The Geologic Time Scale

Geologic Time Scale Overview

• A framework used to describe the timing and relationships between events in Earth’s history.
• Organized into hierarchical segments: Eons, Eras, Periods, and Epochs.

Eons

• Precambrian Eon: Earliest and longest span of geological time, covering 88% of Earth's history.
• Phanerozoic Eon: Encompasses all of the visible life and geological events we know, beginning around 541 million years ago.

Eras within Phanerozoic Eon

• Paleozoic Era: Known as the "Age of Invertebrates," featuring the development of diverse marine life.
  • Key Periods: Cambrian, Ordovician, Silurian, Devonian, Carboniferous, and Pennsylvanian.
• Mesozoic Era: Recognized as the "Age of Reptiles," dominated by dinosaurs and early mammals.
  • Key Periods: Triassic, Jurassic, and Cretaceous.
• Cenozoic Era: Known as the "Age of Mammals," marked by the evolution and dominance of mammals and birds.
  • Key Periods: Paleogene, Neogene, Quaternary.

Periods within Eras

• Cambrian Period: Significant for the first appearances of many major animal phyla, known for abundant fossils like Dickinsonia costata.
• Ordovician Period: Characterized by a diverse marine ecosystem with organisms such as Haikouichthys ercaicunensis.
• Silurian Period: Marked the emergence of jawed vertebrates and terrestrial flora.
• Devonian Period: Known as the "Age of Fish," with significant marine and terrestrial evolutionary developments.
• Carboniferous Period: Notable for extensive coal formation and dominant forests.
• Permian Period: The final period of the Paleozoic, ended with the largest mass extinction.
• Triassic, Jurassic, Cretaceous Periods (Mesozoic): Sequential periods known for the rise and fall of dinosaurs.
• Paleogene, Neogene, Quaternary Periods (Cenozoic): Marked by significant mammalian diversification and ultimately human evolution.

Epochs within Periods

• Holocene Epoch: Current geological epoch, beginning approximately 11,700 years ago after the last Ice Age.
• Pleistocene Epoch: Preceded the Holocene, known for glaciers covering significant parts of the Earth.
• Key earlier epochs in the Cenozoic include Paleocene, Eocene, Oligocene, Miocene, and Pliocene characterized by significant climate and ecological changes.

Noteworthy Fossils

• Dickinsonia costata: One of the earliest multi-cellular organisms; found in Cambrian deposits.
• Haikouichthys ercaicunensis: Early vertebrate, providing insight into the evolution of jawed vertebrates.

Memory Aid

• "Camels often sit down carefully perhaps their joints creak" helps to remember the order of geological periods within the Paleozoic Era: Cambrian, Ordovician, Silurian, Devonian, Carboniferous, and Pennsylvanian.