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Paintings by Charles Knight Fossils & Evolution—Chapter 6 1 Fossils & Evolution—Chapter 6 2 Fossils & Evolution—Chapter 6 3 Fossils & Evolution—Chapter 6 4 Fossils & Evolution—Chapter 6 5 Fossils & Evolution—Chapter 6 6 Fossils & Evolution—Chapter 6 7 Fossils & Evolution—Chapter 6 8 Fossils & Evolution—Chapter 6 9 Late Cretaceous 85 million years ago Fossils & Evolution—Chapter 6 10 Late Cretaceous 75 million years ago Fossils & Evolution—Chapter 6 11 end-Cretaceous 65 million years ago Hell Creek Formation (coastal plain setting) Fossils & Evolution—Chapter 6 12 Fossils & Evolution—Chapter 6 13 Chicxulub crater Earth History, Ch. 17 Impact trajectory 14 Radar image of Chicxulub crater Fossils & Evolution—Chapter 6 15 Chicxulub crater Gravity survey data Fossils & Evolution—Chapter 6 16 Iridium layer at Gubbio, Italy Fossils & Evolution—Chapter 6 17 Iridium layer near Drumheller (southern Alberta, Canada) Earth History, Ch. 17 18 Chapter 6—Key concepts • 99.9% of all organisms that have ever lived are now extinct. “Background” extinction occurs when a species cannot adapt to a change in its environment. • Mass extinctions are episodes when the extinction rate far exceeds the normal background rate. Mass extinctions do not occur at predictable intervals, and each probably was caused by a unique set of circumstances. Fossils & Evolution—Chapter 6 19 Ch. 6—Key terms • Ecologic limiting factors • Signor-Lipps effect • Pulse vs. Press extinction Fossils & Evolution—Chapter 6 20 Extinction! Fossils & Evolution—Chapter 6 21 Chapter 6—Extinction • Two categories of extinction: – Normal (or background) extinction – Mass extinction (dramatically accelerated) Fossils & Evolution—Chapter 6 22 Rates of extinction • Agents of extinction are changes in ecologic limiting factors • Also, population size, number of populations, and geographic range of populations affect the probability of extinction Fossils & Evolution—Chapter 6 23 Limiting factors • Ecologic limiting factors = physical, chemical and biologic properties of the environment that limit the distribution and abundance of a particular species – Temperature – Oxygen – Depth-related variables • Light, pressure, water chemistry, etc. – – – – Salinity Substratum (nature of the seafloor) Food Other biota (competitors, predators, infectious diseases) Fossils & Evolution—Chapter 6 24 Rates of extinction • Probability of extinction vs. No. of populations – Suppose that, in a given interval of time, every population has a 50% chance of becoming extinct – Species with large numbers of populations are unlikely to suffer total extinction Fossils & Evolution—Chapter 6 25 0.5 probability of total extinction 0.45 0.4 0.35 0.3 0.25 0.2 0.15 0.1 0.05 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 number of populations Fossils & Evolution—Chapter 6 26 Probability of background extinction “overspecialization” model or probability of extinction probability of extinction • Are species that have been around a long time more or less resistant to extinction than newly formed species? “resistence” model duration of species existence duration of species existence Fossils & Evolution—Chapter 6 27 Overspecialization model Resistence model Most genera do not survive very long. Importantly, though, the probability of survival does not increase or decrease with a taxon’s longevity. Fossils & Evolution—Chapter 6 28 Mass extinctions Sepkoski (1982) Fossils & Evolution—Chapter 6 29 Mass extinctions • Causes are poorly understood – – – – Global climate change Volcanism Asteroid impact Environmental deterioration • CO2 & methane poisoning • Anoxia Fossils & Evolution—Chapter 6 30 Mass extinctions: 26 Ma periodicity suggests astronomical cause? Fossils & Evolution—Chapter 6 31 Causes of mass extinctions • Causes are extremely difficult to determine • Timing is key to causal analysis – “Press” (gradual) vs. “pulse” (abrupt) extinction • Types of organisms affected is also key to causal analysis – Marine only vs. terrestrial and marine – Physiologic selectivity • e.g., filter feeders only, etc. Fossils & Evolution—Chapter 6 32 Signor-Lipps effect • Consider two species, one rarely preserved (occurs in 10% of samples) and the other commonly preserved (occurs in 80% of samples) • Assume that both became extinct at “extinction level” • Where are we likely to find their highest observed occurrence? actual extinction level 2m Fossils & Evolution—Chapter 6 33 Signor-Lipps effect • Mass extinctions appear gradual when last observed occurrences of taxa are plotted on stratigraphic sections suspected extinction level 2m Fossils & Evolution—Chapter 6 34 False “gradualness” of mass extinctions • Probability of finding abundantly occurring taxa in a given sample is much greater than probability of finding rare taxa • Most taxa whose last observed occurrence is some distance below an extinction horizon are rare taxa Fossils & Evolution—Chapter 6 35 distance of highest observed occurrence below extinction level False “gradualness” of mass extinctions Abundance (% of samples in which each species occurs) Highest occurrence of common species Likely will be at or near extinction level “hollow” distribution curve is consistent with Signor-Lipps effect Highest occurrence of rare species might be at extinction level or much lower Fossils & Evolution—Chapter 6 36 Taskent Section, southern Turkey JT-12 JT-11 JT-10 JT-9 JT-8 approx. 8.69m above base of section 7m JT-7 JT-18 14 cm stromatolitic ls. P-T boundary 6m crinoidal grainstn JT-6 5m 7 cm oolitic ls. JT-17 possible intraclasts JT-16 22 cm oolitic ls. JT-5 JT-4 4m 3m JT-15 8 cm oolitic ls. algal wackestn JT-3 2m 16 cm oolitic ls. JT-14 JT-2 1m JT-13 4 cm packstn dark grey to black wackestn (8 beds in 1 m) approx. 7.98m above base of section JT-1 Fossils & Evolution—Chapter 6 37 18 meters above base of section 16 14 12 10 8 6 4 2 0 0 2 4 6 8 10 12 14 16 18 species diversity Fossils & Evolution—Chapter 6 38 stratigraphic abundance 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 0 last occurrence below P-T boundary 1 2 3 4 5 6 7 8 9 Fossils & Evolution—Chapter 6 39