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.

Geologic Time Scale A Historical Perspective Reference: Tarbuck & Lutgens Pages 10 & 237 Geologic Time Scale A Historical Perspective

Geologic Time Scale A Historical Perspective 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

P H A N E R O Z O IC C E N O Z OI C MESOZOIC P A L E O Z O I C PROTEROZOIC MESOZOIC

Divisions of Geologic Time Geologic Time Scale What do the divisions of the geologic time scale signify? Eons Eras Divisions of Geologic Time Eon, Era, Period, Epoch Largest span of time Smallest span of time

Geologic Time The Precambrian Era (Not an Eon – but you don’t need to know the Eons) – collectively called ‘the precambrian’ 85% of Earths history! uncertainty exists in the time due to its old age and the lack of lifeforms during the time. Not suitable for life (extreme weather and abiotic conditions) **All time before 545 Ma.

P H A N E R O Z O IC C E N O Z OI C MESOZOIC P A L E O Z O I C PROTEROZOIC MESOZOIC

EONS- largest time frames Don’t need to know the Eons in Precambrian (just collectively called Precambrian) Phanerozoic (“visible life”) — The most recent eon, which began about 540 million years ago. Fossils here help us determine the ages About 13% of Earth history. It represents the emergence of more complex life as organisms evolved.

P H A N E R O Z O IC C E N O Z OI C MESOZOIC P A L E O Z O I C PROTEROZOIC MESOZOIC

Geologic Time Scale A Historical Perspective Reference: Tarbuck & Lutgens Pages 10 & 237 Geologic Time Scale A Historical Perspective

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. You do NOT need to know the periods or epochs. Do NOT confuse the Paleozoic Era with the Phanerozoic Eon.

P H A N E R O Z O IC C E N O Z OI C MESOZOIC P A L E O Z O I C PROTEROZOIC MESOZOIC

FOSSILS INDICATED EVOLUTIONARY PATHWAYS: Each Era can be divided based on the fossil evidence found in rocks during those times Precambrian Era – “single celled organisms” 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.

FOSSILS INDICATED EVOLUTIONARY PATHWAYS: Each Era can be divided based on the fossil evidence found in rocks during those times Phanerozoic Eon Fossil evidence is divided in its 3 Era’s 1. Paleozoic (ancient) 2. Mesozoic 3. Cenozoic (recent)

Fossils Indicate Evolutionary Pathways: 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 Middle Paleozoic Late Paleozoic (more recent)

P H A N E R O Z O IC C E N O Z OI C MESOZOIC P A L E O Z O I C PROTEROZOIC MESOZOIC

FOSSILS INDICATE EVOLUTIONARY PATHWAYS: Paleozoic Era Divided Early Paleozoic Era -- “Age of the Invertebrate” Invertebrates evolved into vertebrates. Middle Paleozoic Era – “Age of Fishes” Abundance of fishes and First land plants Late Paleozoic -- “age of amphibians”. Lung fish evolved into amphibians Mass extinctions of invertebrates including trilobites and numerous other marine species occurred at the end of the Paleozoic Era.

Fossils Indicate Evolutionary Pathways: Mesozoic Era -- “Age of the Reptiles” Reptiles - Dinosaurs became dominant. First birds are seen during this time. First Flowering Plants 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)

Fossils Indicate Evolutionary Pathways: **You MUST know the order as well as what time each one was a major player in Single-celled Organisms - Invertebrates – Fish – First Land Plants – Amphibians – Reptiles – Birds – Flowering Plants - Mammals Summers I Fish First And Ride Bikes For Months OR - Since I Found Flying Angels Riding Brooms Forget Medicine.

Mnemonic device Summers I Fish First And Ride Bikes For Months S = Single-celled organisms (Precambrian) I = Invertebrates (Early Paleozoic -1/4) F = Fishes (Middle Paleozoic -2/4) F = First Land Plants (Early To Middle Paleozoic -3/4) A = Amphibians (Late Paleozoic – 4/4) R = Reptiles (Mesozoic – 1/3) B = Birds (Mesozoic – 2/3) F = Flowering Plants (Mesozoic -3/3) M = Mammals (Cenozoic)

Mass Extinctions 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!

Mass Extinctions 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.

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