Geologists use the fossil record to determine natural cut-offs Fossils of similar living things are used to represent a specific time period Transitions from one dominant group of organisms to another can mark the division of time periods (Eras, periods, epochs) Other geologic events present in rock layers are also used to mark transitions between time periods: Climate change Extraterrestrial impact Continental drift Change in dominant rock type
4 major eras in Earth’s history Precambrian 4.5 bya-540 mya Paleozoic 540mya- 250 mya Mesozoic 250 mya- 65 mya Cenozoic 65 mya-present
Longest Era- spans about 4 billion years or 89% of Earth’s total history Lots of volcanism creating Earth’s crust Some very simple organisms- bacteria, algae, protozoa Oldest Precambrian rocks on Earth date to about 3.5 billion years old Asteroids are thought to have formed at approximately same time as all terrestrial planets- all meteorites that have been recovered from Earth’s surface date to around 4.6 billion years.
Paleozoic begins with “Cambrian Explosion”- huge diversity of life Middle of Paleozoic: life made the transition from entirely aquatic to both aquatic and terrestrial Paleozoic ends with the End Permian Mass Extinction- largest loss of life EVER on Earth- over 90% of all living things died out
Age of Dinosaurs” Lizards/reptiles are the dominant life form during Mesozoic Birds and mammals also appeared during the Mesozoic
With dinosaurs gone, mammals and birds flourished Age of Mammals Mammals became larger Global temperatures cooled Apes and humans appeared Still continues today
5 major mass extinctions 1. End Ordovician - global cooling (57% marine genera) 2. Devonian - 20 degree drop in ocean temperatures (50% marine genera) 3. End Permian - global cooling? Global warming? (90% marine genera, 70% land species) 4. End Triassic - global cooling (48% marine genera) 5. Cretaceous (K/T) - extraterrestrial impact (50% marine genera, 18% terrestrial families)
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