Fossils & Evolution—Chapter 61 Paintings by Charles Knight

Fossils & Evolution—Chapter 62

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Fossils & Evolution—Chapter 611 Late Cretaceous 75 million years ago

Fossils & Evolution—Chapter 612 end-Cretaceous 65 million years ago Hell Creek Formation (coastal plain setting)

Fossils & Evolution—Chapter 613

Earth History, Ch Chicxulub crater Impact trajectory

Fossils & Evolution—Chapter 615 Radar image of Chicxulub crater

Fossils & Evolution—Chapter 616 Chicxulub crater Gravity survey data

Fossils & Evolution—Chapter 617 Iridium layer at Gubbio, Italy

Earth History, Ch Iridium layer near Drumheller (southern Alberta, Canada)

Fossils & Evolution—Chapter 619 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 620 Ch. 6—Key terms Ecologic limiting factors Signor-Lipps effect Pulse vs. Press extinction

Fossils & Evolution—Chapter 621 Extinction!

Fossils & Evolution—Chapter 622 Chapter 6—Extinction Two categories of extinction: –Normal (or background) extinction –Mass extinction (dramatically accelerated)

Fossils & Evolution—Chapter 623 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 624 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 625 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 626

Fossils & Evolution—Chapter 627 Probability of background extinction Are species that have been around a long time more or less resistant to extinction than newly formed species? duration of species existence probability of extinction duration of species existence probability of extinction or “overspecialization” model “resistence” model

Fossils & Evolution—Chapter 628 Overspecialization modelResistence 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 629 Mass extinctions Sepkoski (1982) background extinction levels

Fossils & Evolution—Chapter 630 Mass extinctions Causes are poorly understood –Global climate change –Volcanism –Asteroid impact –Environmental deterioration CO 2 & methane poisoning Anoxia

Fossils & Evolution—Chapter 631 Mass extinctions: 26 Ma periodicity suggests astronomical cause?

Fossils & Evolution—Chapter 632 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 633 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 634 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 635 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 636 False “gradualness” of mass extinctions distance of highest observed occurrence below extinction level “hollow” distribution curve is consistent with Signor-Lipps effect Highest occurrence of rare species might be at extinction level or much lower Abundance (% of samples in which each species occurs) Highest occurrence of common species Likely will be at or near extinction level

Fossils & Evolution—Chapter 637 JT-1 1m 2m 3m 4m 5m 6m 7m dark grey to black wackestn (8 beds in 1 m) JT-2 JT-3 algal wackestn JT-4 JT-5 possible intraclasts JT-6 crinoidal grainstn JT-7 JT-8 JT-9 JT-10 JT-11 JT-12 4 cm packstn 16 cm oolitic ls. 14 cm stromatolitic ls. JT-13 approx. 7.98m above base of section approx. 8.69m above base of section JT-14 8 cm oolitic ls. JT cm oolitic ls. JT-16 7 cm oolitic ls. JT-17 JT-18 P-T boundary Taskent Section, southern Turkey

Fossils & Evolution—Chapter 638

Fossils & Evolution—Chapter 639