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Mass Extinctions
Geology 331Paleontology
The Phanerozoic Eons are divided by Mass Extinctions
Permian
K-T
How have physical changes on the earth effected the evolutionary history of life during the Phanerozoic?
Types of Extinction• Background Extinction – when species go
extinct through natural selection during gradual environmental change or competition between species
• Mass Extinction – when large numbers of species go extinct together due to very unusual, environmentally catastrophic conditions. Usually unrelated to their normal adaptations to their environment. Is this natural selection at work?
Victims
• Ordovician – over 50% of marine genera spread over about 100 families from several different groups
• Devonian – about 40% of marine genera including all stromatoporoids, all shallow water corals (new families evolved later), most trilobites
www.trilobites.info/biostratigraphy.htm
Devonian mass extinction of 4 trilobite orders
Phacopids: Phacops rana
Phacops weathering out of the outcrop
Rugose Corals
Victims• Permian – about 50% of families, 84% of
genera, 95% of species (estimated)– All fusulinid forams– Most bryozoans– Most brachiopods– All rugose and tabulate corals– All remaining trilobites– Nearly all crinoids– Nearly all cephalopodsOn Land:– 80% of amphibians and reptiles
http://www.evolution-textbook.org/content/free/figures/10_EVOW_Art/37_EVOW_CH10.jpg
Near extinction of crinoids at the end of the Permian
(Berner, 2007)
Oxygen and CO2 levels during the Phanerozoic, based on geochemical proxies
PAL = Present Atmospheric Level
R = ratio
Did higher oxygen levels during the Carboniferous allow the growth of giant insects? How would a drop in oxygen effect life?
Large Carboniferous Cockroaches
1 cm
Causes of Permian Mass Extinction?
• Hypoxia in atmosphere• Anoxia in oceans led to H2S buildup reaching
into shallow water and released to atmosphere.• Oxidation of coal and hydrocarbons by extensive
erosion of sedimentary rocks, and/or massive volcanic eruptions in Siberia?
CH2O + O2 CO2 + H2O• Release of methane hydrates from continental
shelves?CH4 + 2O2 CO2 + 2H2O
• Caused by a drop in sea level exposing the shelves?
Hallam Curve
Exxon Curve
Phanerozoic Sea LevelsSe
a L
evel
Falling Sea Level
Comparison of atmospheric and sea level curves (with time running to the right).
Sea Level Change: Fall in sea level towards the top, rise in sea level towards the bottom.
Victims
• Triassic – about 48% of marine genera– All conodonts– Many brachiopods– Most cephalopodsOn Land:– All synapsid reptiles (but not their
descendants, the mammals)– Many archaic reptile groups
Did the hypoxia in the Early Mesozoic favor the later adaptive radiation of dinosaurs and birds
because of their greater lung capacities?
Huey and Ward, 2005, Science
Present-day altitude equivalent of partial pressure of oxygen at sea level during the Phanerozoic.
An 8 km change is the equivalent of 25,000 ft. Change from high to very low oxygen levels. (=Death Zone on Mt. Everest)
Victims• Cretaceous – over 40% of marine genera
– All rudist bivalves– All ammonoids and belemnites– Most forams and coccoliths– Marine reptiles: ichthyosaurs, plesiosaurs,
mososaursOn Land:– Pterosaurs– Non-avian dinosaurs– Many mammals
Cretaceous Rudist Bivalves
Mesozoic Ammonites
Mesozoic Belemnites
Cretaceous Forams
Coccoliths surrounding a diatom
Coccoliths are plates on a spheroidal algae
On to the end Cretaceous event, a.k.a., the K/T, or K/P, extinction
event
