Geologic Time Geologic time scale spans 4.5 billion years.

The last ~500 million years are detailed due to the study of fossils.

Fossils are the remains or traces of prehistoric life.

Fossils have been used to divide geologic time into eons, eras, periods, and epochs.

All time before the Phanerozoic Eon is referred to as the Precambrian Era. All time before 545 Ma.

Geologic Time The Precambrian Era has been divided

(Hadean, Archaean, and Proterozoic), however; uncertainty exists in the time due to its old age and the lack of lifeforms during the time.

What is a fossil?

Fossils are the remains or traces of organisms found in sedimentary rocks.

What conditions are necessary for fossils to form?1) Rapid Burial

If the organism is quickly buried by fine-grained sediment it is protected from being eaten by scavengers or decomposed by bacteria (i.e. little to no oxygen).

2) Presence of Hard Body Parts

Fossils of organisms that contained hard parts are abundant in the fossil record, but only rare traces of soft tissue organisms are seen as fossils. Soft-bodied organisms could get buried by volcanic ash.

3) Low Oxygen Environment

In a low oxygen environment, there is less bacteria and as a result, decomposition is slower.

What is the importance of fossils to geologist? What does a fossil indicate?

1) Fossils indicate the age of sedimentary rocks. Approximate age of the rock can be determined if we know when a life form existed on Earth. E.g. Olenellus Trilobites indicate Cambrian Period of Paleozoic Era (540-490 Ma).

2) Fossils indicate the environments in which rocks formed. For example, fossils of coral indicate a warm tropical environment.

3) Fossils are used to correlate (match up) rocks. FOSSIL CORRELATION!

What is the importance of fossils to geologist? what does a fossil indicate?

4) Fossils provide the basis by which the subdivisions of the Geologic Time Scale are made.

Division of the Geologic Time Scale is marked by some significant event in the evolution of Earth.

Example: extinctions marked the end of the Paleozoic Era and the Cenozoic Era. The extinction of trilobites marked the end of the Paleozoic Era (248 Ma). The extinction of dinosaurs marked the end of the Mesozoic Era (65 Ma).

5) Fossils can also indicate evolutionary pathways.

Fossil evidence show the progression (evolution) of life forms with time.

Invertebrates → Fish → Land Plants → amphibians → reptiles → Mammals → Birds → Humans

Fossils are preserved in the rock record in several ways!

1) Petrification (or Petrifaction) By Replacement

2) Carbonization

3) Mold and Cast

4) Preservation (Intact)

Ice, Mummification, and Amber

5) Traces (Indirect Evidence)

Tracks, Burrows/Tunnels, Eggs, gastrolites (stomach stones), and Coprolites (feces).

Information From Trace Fossils

For example, a measurement of the depth and surface area of a footprint when coupled with the supposed soil characteristics in which it was made can give a reasonably accurate measure of the creature’s body mass.

Occurs when the small internal cavities and pores of the original structure are filled with precipitated mineral matter.

Occurs when cell walls and solid material are removed and replaced by mineral material carried by ground water. The process is called replacement.

Sometimes internal details and structures are retained.

Occurs when fine sediment encloses delicate matter such as leaves (e.g. ferns) in a oxygen-poor environment. As time passes, pressure squeezes out the liquid and gaseous components of the organism leaving behind a thin residue of carbon.

Often preserve a replica of a plant or animal in sedimentary rocks.

The mold shows only the original EXTERNAL SHAPE and SURFACE MARKINGS of the organism. It does not reveal the internal structure!

An organism is buried in sediment and then dissolved by groundwater leaving a hollow depression or an impression called a mold.

When minerals or sediment fills the hollow depression or impression it forms a cast.

Original remains can be preserved in ice or in amber (hardened tree sap).

Both ice and amber protects the organism from decay (oxygen-free environment) and from pressures that would cause it to be crushed.

The entire organism has been preserved; even the soft parts, which usually decay and disappear.

Examples: (1) Woolly Mammoths preserved in ice in Alaska and Siberia. (2) Insects preserved in tree sap (amber). Cane in Jurassic Park.

Show traces left in the rock by an organism. Examples include:

Tracks - animal footprints made in soft sediment. The sediment later turns into sedimentary rock.

Coprolites - Fossil dung (feces).

Gastrolites – Stomach stones.

Burrows/Tunnels - Animal trails made in soft sediment. The sediment later turns into sedimentary rock.

Core Lab #2“Estimating Dinosaur Size and Speed from

Trackways”

Fossils of lifeforms which existed during limited segments of geologic time can be used as guides to the age of the rocks in which they are preserved. Example: Paradoxides Trilobites and Olenellus Trilobites were Cambrian lifeforms (~545 Ma).

Conditions for a good index fossil:1. Fossils found over a wide area of Earth’s surface.2. Species must have been short-lived (geologically speaking).

Precambrian Era

Little direct evidence of fossils due to lack of species with hard body parts (and thus lack of lifeforms).

Fossil evidence include algae, bacteria, and traces of soft-bodied organisms.

THE PALEOZOIC ERA REPRESENTS MORE THAN ONE DOMINANT FORM OF LIFE!

An explosion of life took place at the end of the Precambrian Era (Start of the Phanerozoic Eon, Paleozoic Era, Cambrian Period).

Early Paleozoic Era -- “Age of the Invertebrate” Invertebrates evolved into vertebrates. First land plants evolved in the Silurian Period of this era. Abundance of fishes in the Devonian Period (Middle Paleozoic), which is

known as the “age of the fishes”. Lung fish evolved into amphibians throughout the the Mississippian and

Pennsylvanian Periods. Late Paleozoic = “age of amphibians”. Amphibians evolved into reptiles in the Permian Period and reptiles are known

as the first true land dwellers. Hard shelled eggs made this possible. Mass extinctions of invertebrates including trilobites and numerous other

marine species occurred at the end of the Paleozoic Era.

Mesozoic Era -- “Age of the Reptiles”

Dinosaurs became dominant.

First birds are seen during this time.

The end of the Mesozoic Era was marked by mass extinctions of reptiles including dinosaurs and numerous other species. Meteorite!

Cenozoic Era -- “Age of the Mammals”

Mammals evolve and dominate during this time.

Flowering plants are the dominant land plant.

Some mammals became extinct during the late Cenozoic (11,000 years ago – ICE AGE). These include the mastodon, mammoth, saber-tooth cat, large ground sloth, and giant bison. (and others)

Single-celled Organisms - Invertebrates – Fish – First Land Plants – Amphibians – Reptiles – Birds – Flowering Plants - Mammals

Summers I Fly Fish And Ride Bikes For Months

OR Since I Found Flying Angels Riding Brooms

Forget Medicine. S = Single-celled organisms (Precambrian) I = Invertebrates (Early Paleozoic) F = Fishes (Middle Paleozoic) F = First Land Plants (Early To Middle

Paleozoic) A = Amphibians (Late Paleozoic) R = Reptiles (Mesozoic) B = Birds (Mesozoic) F = Flowering Plants (Mesozoic) M = Mammals (Cenozoic)

Reference:

Tarbuck & Lutgens Pages 10 & 237

Scientist and their contributions to the Geologic Time Scale:

Nicolaus Steno

Principle of Original Horizontality.

Principle of Superposition.

James Hutton and Charles Lyell

Principle of Uniformitarianism

William Smith

Principle of Faunal (Fossil) Succession

What do the divisions of the geologic time scale signify?

Divisions of Geologic Time

Eon, Era, Period, Epoch

Largest span of time

Smallest span of time

Eons Eras

PHANEROZOIC

CENOZOIC

PALEOZOIC

PROTEROZOIC

MESOZOIC

Names of the EONS

Phanerozoic (“visible life”) — The most recent eon, which began about 540 million years ago. It represents the emergence of more complex life as organisms evolved.

ProterozoicArcheanHadean — The oldest eon.

Names of the Eras Era—Subdivision of an eon. Eras of the Phanerozoic eon include:

Cenozoic (“recent life”)Mesozoic (“middle life”)Paleozoic (“ancient life”)

Eras are subdivided into periods. Periods are subdivided into epochs. All time before the above three eras

(i.e. Phanerozoic Eon) is called the Precambrian Era.

Do NOT confuse the Paleozoic Era with the Phanerozoic Eon.

Precambrian Era Nearly 4 billion years prior to the

Cambrian period (of the Phanerozoic Eon). It is the greatest part of Earth’s history.

Not divided into smaller time units (no periods or epochs) because the events are not known in great detail. This is due to age, and weathering and erosion. This is also due to the lack of fossil lifeforms, which is due to physical and chemical environmental conditions (i.e. extreme UV levels, extreme temperatures, metamorphism of rocks, and low levels of free oxygen in the atmosphere). First abundant fossil evidence does not

appear until the beginning of the Cambrian Period (~545 Ma).

Mass extinctions are episodes in geologic history where mass amounts of organisms (species) are killed off.

Two major periods of extinction are recognized in Earth’s history, which include:

1) Permian Period – Triassic Period Boundary (End of Paleozoic Era and Beginning of Mesozoic Era)2) Cretaceous Period – Tertiary Period Boundary (End of Mesozoic Era and Beginning of Cenozoic Era)

Reference: Tarbuck and Lutgens Pages 298 & 304

Some species flourished as other species went extinct!

The most widely accepted hypothesis for the extinction at the end of the Paleozoic Era is the plate tectonic assembly of Pangaea and the loss of habitat. This was 245 Ma and 96 % of marine species disappeared. Trilobites are one example!

The most widely accepted hypothesis for the extinction at the end of the Mesozoic Era is the impact of a great meteorite and the corresponding disruption of climate. This was 65 Ma and over 50 % of all species went extinct. This ended dinosaurs (or large reptiles).

Other possible explanations include:

1) falls in sea levels;

2) climatic changes;

3) prolonged volcanic eruptions; and

4) periods of lack of oxygen in oceans.