The Fossil Record and the Geological Timescale

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The Fossil Record and the Geological Timescale

• Chapter 12
• In your textbook

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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.

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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

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Fossil Formation

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

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Fossil Formation, contd

• Trace fossils record an organisms 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

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Fossil Formation, contd

1. Amber-preserved fossils preserved in tree resin
   that hardens around the organism
2. Preserved remains when an entire organism
   becomes encased in ice or volcanic ash, or
   immersed in a bog

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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

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Age of Fossils, contd

• 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

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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

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Radiometric Dating

• The discovery of a way to estimate a fossils
  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

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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

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Half-Life, contd

• By comparing the proportions of parent to
  daughter element in a fossil, and knowing the
  half-life, the fossils 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

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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

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Geological Eras

• Geological time is broken down into segments,
  divided by events in Earths 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

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Precambrian Time

• From 4.6 BYA to approx 540 MYA (87 of Earths
  history)
• First simple, anaerobic (no O2 required)
  organisms
• Then photosynthetic organisms appear (and add
  O2 to atmosphere)

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Precambrian Time

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

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Precambrian, contd

• 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

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Paleozoic Era

• From 540 245 MYA
• Cambrian Explosion, at beginning of this era,
  leads to appearance of many new types of plants
  and animals

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Paleozoic Era, contd

• 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

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Paleozoic Era, contd

• Near end of Paleozoic, continents drift together
  (Pangaea) and seas drop to lowest level ever

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Paleozoic Era, contd

• 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

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Mesozoic Era

• From 245 65 MYA

• During Triassic (first third)
• Mammals and dinosaurs appear
• Also present - fish, insects, reptiles
• Cone-bearing plants

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Mesozoic, contd

• 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)

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Mesozoic, contd

• 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

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Cenozoic Era

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

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Cenozoic, contd

• 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

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