Fossils and Evolution 870:125

Transcription

1 Fossils and Evolution 870:125 Review syllabus Text Supplemental resources Objectives Tests and grading Trip to Ashfall (NE) Fossils & Evolution Chapter 1 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 finegrained 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 2

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

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 4

5 Modes of preservation 1. Unaltered remains (frozen mammoths; insects in amber; unaltered shells & bones) 2. Permineralization (infilling of void spaces) 3. Replacement (molecule by molecule substitution) 4. Impressions 5. Carbonization 6. Molds / casts Fossils & Evolution Chapter 1 5

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

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) Necrolysis (scavenging, decay) Death assemblage (thanatocenosis) Biostratinomy (break-up, scattering and shallow burial of remains) Initial fossil assemblage Diagenesis (deep burial, recrystallization, dissolution, metamorphism, etc.) Final fossil assemblage Fossils & Evolution Chapter 1 7

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 8

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 9

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 < 0.5% 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 10

11 Standing crop in ¼ m 2 (offshore Japan) ½ m 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.) Fossils & Evolution Chapter 1 11

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

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 13

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

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 15

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

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

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 18

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 19

20 Relative chemical stability <-- low stability high stability --> fossil groups radiolarians diatoms sponges calcareous algae calcareous forams mollusks cnidarians tabulates rugosans scleractinians stromatoporoids echinoderms brachiopods bryozoa arthropods conodonts vertebrates graptolites dinoflagellates pollen/spores opaline silica aragonite most hi-mg calcite low-mg calcite oysters???? apatite (CaPO4) organic Fossils & Evolution Chapter 1 20

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 21

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

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 23

24 Environment and preservability Best preservation generally occurs in calm, aquatic environments Exceptional preservation occurs in finegrained sediments in the absence of oxygen, ( biologically inert burial conditions) Fossils & Evolution Chapter 1 24

25 Environment and preservability Lagerstätten ( storage places ) = deposits that contain unusually large numbers of fossils and/or 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 25

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

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

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

29 Tully monster (Mazon Creek Shale, Pennsylvanian of Illinois) Check out U-Haul website Fossils & Evolution Chapter 1 29

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 30

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

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 32

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 33

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 34

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

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 36