Fossils and Evolution 870:125

1. Fossils and Evolution870:125 • Review syllabus • Text • Supplemental resources • Objectives • Tests and grading • Trip to Ashfall (NE) Fossils & Evolution—Chapter 1

2. Ch. 1—Key concepts to know • The fossil record is incomplete. Biological, mechanical and diagenetic agents all destroy potential fossils. • Normal fossil preservation is favored by rapid burial of durable hard parts. • Exceptional preservation occurs via rapid burial in fine-grained sediments under low oxygen conditions. • The fossil record is biased because of differential preservation and uneven sampling. It is biased in favor of: 1) durably skeletonized organisms; 2) marine organisms; 3) geologically recent organisms • Because of biases, knowledge of past life is far better at higher taxonomic levels than at lower taxonomic levels Fossils & Evolution—Chapter 1

3. Key terms(know definitions) • Taphonomy • Biocenosis • Thanatocenosis • Necrolysis • Biostratinomy • Diagenesis • Lagerstätten Fossils & Evolution—Chapter 1

4. Why study fossils? • Fossil = remains or traces of a once-living organism • Paleontology = the study of fossils • Importance of paleontology • Biostratigraphy (age dating of rocks) • Evolution • Paleoecology/paleoenvironmental interpretation • Paleogeography/paleobiogeography • Simple fascination Fossils & Evolution—Chapter 1

5. Modes of preservation • Unaltered remains (frozen mammoths; insects in amber; unaltered shells & bones) • Permineralization (infilling of void spaces) • Replacement (molecule by molecule substitution) • Impressions • Carbonization • Molds / casts Fossils & Evolution—Chapter 1

6. Eocene mammal with partially preserved fur and flesh (Germany) Carbonized Jurassic leaf Fossils & Evolution—Chapter 1

7. Taphonomy = science of fossilization • Many steps in the process of fossilization, with significant removal of specimens at every step • Once fossilized, the odds of being collected are low (uplift and exposure; weathering; discovery; chance, etc.) Life assemblage (biocenosis) Death assemblage (thanatocenosis) Necrolysis (scavenging, decay) Biostratinomy (break-up, scattering and shallow burial of remains) Diagenesis (deep burial, recrystallization, dissolution, metamorphism, etc.) Initial fossil assemblage Final fossil assemblage Fossils & Evolution—Chapter 1

8. Life assemblage (biocenosis) Destruction of most soft tissues Death assemblage (thanatocenosis) Destruction of most hard tissues Total fossil assemblage Destruction of most fossils Fossils actually discovered Fossils & Evolution—Chapter 1

9. Quality of the fossil record • The fossil record is highly biased • Number of fossils is but a fraction of the number of once-living plants and animals Fossils & Evolution—Chapter 1

10. Fossilization is a rare event! • Some estimates: • > 4,500,000 living species of plants and animals • 250,000 described fossil species • Thus, all described fossil species represent < 5% of the total number of living species • Yet, fossil record covers billions of years and today’s biota is but a snapshot • If preservation were truly efficient, then number of fossil species should dwarf number of extant species Fossils & Evolution—Chapter 1

11. Standing crop in ¼ m2(offshore Japan) ½ m • 197 shells (~ 200) • Average lifespan = 2 years, thus:……… • 1000 empty shells in 10 years • 100,000,000 empty shells in 1 m.y. • A stack of shells 1000 m high if a layer of 1000 shells is 1 cm thick (actual sedimentary thickness is ~320 m/m.y.) ½ m Fossils & Evolution—Chapter 1

12. Sources of bias • Uneven preservation potential • Sampling bias Fossils & Evolution—Chapter 1

13. Uneven preservation potential • Preservation potential of organisms is goverened by • Resistance to destruction • Biological, mechanical, chemical • Hard parts vs. soft parts • Habitat (during life) Fossils & Evolution—Chapter 1

14. Destruction • Biologic destruction includes • Predation • Scavenging • Boring • Bacterial decay Example: Radiograph of heavily bored gastropod Fossils & Evolution—Chapter 1

15. Destruction • Mechanical destruction includes breakage and abrasion due to particle interactions caused by wind, waves, currents • Some shells and bones are more resistant to abrasion and breakage than others • Different sizes of the same shells vary in their resistance to abrasion and breakage Fossils & Evolution—Chapter 1

16. Abrasion experiment (multitaxa) gastropod coral alga coral gastropod Fossils & Evolution—Chapter 1

17. Abrasion experiment (marine bivalves) large shells small shells Fossils & Evolution—Chapter 1

18. Durability of vertebrate bones • Durability is governed by bone density and thickness; also by surface area-to-volume ratio: • Least durable • Ribs, vertebrae, breastbone, hip (part), shoulder blade, fingers, toes • Intermediate • Thigh, shin, upper and lower arms, ankles and wrists, hip (part) • Most durable • Teeth, jaws, skull Fossils & Evolution—Chapter 1

19. Destruction • Chemical destruction varies with: • the original skeletal mineralogy of a fossil • the chemistry of subsurface fluids • temperature of burial environment Fossils & Evolution—Chapter 1

20. Relative chemical stability Fossils & Evolution—Chapter 1

21. Destruction • Chemical stability vs. temperature and pressure • Silica is more stable in cold water • Carbonate is more stable in warm water and under low pressures • Dissolution occurs under high pressure and low temperature conditions Fossils & Evolution—Chapter 1

22. Distribution of modern deep sea pelagic sediments Fossils & Evolution—Chapter 1

23. Resistance to destruction • Hard parts are much more likely to be preserved than soft parts (but soft parts and even pigments can be preserved) Fossils & Evolution—Chapter 1

24. Environment and preservability • Best preservation generally occurs in calm, aquatic environments • Exceptional preservation occurs in fine-grained sediments in the absence of oxygen, (“biologically inert” burial conditions) Fossils & Evolution—Chapter 1

25. Environment and preservability • Lagerstätten (“Mother lode”) = deposits that contain large numbers of unusually well preserved fossils • Burgess Shale (Cambrian, Canada) • Hunsrück Shale (Devonian, Germany) • Mazon Creek Shale (Pennsylvanian, Illinois) • Solnhofen Limestone (Jurassic, Germany) • Baltic amber (Oligocene, Germany) • La Brea tar deposits (Pleistocene, California) Fossils & Evolution—Chapter 1

26. Lagerstätten(Hunsrück Shale, Devonian of Germany) Fossils & Evolution—Chapter 1

27. Lagerstätten(Solnhofen Limestone, Germany) Fossils & Evolution—Chapter 1

28. Lagerstätten(Burgess Shale, Cambrian of Alberta) Fossils & Evolution—Chapter 1

29. Tully monster(Mazon Creek Shale,Pennsylvanian of Illinois) Check out U-Haul website http://www.uhaul.com/supergraphics/tully/the_graphic.html Fossils & Evolution—Chapter 1

30. Sampling bias • Fossil record is best in most recent geologic systems • Younger rocks are less likely to be covered or obscured by other rocks • Younger rocks are less likely to have been eroded, metamorphosed or subducted Fossils & Evolution—Chapter 1

31. Fossil species diversity vs sediment volume/exposure Fossils & Evolution—Chapter 1

32. Consequences of preservation and sampling bias • Knowledge of past life is far better at higher taxonomic levels than at lower taxonomic levels • In a given sample, you’d only need to look at a small number of specimens to find all of the phyla present, but you’d have to look at a lot of specimens to find all of the species present! Fossils & Evolution—Chapter 1

33. Sampling bias:Danish Miocene mollusks Phyla 1 Classes 3 Orders 12 Families 44 Genera 64 Species 86 Individual shells 2,954 Fossils & Evolution—Chapter 1

34. Sampling bias:Danish Miocene mollusks • If sample size were larger, then more species and possibly more genera might have been found, but probably no more classes or phyla • If sample size were smaller, then fewer genera and species would have been found, but probably no fewer classes or phyla Fossils & Evolution—Chapter 1

35. Rarefaction curve[How many taxa would have been found had the sample been smaller?] Fossils & Evolution—Chapter 1

36. Conclusions • Every assemblage of fossils represents an extremely biased sample of the organisms once living in an area • Lack of fossils in a rock cannot be taken to mean that organisms were not living in the area • “Absence of evidence is not evidence of absence” Fossils & Evolution—Chapter 1