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The Fossil Record and the Geological Timescale. Chapter 12 In your textbook. Fossils. The preserved remains of anything that was once living When an animal or plant dies, its remains usually rot away to nothing.
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The Fossil Record and the Geological Timescale Chapter 12 In your textbook
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.
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
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
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
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
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
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
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
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
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
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
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
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
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)
Precambrian Time • Oldest fossils, in rocks dated to approx. 3.5 BYA, include: • Cyanobacteria (left) • Stromatolites (mats of cyanobacteria; right)
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
Paleozoic Era • From ~540 – 245 MYA • “Cambrian Explosion,” at beginning of this era, leads to appearance of many new types of plants and animals
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
Paleozoic Era, cont’d • Near end of Paleozoic, continents drift together (Pangaea) and seas drop to lowest level ever
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
Mesozoic Era • From ~245 – 65 MYA • During Triassic (first third): • Mammals and dinosaurs appear • Also present - fish, insects, reptiles • Cone-bearing plants
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)
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
Cenozoic Era • From ~65 MYA to present • Fossils from this era are common and well-preserved • Mammals, especially primates, diversify and spread
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