Lecture 6 Extinctions through geologic time Extinction: causes and processes - Extinct: no member of the species remains alive anywhere in the world (dodo, passenger pigeon, Tasmanian 'tiger') - Extinct in the wild: individuals remain alive but only in captivity (Franklin tree, African black rhino) - Extirpated - locally or regionally extinct: species is no longer found in an area it once inhabited, but still is found elsewhere in the wild (beaver and bison in Britain) Quaternary : 0.01 Tertiary 65 Cretaceous Jurassic 180 Triassic 250 Permian Carboniferous 345 Devonian Silurian Ordovician 500 Cambrian Extinction Pleistocene: large mammals and birds Extinction Cretaceous: Dinosaurs, foraminifera, mollusks Extinction Extinction Triassic: 35% of animal families: many reptiles and marine mollusks Permian: 50% of animal families: >95% of marine species, amphibians, most byozoans & brachiopods, trilobites Extinction Devonian: 30% of animal families: agnathan and placoderm fishes, many trilobites Extinction Ordovician: 50% of animal families: many trilobites Bar width represents relative number of living groups - Ecologically extinct: species persists in such small numbers that its effects on other species in community are negligible (tiger, Asian rhino) Extinction events: possible causes Past Mass Extinction events: Quaternary . 0.01 Tertiary 65 - Species diversity generally increased with time since life began - History of marine life easier to trace than terrestrial life, because fossils preserve better in anoxic, water-lain sediments Cretaceous Jurassic 180 Triassic 250 Permian - Marine life began ca 600 mya Carboniferous 345 Devonian Silurian Ordovician Cambrian 1-500 - New groups of marine animals arose for next 150 mya, then 1st major extinction event Extinction Pleistocene: climate change or human predation Extinction Cretaceous: Asteroid impact Extinction Triassic: Climate change or mid-Atlantic volcanic eruptions Extinction Permian: Climate change, plate tectonics, or Siberian volcanic eruptions Extinction Devonian: Climate change, plate tectonics, asteroid impact, or the 'Devonian Plant Hypothesis" Extinction Ordovician: Global cooling or oxygen depletion in ocean waters Bar width represents relative number of living groups Devonian Plant Hypothesis - Proposed by Algeo et al. (1995) - Devonian marine deposits have black shales from shallow inland seas: indicate anoxic bottom waters - time indicating a thin spread of vegetation - Time of major spread of terrestrial vegetation Permian Extinction Event Rise of terrestrial plant cover Increased organic input causes eutrophication of waters ! Decrease in oxygen levels in waters ! Decline in diversity of aquatic life Accelerated soil formation on land Accelerated silicate weathering ! Formation of calcium and magnesium carbonates Removal of CO2 from atmosphere - Took place 250 mya - 77-96% of all marine animal species died out - Half of all animal families died out - Earth's biota took 50 million years to recover same level of diversity Global cooling
Possible causes of this event: - (1) glaciation on Gondwana brought global cooling and lowering of sea level - (2) reduction of shallow continental shelves due to formation of Pangea (most oceanic biological diversity centred in shallow waters) - (3) Global environmental change: glaciation at both poles, aridity in temperate zones - (4)