1. Fossils & Evolution—Chapter 61 Paintings by Charles Knight

2. Fossils & Evolution—Chapter 62

3. 3

4. 4

5. 5

6. 6

7. 7

8. 8

9. 9

10. 10 Late Cretaceous 85 million years ago

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

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

13. Fossils & Evolution—Chapter 613

14. Earth History, Ch. 1714 Chicxulub crater Impact trajectory

15. Fossils & Evolution—Chapter 615 Radar image of Chicxulub crater

16. Fossils & Evolution—Chapter 616 Chicxulub crater Gravity survey data

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

18. Earth History, Ch. 1718 Iridium layer near Drumheller (southern Alberta, Canada)

19. 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.

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

21. Fossils & Evolution—Chapter 621 Extinction!

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

23. 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

24. 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)

25. 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

26. Fossils & Evolution—Chapter 626

27. 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

28. 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.

29. Fossils & Evolution—Chapter 629 Mass extinctions Sepkoski (1982) background extinction levels

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

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

32. 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.

33. 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

34. 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

35. 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

36. 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

37. 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-15 22 cm oolitic ls. JT-16 7 cm oolitic ls. JT-17 JT-18 P-T boundary Taskent Section, southern Turkey

38. Fossils & Evolution—Chapter 638

39. Fossils & Evolution—Chapter 639