: failure to adapt to changes in environmentchanges in environment

How do you define a species?How do you define a species?

Defining species Defining species Biological species concept – species are groups of Biological species concept – species are groups of

actually/potentially interbreeding natural actually/potentially interbreeding natural populations reproductively isolated from other populations reproductively isolated from other such groups (Mayr 1942)such groups (Mayr 1942)

Evolutionary species concept – a species is a Evolutionary species concept – a species is a single lineage of populations that maintains an single lineage of populations that maintains an identity separate from other such lineages and identity separate from other such lineages and has its own evolutionary tendencies (Wiley 1978)has its own evolutionary tendencies (Wiley 1978)

Phylogenetic species concept – a species is an Phylogenetic species concept – a species is an irreducible cluster of organisms that is irreducible cluster of organisms that is diagnosably distinct from other such clusters, and diagnosably distinct from other such clusters, and within which there is a parental pattern of within which there is a parental pattern of ancestry and descent (Cracraft 1989)ancestry and descent (Cracraft 1989)

Species LifespanSpecies Lifespan

Mammals ~ 1million year lifespanMammals ~ 1million year lifespan 5000 currently alive5000 currently alive Background rate ~ 1 species/200 Background rate ~ 1 species/200

yearsyears 89 species extinct in last 400 years89 species extinct in last 400 years

Geologic Time ScaleGeologic Time Scale EraEra Time (mya)Time (mya) PeriodPeriod 1.8 – 0.011.8 – 0.01 QuaternaryQuaternary CenozoicCenozoic 65 – 1.865 – 1.8 TertiaryTertiary 144 – 65144 – 65 CretaceousCretaceous MesozoicMesozoic 206 – 144206 – 144 JurassicJurassic 248 – 206248 – 206 TriassicTriassic 290 – 248290 – 248 PermianPermian 354 – 290354 – 290 CarboniferousCarboniferous PaleozoicPaleozoic 417 – 354417 – 354 DevonianDevonian 443 – 417443 – 417 SilurianSilurian 490 – 443490 – 443 OrdovicianOrdovician 543 – 490543 – 490 CambrianCambrian

OrdovicianOrdovicianEchinoderm TimeEchinoderm Time

490 – 443 mya490 – 443 mya

great diversification of phyla, including 21 great diversification of phyla, including 21 classes of echinodermsclasses of echinoderms

formation of reefsformation of reefs

evolution of agnathans and trilobitesevolution of agnathans and trilobites

plants invade landplants invade land

Gondwanaland centered in southern Gondwanaland centered in southern hemispherehemisphere

Gondwanaland (~ 400 mya)Gondwanaland (~ 400 mya)

End-Ordovician ExtinctionEnd-Ordovician Extinction

57% of marine genera wiped out57% of marine genera wiped out• Trilobites take a dirt napTrilobites take a dirt nap

Probable cause: Global coolingProbable cause: Global cooling Glaciation over Gondwanaland led to Glaciation over Gondwanaland led to

cooling and drop in sea levelscooling and drop in sea levels Time span: 2 myTime span: 2 my Result: Paved the way Result: Paved the way

for ostracodermsfor ostracoderms

DevonianDevonianConquest of LandConquest of Land

417 – 354 mya417 – 354 mya

Osteichthyes develop (Sarcopterygii Osteichthyes develop (Sarcopterygii and Actinopterygii)and Actinopterygii)

Terrestrial arthropods evolveTerrestrial arthropods evolve

Land plants diversify (ferns, horsetails)Land plants diversify (ferns, horsetails)

Ichthyostegid amphibians evolve from Ichthyostegid amphibians evolve from rhipidistiansrhipidistians

Late-Devonian ExtinctionLate-Devonian Extinction

22% of marine families and 57% of 22% of marine families and 57% of marine generamarine genera• Loss of coral reefs and ostracodermsLoss of coral reefs and ostracoderms

Once again: global cooling from Once again: global cooling from glaciation,glaciation,

Time span: 4 million Time span: 4 million

yearsyears

PermianPermian

First amniotes - captorhinomorphsFirst amniotes - captorhinomorphs Fish/Insects/Reptiles continue to Fish/Insects/Reptiles continue to

diversifydiversify Amphibians declineAmphibians decline Formation of Formation of

PangeaPangea

Permian-Triassic ExtinctionPermian-Triassic Extinction

Mother of Mass ExtinctionsMother of Mass Extinctions Say goodbye to 54% of all marine speciesSay goodbye to 54% of all marine species Lost 75% of all land vertebrate familiesLost 75% of all land vertebrate families Estimated that 84% of all genera on Earth Estimated that 84% of all genera on Earth

became extinctbecame extinct Time span: 1 million years Time span: 1 million years Results: so long frogs, Results: so long frogs,

hello dinoshello dinos

Cause:Cause:

Continental drift created a non-fatal but Continental drift created a non-fatal but precariously balanced global environment, precariously balanced global environment, a supernova weakened the ozone layer, a supernova weakened the ozone layer, and then a large meteor impact triggered and then a large meteor impact triggered the eruption of the Siberian Traps. The the eruption of the Siberian Traps. The resultant global warming eventually was resultant global warming eventually was enough to melt the methane hydrate enough to melt the methane hydrate deposits on continental shelves of the deposits on continental shelves of the world-ocean. world-ocean.

CausesCauses

Plate Tectonics – Pangea formationPlate Tectonics – Pangea formation Supernova – destroy ozone layerSupernova – destroy ozone layer Asteroid impact – trigger volcanoesAsteroid impact – trigger volcanoes Siberian traps – 200,000 cubic Siberian traps – 200,000 cubic

kilometerskilometers Methane hydrate released from Methane hydrate released from

oceansoceans Oxygen depleted from oceansOxygen depleted from oceans

TriassicTriassicAge of the ReptilesAge of the Reptiles

Pangea would soon break up into Pangea would soon break up into Laurasia and GondwanalandLaurasia and Gondwanaland

Marine groups diversifiedMarine groups diversified• Forams, modern corals, bony fishForams, modern corals, bony fish

Gymnosperms dominate on landGymnosperms dominate on land

First dinosaurs and first mammals arriveFirst dinosaurs and first mammals arrive

End-Triassic ExtinctionEnd-Triassic Extinction

Loss of 25% of all familiesLoss of 25% of all families• Decimated sponges, marine reptiles and Decimated sponges, marine reptiles and

conodontsconodonts Cause: largest volcanic event from Cause: largest volcanic event from

Pangea riftPangea rift Results: Paved the way for more Results: Paved the way for more

dinosaurs in more nichesdinosaurs in more niches

CretaceousCretaceousAge of the DinosaursAge of the Dinosaurs

Continents have fully separatedContinents have fully separated Angiosperms have developed and Angiosperms have developed and

taken overtaken over Birds evolve from theropod dinosaursBirds evolve from theropod dinosaurs

==

Cretaceous-Tertiary ExtinctionCretaceous-Tertiary Extinction

Death to the DinosaursDeath to the Dinosaurs 50% of all genera50% of all genera

• Pterosaurs, dinosaurs, sea reptilesPterosaurs, dinosaurs, sea reptiles Cause: Celestial impact reduced Cause: Celestial impact reduced

photosynthesisphotosynthesis Results: Welcome bug eatersResults: Welcome bug eaters

(aka mammals)(aka mammals)

Future ExtinctionsFuture Extinctions

Probable Cause: UsProbable Cause: Us Habitat destruction - estimated that Habitat destruction - estimated that

5-10% of species will be extinct in 5-10% of species will be extinct in thirty yearsthirty years

Global warming – estimated that Global warming – estimated that ~35% of species will become ~35% of species will become committed to extinction in the next committed to extinction in the next fifty yearsfifty years

Evolving past ExtinctionEvolving past Extinction

Possible to become resistant to Possible to become resistant to extinction?extinction?

Probably notProbably not

Natural selection cannot plan ahead Natural selection cannot plan ahead Extinction causes are numerous, thus Extinction causes are numerous, thus

there is no carryover of extinction there is no carryover of extinction resistanceresistance

Just as likely for old taxa to go Just as likely for old taxa to go extinct as for new taxaextinct as for new taxa

Why do we see a decline in the Why do we see a decline in the background extinction rate during background extinction rate during the Phanerozoic (Cambrian – today)?the Phanerozoic (Cambrian – today)?

Two TheoriesTwo Theories

1. average # of species per family 1. average # of species per family has increased over timehas increased over time

(Flessa and Jablonski 1985)(Flessa and Jablonski 1985)

2. Taxa more likely to go extinct went 2. Taxa more likely to go extinct went extinct early in the Phanerozoicextinct early in the Phanerozoic

(Erwin et al. 1987)(Erwin et al. 1987)

Importance of Mass ExtinctionsImportance of Mass Extinctions

Adaptive radiationAdaptive radiation Release from competition, predationRelease from competition, predation