The Fossil Record and the Geological Timescale

1. The Fossil Record and the Geological Timescale Chapter 12 In your textbook

2. Fossils • The preserved remains of anything that was once living • When an animal or plant dies, its remains usually rot away to nothing. • Sometimes, when the conditions are just right and the remains are covered up quickly, a fossil may form.

3. Paleontology • Paleonotologists = scientists who study fossils • Studying the fossil record helps scientists determine: • How long life has existed on Earth • How different organisms are related to each other • How and where different organisms lived

4. Fossil Formation • Permineralization (aka petrification) = when minerals carried by H2O are deposited around a hard structure, filling every cavity • Natural casts & molds = when water removes the original bone / tissue, leaving an impression behind

5. Fossil Formation, cont’d • Trace fossils = record an organism’s activity; typically form when an organism moves over the surface of soft sediment & leaves behind an impression • Impressions = two-dimensional imprint most commonly found in silt or clay; no organic material present

6. Fossil Formation, cont’d • Amber-preserved fossils = preserved in tree resin that hardens around the organism • Preserved remains = when an entire organism becomes encased in ice or volcanic ash, or immersed in a bog

7. Determining the Age of Fossils • Fossil sequences were recognized and established long before Darwin had even thought of natural selection • Early geologists – in the 1700s and 1800s – noticed how fossils seemed to occur in sequences, such that certain groups of fossils were always found below other groups of fossils

8. Age of Fossils, cont’d • From the 1830s onwards, geologists noted how fossils became more complex through time • The oldest rocks contained no fossils • Then, simple creatures  fish  life on land  reptiles  mammals  humans • Clearly, there was some kind of “progress” happening

9. Relative Dating • This method is conducted by observing fossils and recording which is younger and which is older based upon how deep the fossil is found • Older fossils should be buried deeper than younger fossils

10. Radiometric Dating • The discovery of a way to estimate a fossil’s actual – or absolute – age in the early 1900s was a huge advance • These techniques are based on the natural decay rate of unstable, radioactive isotopes • Most elements have several isotopes (versions of the element with different numbers of neutrons) • Isotopes are named by their number of protons + neutrons

11. Half-Life • Some unstable isotopes decay at predictable rates • Chemists measure the half-life of these isotopes – the time it takes for half of the radioactive “parent” isotope to break down to the stable “daughter” isotope

12. Half-Life, cont’d • By comparing the proportions of parent to daughter element in a fossil, and knowing the half-life, the fossil’s age can be calculated • The best-known technique uses Carbon-14; however, with a half-life of ~5700 years, C-14 cannot be used for fossils that are more than 70,000 years old

13. Index Fossils • Fossils of organisms that existed in only certain spans of time over wide geographic areas • Help scientists date the rock in which the fossil is found • Also help scientists figure out which rock layers correspond to each other in different areas

14. Geological Eras • Geological time is broken down into segments, divided by events in Earth’s history such as a major climatic shift, a mass extinction, or the advent of a new group of organisms • Era = longest unit of geological time; consists of 2+ periods • Period = last tens of millions of years • Epoch = smallest unit of geological time

15. Precambrian Time • From 4.6 BYA to approx 540 MYA (~87% of Earth’s history) • First – simple, anaerobic (no O2 required) organisms • Then – photosynthetic organisms appear (and add O2 to atmosphere)

16. Precambrian Time • Oldest fossils, in rocks dated to approx. 3.5 BYA, include: • Cyanobacteria (left) • Stromatolites (mats of cyanobacteria; right)

17. Precambrian, cont’d • First fossil evidence of eukaryotes dates to approx. 2 BYA • Few fossils exist from this time • Most organisms were soft-bodied • No life on land, all on sea

18. Paleozoic Era • From ~540 – 245 MYA • “Cambrian Explosion,” at beginning of this era, leads to appearance of many new types of plants and animals

19. Paleozoic Era, cont’d • First organisms w/ shells and “bones” (mostly in the form of outer skeletons, aka exoskeletons) • Fish (early vertebrates) appear in 1st half of era • Plants move to land • Amphibians, then reptiles, appear in 2nd half of era

20. Paleozoic Era, cont’d • Near end of Paleozoic, continents drift together (Pangaea) and seas drop to lowest level ever

21. Paleozoic Era, cont’d • Three mass extinctions occurred during this era: 440 MYA, 330 MYA, 245 MYA • At 245 MYA, approx. 96% of all animal species living at the time became extinct • Many fish and reptiles were not affected • Many amphibians were wiped out • Suspected causes include: dramatic changes to climate, volcanic activity, asteroid / comet impact

22. Mesozoic Era • From ~245 – 65 MYA • During Triassic (first third): • Mammals and dinosaurs appear • Also present - fish, insects, reptiles • Cone-bearing plants

23. Mesozoic, cont’d • During Jurassic (middle third): • Height of dinosaurs • Birds appear • During Cretaceous (final third): • Reptiles – including dinosaurs – still dominant • Spread of mammals • Appearance of flowering plants, including flowering trees (angiosperms)

24. Mesozoic, cont’d • Two mass extinctions: • ~210 MYA; mild by comparison • ~65 MYA; led to extinction of 2/3 of all land species, including the dinosaurs, and 3/4 of all marine species • Suspected causes: asteroid / comet impact; volcanic activity

25. Cenozoic Era • From ~65 MYA to present • Fossils from this era are common and well-preserved • Mammals, especially primates, diversify and spread

26. Cenozoic, cont’d • During Tertiary period: • Climates generally warm • Marine mammals appear • Flowering plants dominate on land • Land mammals increase in size • During Quarternary period: • Climates cool (series of ice ages) until 20,000 yrs ago – warming trend, glaciers melt, seas rise • Modern humans may have first appeared ~100,000 yrs ago