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

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

Fossil Formation, cont’d

3. 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

4. Amber-preserved fossils = preserved in tree resin that hardens around the organism

5. 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